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Functionalization vs. fragmentation: N-aldehyde oxidation mechanisms and secondary organic aerosol formation
Abstract
Because of their relatively well-understood chemistry and atmospheric relevance, aldehydes represent a good model system for carbon-carbon fragmentation reactions in organic-aerosol aging mechanisms. Small aldehydes such as ethanal and propanal react with OH radicals under high NO(x) conditions to form formaldehyde and ethanal, respectively, with nearly unit yield. CO(2) is formed as a coproduct. This path implies the formation of the C(n-1) aldehyde, or an aldehyde with one fewer methylene group than the parent. However, as the carbon number of the n-aldehyde increases, reaction with the carbon backbone becomes more likely and the C(n-1) formation path becomes less important. In this work we oxidized n-pentanal, n-octanal, n-undecanal and n-tridecanal with OH radicals at high NO(x). The C(n-1) aldehyde molar yields after the peroxyl radical + NO reaction were 69 ± 15, 36 ± 10, 16 ± 5 and 4 ± 1%, respectively. Complementary structure-activity relationship calculations of important rate constants enable estimates of branching ratios between several intermediates of the C(n)n-aldehyde reaction with OH: C(n) peroxyacyl nitrate versus C(n) alkoxyacyl radical formation, C(n-1) alkyl nitrate versus C(n-1) alkoxy radical, and C(n-1) aldehyde formation versus isomerization products. We also measured SOA mass yields, which we compare with analogous n-alkanes to understand the effect of fragmentation on organic-aerosol formation.
... Due to the weaker bond strength of the aldehydic hydrogen, the oxidation of aldehydes is frequently initiated by the abstraction of that atom (Calvert et al., 2011;Mellouki et al., 2003Mellouki et al., , 2015. Under high-NO x conditions, this commonly leads to C n−1 alkyl nitrates, C n−1 aldehydes, and C n−1 alkoxy isomerization products (Chacon-Madrid et al., 2010) via scission of the carbon chain (red arrows in Fig. 1) through acyl (i.e., CO loss) and acyloxy (i.e., CO 2 loss) intermediates Vereecken and Peeters, 2009). The exceptions are the C n peroxy acids (PA) and C n peroxyacyl nitrates (PAN) (Calvert et al., 2011;Mellouki et al., 2003Mellouki et al., , 2015Chacon-Madrid et al., 2010) formed in reactions of acyl peroxy radicals (APR) with HO 2 and NO 2 , respectively (see Fig. 1). ...
... Under high-NO x conditions, this commonly leads to C n−1 alkyl nitrates, C n−1 aldehydes, and C n−1 alkoxy isomerization products (Chacon-Madrid et al., 2010) via scission of the carbon chain (red arrows in Fig. 1) through acyl (i.e., CO loss) and acyloxy (i.e., CO 2 loss) intermediates Vereecken and Peeters, 2009). The exceptions are the C n peroxy acids (PA) and C n peroxyacyl nitrates (PAN) (Calvert et al., 2011;Mellouki et al., 2003Mellouki et al., , 2015Chacon-Madrid et al., 2010) formed in reactions of acyl peroxy radicals (APR) with HO 2 and NO 2 , respectively (see Fig. 1). The subsequent branching of the C n−1 alkoxy radical towards isomerization, decomposition, and reaction with O 2 depends on the size and substitution of the alkyl chain, with the longer chains (≥ C 7 ) favoring the isomerization paths (Atkinson and Arey, 2003;Vereecken and Peeters, 2010;Ziemann, 2005, 2009;Kwok et al., 1996;Atkinson, 2007). ...
... Aldehydes are common first-generation products in several hydrocarbon oxidation sequences (Atkinson and Arey, 2003), and thus their tendency to form HOMs is of special interest. Chacon-Madrid et al. (2010) have studied the SOA yields from several n-aldehyde oxidation systems under high-NO x conditions and contrasted the findings with similar n-alkane oxidation. Under their high-NO x reaction conditions, they found significantly lower SOA yields for the aldehydes and attributed it to the relatively volatile PAN formation (see Fig. 1). ...
Aldehydes are common constituents of natural and polluted atmospheres, and their gas-phase oxidation has recently been reported to yield highly oxygenated organic molecules (HOMs) that are key players in the formation of atmospheric aerosol. However, insights into the molecular-level mechanism of this oxidation reaction have been scarce. While OH initiated oxidation of small aldehydes, with two to five carbon atoms, under high-NOx conditions generally leads to fragmentation products, longer-chain aldehydes involving an initial non-aldehydic hydrogen abstraction can be a path to molecular functionalization and growth. In this work, we conduct a joint theoretical–experimental analysis of the autoxidation chain reaction of a common aldehyde, hexanal. We computationally study the initial steps of OH oxidation at the RHF-RCCSD(T)-F12a/VDZ-F12//ωB97X-D/aug-cc-pVTZ level and show that both aldehydic (on C1) and non-aldehydic (on C4) H-abstraction channels contribute to HOMs via autoxidation. The oxidation products predominantly form through the H abstraction from C1 and C4, followed by fast unimolecular 1,6 H-shifts with rate coefficients of 1.7×10-1 and 8.6×10-1 s−1, respectively. Experimental flow reactor measurements at variable reaction times show that hexanal oxidation products including HOM monomers up to C6H11O7 and accretion products C12H22O9−10 form within 3 s reaction time. Kinetic modeling simulations including atmospherically relevant precursor concentrations agree with the experimental results and the expected timescales. Finally, we estimate the hexanal HOM yields up to seven O atoms with mechanistic details through both C1 and C4 channels.
... More oxidized molecules can fragment more easily, as shown by Kroll et al. (2009), reducing their ability to form organic aerosol when reacting with the OH radical. Chacon-Madrid et al. (2010) showed that naldehydes fragment significantly more than n-alkanes with similar vapor pressures, thus forming less SOA. When examining the gas-phase chemistry of different volatile 20 organic compounds (VOCs) with the OH radical in the presence of NO x (Atkinson and Arey, 2003;Atkinson, 2000Atkinson, , 2007, it is clear the alkoxy radical is the leading intermediate that fragments molecules especially when other functionalities are already present (Atkinson, 2007;Kwok et al., 1996). ...
... A key assumption is that interferences from other species were minimal. An exception to this was n-tridecanal, explained in Chacon-Madrid et al. (2010), where a C n−1 dycarbonyl is formed due to isomerization. ...
... SOA mass yields for the 10 5 µg m −3 sequence (n-tridecanal, pinonaldehyde, 2-and 7tridecanone, and n-pentadecane) are presented in Figs. 3 and 4. All of these species are exposed to similar OH and NO x concentrations, and none of the reagents showed significant losses to the walls before the OH-radical source was turned on, indicating 5 that wall losses such as those reported by Matsunaga and Ziemann (2010), were not a problem. n-Pentadecane SOA mass yields are shown as a function reproducing data from Presto et al. (2010) and n-tridecanal yields are from Chacon-Madrid et al. (2010). We shall use the n-pentadecane mass yield curve for reference throughout this discussion. ...
The transformation process that a carbon backbone undergoes in the atmosphere is complex and dynamic. Understanding all these changes for all the species in detail is impossible; however, choosing different molecules that resemble progressively higher stages of oxidation or aging and studying them can give us an insight into general characteristics and mechanisms. Here we determine secondary organic aerosol (SOA) mass yields of two sequences of molecules reacting with the OH radical at high NO<sub>x</sub>. Each sequence consists of species with similar vapor pressures but a succession of oxidation states. The first sequence consists of n -pentadecane, n -tridecanal, 2-, 7-tridecanone, and pinonaldehyde. The second sequence consists of n -nonadecane, n -heptadecanal and cis -pinonic acid. Oxidized molecules tend to have lower relative SOA mass yields; however, oxidation state alone was not enough to predict how efficiently a molecule forms SOA. Certain functionalities are able to fragment more easily than others, and even the position of these functionalities on a molecule can have an effect. n -Alkanes tend to have the highest yields, and n -aldehydes the lowest. n -Ketones have slightly higher yields when the ketone moiety is located on the side of the molecule and not in the center. In general, oxidation products remain efficient SOA sources, though fragmentation makes them less effective than comparable alkanes.
... For instance, a commonly used gas-phase aging scheme 17 in atmospheric models only considers functionalization reactions despite evidence that fragmentation reactions become increasingly relevant with oxidation. [22][23][24] This scheme has been shown to overestimate ambient SOA mass concentrations. [25][26][27] Heterogeneous chemistry is rarely simulated in chamber and atmospheric models, which is likely because this oxidation pathway is believed to be signicantly slower than gas-phase chemistry. ...
... Reaction with OH also leads to the fragmentation of the precursor into higher volatility species 24 and this is characterized using the probability of fragmentation (P frag ). Since the likelihood of fragmentation increases as a molecule becomes more functionalized (i.e. has more oxygen atoms), 23 and since it is oxygen addition (functionalization) that leads to decreases in c*, the P frag values are parameterized as: ...
Secondary organic aerosol (SOA) is an important fraction of the fine-mode atmospheric aerosol mass. Frameworks used to develop SOA parameters from laboratory experiments and subsequently used to simulate SOA formation...
... Carbonyl compounds are classified as polar organic compounds, constituting a portion of the oxygenated organic compounds in atmospheric particulate matter (PM). Aliphatic carbonyl compounds are directly emitted into the atmosphere from primary biogenic and anthropogenic sources (Schauer et al., 2001(Schauer et al., , 2002a, as well as being secondary products of the atmospheric oxidation of hydrocarbons (Chacon-Madrid et al., 2010;Zhang et al., 2015;Han et al., 2016). ...
... The similarity of the n-alkanes / n-alkanal ratio between MR and the engine studies (after DPF) strongly suggests that diesel vehicle emissions were the main source of alkanals at MR. The higher ratios at the other sites may be due to greater air mass ageing and loss of alkanals due to their higher reactivity (Chacon-Madrid and Donahue, 2011;Chacon-Madrid et al., 2010). ...
Three groups of aliphatic carbonyl compounds, the n-alkanals
(C8–C20), n-alkan-2-ones (C8–C26), and
n-alkan-3-ones (C8–C19), were measured in both particulate
and vapour phases in air samples collected in London from January to
April 2017. Four sites were sampled including two rooftop background sites,
one ground-level urban background site, and a street canyon location on
Marylebone Road in central London. The n-alkanals showed the highest
concentrations, followed by the n-alkan-2-ones and the n-alkan-3-ones, the
latter having appreciably lower concentrations. It seems likely that all
compound groups have both primary and secondary sources and these are
considered in light of published laboratory work on the oxidation
products of high-molecular-weight n-alkanes. All compound groups show
a relatively low correlation with black carbon and NOx in the
background air of London, but in street canyon air heavily impacted by
vehicle emissions, stronger correlations emerge, especially for the
n-alkanals. It appears that vehicle exhaust is likely to be a major
contributor for concentrations of the n-alkanals, whereas it is a much smaller
contributor to the n-alkan-2-ones and n-alkan-3-ones. Other primary sources
such as cooking or wood burning may be contributors for the ketones but were
not directly evaluated. It seems likely that there is also a significant
contribution from the photo-oxidation of n-alkanes and this would be consistent
with the much higher abundance of n-alkan-2-ones relative to
n-alkan-3-ones if the formation mechanism were through the oxidation of
condensed-phase alkanes. Vapour–particle partitioning fitted the Pankow model
well for the n-alkan-2-ones but less well for the other compound groups,
although somewhat stronger relationships were seen at the Marylebone Road
site than at the background sites. The former observation gives support to
the n-alkane-2-ones being a predominantly secondary product, whereas primary
sources of the other groups are more prominent.
... Schauer et al. (2002) estimated that cooking seed oils might contribute a significant fraction of lighter n-alkanoic acids such as nonanoic acid in the atmosphere. The VOCs emitted from heated cooking oils were dominated by aldehydes (Klein et al., 2016a), which were suggested to be potential SOA precursors (Chacon-Madrid et al., 2010). Despite these previous efforts, there are still no available data regarding SOA formation from heated cooking oils. ...
... Omega-6 fatty acids are a family of poly-unsaturated fatty acids that have in common a final carbon-carbon double bond in the n-6 position, counting from the methyl end (Simopoulos, 2002). The peroxyl radical reactions of omega-6 fatty acids might emit long-chain aldehydes (Gardner, 1989), which have been suggested as potential SOA precursors (Chacon-Madrid et al., 2010). ...
Cooking emissions can potentially contribute to
secondary organic aerosol (SOA) but remain poorly understood. In this study,
formation of SOA from gas-phase emissions of five heated vegetable oils
(i.e., corn, canola, sunflower, peanut and olive oils) was investigated in a
potential aerosol mass (PAM) chamber. Experiments were conducted at
19–20 °C and 65–70 % relative humidity (RH). The
characterization instruments included a scanning mobility particle sizer
(SMPS) and a high-resolution time-of-flight aerosol mass spectrometer
(HR-TOF-AMS). The efficiency of SOA production, in ascending order, was
peanut oil, olive oil, canola oil, corn oil and sunflower oil. The major SOA
precursors from heated cooking oils were related to the content of
monounsaturated fat and omega-6 fatty acids in cooking oils. The average
production rate of SOA, after aging at an OH exposure of 1. 7 × 1011 molecules cm−3 s, was 1. 35 ± 0. 30 µg min−1, 3 orders of magnitude lower compared with
emission rates of fine particulate matter (PM2. 5) from heated cooking
oils in previous studies. The mass spectra of cooking SOA highly resemble
field-derived COA (cooking-related organic aerosol) in ambient air, with
R2 ranging from 0.74 to 0.88. The average carbon oxidation state
(OSc) of SOA was −1.51 to −0.81, falling in the range between
ambient hydrocarbon-like organic aerosol (HOA) and semi-volatile oxygenated
organic aerosol (SV-OOA), indicating that SOA in these experiments was
lightly oxidized.
... Fragmentation can generate high-volatility species thus promoting evaporation. Since fragmentation increased with O / C and the role of functionalization decreased Chacon-Madrid and Donahue, 2011;Chacon-Madrid et al., 2010), the role of fragmentation became more and more significant as the reaction proceeded. When the fragmentation dominated over functionalization, the overall volatility of the products increased, i.e., the saturated vapor pressures increased. ...
... This indicates an increasing role of fragmentation since fragmentation cleaved the carbon frame and formed some smaller molecules with higher volatility. As the reaction proceeded, the products got more oxidized and the O / C ratio of products increased; the fragmentation of the compounds became more and more significant Chacon-Madrid and Donahue, 2011;Chacon-Madrid et al., 2010). After the continuous decrease, GE OH (t) decreased to almost zero or even negative for the limonene case (Fig. 3c). ...
Oxidation by hydroxyl radical (OH) and ozonolysis are the two major pathways
of daytime biogenic volatile organic compound (BVOC) oxidation and
secondary organic aerosol (SOA) formation. In this study, we investigated
the particle formation of several common monoterpenes (α-pinene,
β-pinene and limonene) by OH-dominated oxidation, which has seldom
been investigated. OH oxidation experiments were carried out in the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction)
chamber in Jülich, Germany, at low NOx (0.01 ~ 1 ppbV)
and low ozone (O3) concentration (< 20 ppbV). OH concentration
and total OH reactivity (kOH) were measured directly, and through this
the overall reaction rate of total organics with OH in each reaction system
was quantified. Multi-generation reaction process, particle growth, new
particle formation (NPF), particle yield and chemical composition were analyzed
and compared with that of monoterpene ozonolysis. Multi-generation products
were found to be important in OH-dominated SOA formation. The relative role
of functionalization and fragmentation in the reaction process of OH
oxidation was analyzed by examining the particle mass and the particle size
as a function of OH dose. We developed a novel method which quantitatively
links particle growth to the reaction rate of OH with total organics in a
reaction system. This method was also used to analyze the evolution of
functionalization and fragmentation of organics in the particle formation by
OH oxidation. It shows that functionalization of organics was dominant in
the beginning of the reaction (within two lifetimes of the monoterpene) and
fragmentation started to play an important role after that. We compared
particle formation from OH oxidation with that from pure ozonolysis. In
individual experiments, growth rates of the particle size did not
necessarily correlate with the reaction rate of monoterpene with OH and
O3. Comparing the size growth rates at the similar reaction rates of
monoterpene with OH or O3 indicates that, generally, OH oxidation and
ozonolysis had similar efficiency in particle growth. The SOA yield of
α-pinene and limonene by ozonolysis was higher than that of OH
oxidation. Aerosol mass spectrometry (AMS) shows SOA elemental composition
from OH oxidation follows a slope shallower than −1 in the O / C vs. H / C
diagram, also known as Van Krevelen diagram, indicating that oxidation proceeds without significant loss of
hydrogen. SOA from OH oxidation had higher H / C ratios than SOA from
ozonolysis. In ozonolysis, a process with significant hydrogen loss seemed
to play an important role in SOA formation.
... If the second mechanism dominates over the first mechanism, there must be a significant amount of fragmentation in the overall process, resulting in highly-oxidized products with C * in the LVOC and SVOC range and thus 7-10 carbon atoms. This fragmentation is consistent with our overall understanding of hydrocarbon oxidation -as substitution around carbon bonds increases, the probability of carbon bond fragmentation increases as well (Lim and Ziemann, 2009;Kroll et al., 2009;Chacon Madrid et al., 2010). For lighter hydrocarbons, this fragmentation leads to very volatile products such as formaldehyde, acetone, and other highly oxidized vapors. ...
... Sequences at roughly constant C * consisting of progressively more oxygenated compounds show decreased SOA formation with increasing oxygenation, along with gas-phase product molecules consistent with fragmentation (Chacon Madrid et al., 2010;Chacon Madrid and Donahue, 2011). All of these experiments support the general finding that fragmentation processes become more important with increasing substitution associated with increasing oxygenation during OA aging. ...
We discuss the use of a two-dimensional volatility-oxidation space (2-D-VBS) to describe organic-aerosol chemical evolution. The space is built around two coordinates, volatility and the degree of oxidation, both of which can be constrained observationally or specified for known molecules. Earlier work presented the thermodynamics of organics forming the foundation of this 2-D-VBS, allowing us to define the average composition (C, H, and O) of organics, including organic aerosol (OA) based on volatility and oxidation state. Here we discuss how we can analyze experimental data, using the 2-D-VBS to gain fundamental insight into organic-aerosol chemistry. We first present a well-understood "traditional" secondary organic aerosol (SOA) system – SOA from α -pinene + ozone, and then turn to two examples of "non-traditional" SOA formation – SOA from wood smoke and dilute diesel-engine emissions. Finally, we discuss the broader implications of this analysis.
... Non-oxidative association reactions leading to high-molecular-weight products (oligomers) clearly occur, [23,24] but when the carbon number is followed in environments where strong oxidative aging occurs, the general tendency is for carbon number to decrease due to fragmentation. [1,[25][26][27][28] The bottom line is that most organic emissions have OS C , ,À1.5, whereas a large majority of the organics (especially organic aerosol), is highly oxidised. [29] It follows that oxidation chemistry is a crucial, inevitable, monotonic driver of organic properties in the atmosphere. ...
... Thus, aerosol species would evolve through upwards of five generations of chemistry in 24 h, if they were in the gas phase. Because the probability of fragmentation and consequent sharp increases in volatility rises rapidly with increasing oxidation state, [25][26][27] this unrestrained oxidation would sweep the system clean of organic aerosols within a day or two. For example, a C 10 backbone, with on average 1.5 oxygen atoms added per generation, will have a simple fragmentation probability (O : C) 1/4 of 0, 0.62, 0.74, 0.82, 0.88 and 0.93 for generations 0-5. ...
Organic aerosols play a critical role in atmospheric chemistry, human health and climate. Their behaviour is complex. They consist of thousands of organic molecules in a rich, possibly highly viscous mixture that may or may not be in phase equilibrium with organic vapours. Because the aerosol is a mixture, compounds from all sources interact and thus influence each other. Finally, most ambient organic aerosols are highly oxidised, so the molecules are secondary products formed from primary emissions by oxidation chemistry and possibly non-oxidative association reactions in multiple phases, including gas-phase oxidation, aqueous oxidation, condensed (organic) phase reactions and heterogeneous interactions of all these phases. In spite of this complexity, we can make a strong existential statement about organic aerosol: They exist throughout the troposphere because heterogeneous oxidation by OH radicals is more than an order of magnitude slower than comparable gas-phase oxidation.
... It is widely recognized that gas-phase VOC oxidation products (or more generically organic vapors) can undergo multi-generational oxidation given sufficient time in the atmosphere, which may substantially alter the mass and properties of SOA. For example, chamber studies using surrogate molecules -aldehydes to represent gas-phase oxidation products 15 of alkanes (Chacon-Madrid et al., 2010) and biogenic VOCs (Chacon-Madrid et al., 2012) and phenols to represent those from aromatics -have highlighted the potential of VOC oxidation products to undergo multi-generational oxidation to form SOA. In chamber experiments conducted at four different facilities, Donahue et al. (2012). showed that semi-volatile organic vapors, formed from the ozonolysis of 20 alpha-pinene, subsequently reacted with the hydroxyl radical (OH) to enhance SOA mass concentrations. ...
... The carbon number and structure of an alkane influences its SOA mass yield; for the same structure the SOA potential increases with carbon number (Lim and Ziemann, 2009;Presto et al., 2010), while for the same carbon number cyclic alkanes form the most SOA followed by linear and then branched alkanes (Lim and Ziemann, 2009;Tkacik et al., 2012). However, in 3-D models that employ SAPRC-11, a single model 10 VOC species, ALK5, is used to describe the SOA formation from alkanes roughly larger than a carbon number of 6. ...
Multi-generational gas-phase oxidation of organic vapors can influence the abundance, composition and properties of secondary organic aerosol (SOA). Only recently have SOA models been developed that explicitly represent multi-generational SOA formation. In this work, we integrated the statistical oxidation model (SOM) into SAPRC-11 to simulate the multi-generational oxidation and gas/particle partitioning of SOA in the regional UCD/CIT air quality model. In SOM, evolution of organic vapors by reaction with the hydroxyl radical is defined by (1) the number of oxygen atoms added per reaction, (2) the decrease in volatility upon addition of an oxygen atom and (3) the probability that a given reaction leads to fragmentation of the organic molecule. These SOM parameter values were fit to laboratory "smog chamber" data for each precursor/compound class. The UCD/CIT model was used to simulate air quality over two-week periods in the South Coast Air Basin of California and the eastern United States. For the regions and episodes tested, the traditional two-product SOA model and SOM produce similar SOA concentrations but a modestly different SOA chemical composition. Predictions of the oxygen-to-carbon ratio qualitatively agree with those measured globally using aerosol mass spectrometers. Overall, the implementation of the SOM in a 3-D model provides a comprehensive framework to simulate the atmospheric evolution of OA.
... been studied under marine-relevant conditions where NOx <50 ppt (Lee et al., 2009). Similarly, much of the research investigating SOA yields of individual primary-emitted aldehydes is completed under high NOx conditions (Chhabra et al., 455 2011;Chacon-Madrid et al., 2010;Chacon-Madrid and Donahue, 2011), with the only studies in the low NOx regime focused on aldehydes like pinonaldehyde that are intermediates in the oxidation of common BVOC (Chacon-Madrid et al., 2013). The long-chain acyclic aldehydes that contribute to measured RT-Vocus ions in this study have fast reaction rates with OH, are susceptible to photolysis, and are expected to form SOA based on observed new particle formation and growth during ozonolysis of an SSML in Schneider et al. (2019). ...
Dry deposition of ozone (O3) to the ocean surface and the ozonolysis of organics in the sea surface microlayer (SSML) is a potential source of volatile organic compounds (VOC) to the marine atmosphere. We use a gas chromatography system coupled to a Vocus proton transfer reaction time-of-flight mass spectrometer to determine the chemical composition and product yield of select VOC formed from ozonolysis of coastal seawater collected from Scripps Pier in La Jolla, California. Laboratory-derived results are interpreted in the context of direct VOC vertical flux measurements made at Scripps Pier. The dominant products of laboratory ozonolysis experiments and the largest non-sulfur emission fluxes measured in the field correspond to Vocus CxHy+ and CxHyOz+ ions. GC analysis suggests that C5–C11 oxygenated VOC, primarily aldehydes, are the largest contributors to these ion signals. In the laboratory, using a flow reactor experiment, we determine a VOC yield of 0.43–0.62. In the field at Scripps Pier, we determine a maximum VOC yield of 0.04–0.06. Scaling the field and lab VOC yields for an average O3 deposition flux and an average VOC structure results in an emission source of 12.6 to 136 Tg C yr-1, competitive with the DMS source of 21.1 Tg C yr-1. This study reveals that O3 reactivity to dissolved organic carbon can be a significant carbon source to the marine atmosphere and warrants further investigation into the speciated VOC composition from different seawater samples, and the reactivities and secondary organic aerosol yields of these molecules in marine-relevant, low NOx conditions.
... The formation pathways of these oxidation products are proposed here and details can be found in Text S6. † Besides, small aldehydes, such as NH 2 CH 2 CHO formed as the primary oxidation product of MEA, are easily fragmented when reacting with OH radicals. 61,62 The characteristic peak of MEA at ∼867 cm −1 decreased during NO 3 − photolysis (Fig. S2 †), also suggesting that the cleavage of C-C bonds in MEA took place in our system. We have conrmed formic acid (HCOOH) and NH 4 + as reaction products by IC analysis (boxed in orange in Fig. 3). ...
The massive industrial release of monoethanolamine (MEA) into the atmosphere highlights MEA as a potential environmental risk. Nitrate (NO3⁻) is one of the most abundant inorganic compounds and has been found to co-exist with amines in ambient particles. The photolysis of NO3⁻ can produce oxidants (OH radicals, NO2, O(³P), and N(iii)), which lead to particulate MEA decay. Furthermore, MEA degradation products are likely to yield brown carbon (BrC) due to the formation of carbonyl species. Here, we investigated the aging of MEA-containing particles mediated by NO3⁻ photolysis. Particles under different relative humidity (RH) and initial pH conditions were irradiated with 300 nm UV light. After reactions, the more acidic particles (MEA : H2SO4 : NaNO3 : HNO3 molar ratio = 4 : 1:1 : 3 and 4 : 0.75 : 1:3) show an increase in pH, while the 4 : 0.5 : 1:3 particles show a decrease in pH. We attributed these contrary pH changes to the combined results of HONO evaporation which increases the pH against MEA reactions which decreases the pH. NO3⁻ and MEA decay rates are more sensitive to the initial pH than RH. Unlike the monotonically slow decay trends at all RH for the 4 : 0.5 : 1:3 particles, NO3⁻ and MEA in more acidic 4 : 1:1 : 3 and 4 : 0.75 : 1:3 particles decay rapidly in the first few hours but followed by a slower decay. MEA reaction mechanisms in the presence of oxidants produced from NO3⁻ photolysis were proposed by combining quantum chemistry computations and speciation of the products. Furthermore, water-soluble BrC and an organic phase were formed as potential secondary organic aerosols (SOAs). This study reveals the particulate sink of MEA and its potential in BrC and SOA formation mediated by NO3⁻ photolysis in the atmosphere, which may give a new insight into the aging of amines in atmospheric aerosols.
... These small molecules mainly contribute to secondary gaseous pollutants, such as PANs, an important class of active nitrogen-containing PANs are very stable and can also be a temporary storage of NOx for long-distance transmission [78] . Heterogeneous reactions of aldehydes and other oxygenated VOCs(OVOCs) could form particulate matter, while some OVOCs, such as npentanal, n-octanal, and n-undecanal could generate SOA through gas-phase reaction either [79] . Compared with summer, the light intensity in winter is much weak, ozone pollution will also be reduced, and with the large-scale application of clean energy, the NOx concentration can be better controlled, but this does not mean that the ozone pollution will be significantly alleviated [10] . ...
Secondary air pollutants, originating from gaseous pollutants and primary particulate matter emitted by natural sources and human activities, undergo complex atmospheric chemical reactions and multiphase processes. Secondary gaseous pollutants represented by ozone and secondary particulate matter, including sulfates, nitrates, ammonium salts, and secondary organic aerosols, are formed in the atmosphere, affecting air quality and human health. This paper summarizes the formation pathways and mechanisms of important atmospheric secondary pollutants. Meanwhile, different secondary pollutants’ toxicological effects and corresponding health risks are evaluated. Studies have shown that secondary pollutants are generally more toxic than primary ones. However, due to their diverse source and complex generation mechanism, the study of the toxicological effects of secondary pollutants is still in its early stages. Therefore, this paper first introduces the formation mechanism of secondary gaseous pollutants and focuses mainly on ozone’s toxicological effects. In terms of particulate matter, secondary inorganic and organic particulate matters are summarized separately, then the contribution and toxicological effects of secondary components formed from primary carbonaceous aerosols are discussed. Finally, secondary pollutants generated in the indoor environment are briefly introduced. Overall, a comprehensive review of secondary air pollutants may shed light on the future toxicological and health effects research of secondary air pollutants.
... The cooling may result in the partitioning of gaseous species in the vapor/aerosol phase yielding the formation of particles by condensation 14 . Concurrently, chemical reactions of gases may occur rapidly including the production of hydroxyl radicals that rapidly react with unsaturated and oxygenated compounds to form low volatility carbonyl or carboxyl species that can partition to the aerosol phase [24][25][26] . ...
The size and chemical content of particles in electronic cigarette vapors (e-vapors) dictate their fate in the human body. Understanding how particles in e-vapors are formed and their size is critical to identifying and mitigating the adverse consequences of vaping. Thermal decomposition and reactions of the refill liquid (e-liquid) components play a key role in new particles formation. Here we report the evolution of particle number concentration in e-vapors over time for variable mixtures of refill e-liquids and operating conditions. Particle with aerodynamic diameter < 300 nm accounted for up to 17% (or 780 μg/m ³ ) of e-vapors particles. Two events of increasing particle number concentration were observed, 2–3 s after puff completion and a second 4–5 s later. The intensity of each event varied by the abundance of propylene glycol, glycerol, and flavorings in e-liquids. Propylene glycol and glycerol were associated with the first event. Flavorings containing aromatic and aliphatic unsaturated functional groups were strongly associated with the second event and to a lesser extent with the first one. The results indicate that particles in e-vapors may be formed through the heteromolecular condensation of propylene glycol, glycerol, and flavorings, including both parent chemicals and/or their thermal decomposition products.
... Hence, due to the significant abundances in NMOG emissions, pentanal, hexanal and heptanal were also included in our analysis. Chacon-Madrid et al. (2010) reported no significant difference between SOA mass yields for C 5 , C 8 and C 11 alkanals, and therefore we assumed the SOA yields of all alkanals were the same as that of heptanal in Takhar et al. (2020). Available data of SOA yields from various aldehyde precursors is still scarce, and hence future studies are warranted. ...
Cooking is an important source of primary organic aerosol (POA) in urban areas, and it may also generate abundant non-methane organic gases (NMOGs), which form oxidized organic aerosol (OOA) after atmospheric oxidation. Edible fats play an important role in a balanced diet and are part of various types of cooking. We conducted laboratory studies to examine the primary emissions of POA and NMOGs and OOA formation using an oxidation flow reactor (OFR) for three animal fats (i.e., lard, beef and chicken fats) heated at two different temperatures (160 and 180 °C). Positive matrix factorization (PMF) revealed that OOA formed together with POA loss after photochemical aging, suggesting the conversion of some POA to OOA. The maximum OOA production rates (PRs) from heated animal fats, occurring under OH exposures (OHexp) of 8.3-15 × 1010 molecules cm-3 s, ranged from 8.9 to 24.7 μg min-1, 1.6-14.5 times as high as initial POA emission rates (ERs). NMOG emissions from heated animal fats were dominated by aldehydes, which contributed 14-71% of the observed OOA. We estimated that cooking-related OOA could contribute to as high as ~10% of total organic aerosol (OA) in an urban area in Hong Kong, where cooking OA (COA) dominated the POA. This study provides insights into the potential contribution of cooking to urban OOA, which might be especially pronounced when cooking contributions dominate the primary emissions.
... In contrast, the steeper increase in the case of decalin HOMs can be explained by requiring two RO steps (Supplementary Fig. 14). To an extent, the aldehydes (which are firstgeneration oxidation products of alkanes) "short circuit" one step of RO 2 → RO conversion by providing an oxygenated moiety that is primed for autoxidation but also vulnerable to fragmentation via C-C bond scission; this is consistent with the observed lower but non-zero SOA mass yields from aldehydes compared to alkane precursors of similar volatility 25 . Our findings on the importance of RO chemistry support earlier interpretations explaining the large differences in SOA yields observed from the different alkane groups 16 . ...
Oxidation chemistry controls both combustion processes and the atmospheric transformation of volatile emissions. In combustion engines, radical species undergo isomerization reactions that allow fast addition of O2. This chain reaction, termed autoxidation, is enabled by high engine temperatures, but has recently been also identified as an important source for highly oxygenated species in the atmosphere, forming organic aerosol. Conventional knowledge suggests that atmospheric autoxidation requires suitable structural features, like double bonds or oxygen-containing moieties, in the precursors. With neither of these functionalities, alkanes, the primary fuel type in combustion engines and an important class of urban trace gases, are thought to have minor susceptibility to extensive autoxidation. Here, utilizing state-of-the-art mass spectrometry, measuring both radicals and oxidation products, we show that alkanes undergo autoxidation much more efficiently than previously thought, both under atmospheric and combustion conditions. Even at high concentrations of NOX, which typically rapidly terminates autoxidation in urban areas, the studied C6–C10 alkanes produce considerable amounts of highly oxygenated products that can contribute to urban organic aerosol. The results of this inter-disciplinary effort provide crucial information on oxidation processes in both combustion engines and the atmosphere, with direct implications for engine efficiency and urban air quality.
... This result was consistent with Section 3.5; it was found that the chromatogram of Sections 2 and 3, which contained alkanals (C 15 ≤ C n ≤ C 25 ) and alkanones (C 15 ≤ C n ≤ C 25 ), has slightly higher concentrations on haze days than non-haze days. However, the low-ratio alkanal and alkanone compounds are quite readily oxidized (Chacon-Madrid et al., 2010;Chacon-Madrid and Donahue, 2011), and a low ratio may reflect a high degree of further processing to form more oxidized species on the haze days, compensating for enhanced formation. ...
Organic matter is a major component of PM2.5 in megacities. In order to understand the detailed characteristics of organic compounds (≥ C6) at a molecular level on non-haze and haze days, we determined more than 300 organic compounds in the PM2.5 from an urban area of Beijing collected in November–December 2016 using two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC-ToF-MS). The identified organic compounds have been classified into groups, and quantitative methods were used to calculate their concentrations. Primary emission sources make significant contributions to the atmospheric organic compounds, and six groups (including n-alkanes, polycyclic aromatic hydrocarbons – PAHs, levoglucosan, branched alkanes, n-alkenes and alkyl-benzenes) account for 66 % of total identified organic compound mass. In addition, PAHs and oxygenated PAHs (O-PAHs) were abundant amongst the atmospheric organic compounds on both haze and non-haze days. The most abundant hydrocarbon groups were observed with a carbon atom range of C19–C28. In addition, the total concentration of unidentified compounds present in the chromatogram was estimated in the present study. The total identified compounds account for approximately 47 % of total organic compounds (≥ C6) in the chromatogram on both the non-haze and haze days. The total mass concentrations of organic compounds (≥ C6) in the chromatogram were 4.0 and 7.4 µg m⁻³ on the non-haze and haze days, respectively, accounting for 26.4 % and 18.5 % of organic matter, respectively, on those days estimated from the total organic carbon concentration. Ratios of individual compound concentrations between haze and non-haze days do not give a clear indication of the degree of oxidation, but the overall distribution of organic compounds in the chromatogram provides strong evidence that the organic aerosol is less GC volatile and hence more highly oxidized on haze days.
... Recent modeling studies have shown significant impacts on the SOA budget when fragmentation reactions were included relative to the assumption that all products were purely functionalized (e.g., Shrivistava et al., 2013Shrivistava et al., , 2014Shrivistava et al., , 2016. Several recent laboratory studies point to the likely increasing importance of fragmentation reactions as organic vapors age and become more functionalized (Jimenez et al., 2009;Kroll et al., 2009Kroll et al., , 2011Chacon-Madrid et al., 2010;Chacon-Madrid and Donahue, 2011;Lambe et al., 2012;Wilson et al., 2012). Reduced organic vapors generally functionalize without fragmentation upon oxidation, decreasing their volatility. ...
Oxidation flow reactors (OFRs) allow the concentration of a given atmospheric oxidant to be increased beyond ambient levels in order to study secondary organic aerosol (SOA) formation and aging over varying periods of equivalent aging by that oxidant. Previous studies have used these reactors to determine the bulk OA mass and chemical evolution. To our knowledge, no OFR study has focused on the interpretation of the evolving aerosol size distributions. In this study, we use size-distribution measurements of the OFR and an aerosol microphysics model to learn about size-dependent processes in the OFR. Specifically, we use OFR exposures between 0.09 and 0.9 equivalent days of OH aging from the 2011 BEACHON-RoMBAS and GoAmazon2014/5 field campaigns. We use simulations in the TOMAS (TwO-Moment Aerosol Sectional) microphysics box model to constrain the following parameters in the OFR: (1) the rate constant of gas-phase functionalization reactions of organic compounds with OH, (2) the rate constant of gas-phase fragmentation reactions of organic compounds with OH, (3) the reactive uptake coefficient for heterogeneous fragmentation reactions with OH, (4) the nucleation rate constants for three different nucleation schemes, and (5) an effective accommodation coefficient that accounts for possible particle diffusion limitations of particles larger than 60nm in diameter.
We find the best model-to-measurement agreement when the accommodation coefficient of the larger particles (Dp>60nm) was 0.1 or lower (with an accommodation coefficient of 1 for smaller particles), which suggests a diffusion limitation in the larger particles. When using these low accommodation-coefficient values, the model agrees with measurements when using a published H2SO4-organics nucleation mechanism and previously published values of rate constants for gas-phase oxidation reactions. Further, gas-phase fragmentation was found to have a significant impact upon the size distribution, and including fragmentation was necessary for accurately simulating the distributions in the OFR. The model was insensitive to the value of the reactive uptake coefficient on these aging timescales. Monoterpenes and isoprene could explain 24%–95% of the observed change in total volume of aerosol in the OFR, with ambient semivolatile and intermediate-volatility organic compounds (S/IVOCs) appearing to explain the remainder of the change in total volume. These results provide support to the mass-based findings of previous OFR studies, give insight to important size-distribution dynamics in the OFR, and enable the design of future OFR studies focused on new particle formation and/or microphysical processes.
... However, both the low and high values of this range are reasonable for lightly and highly substituted organic compounds (Donahue et al., 2012a). There have been a number of studies that have tried to constrain this process for selected systems like squalene particles (Kroll et al., 2009) and a series of alkanes, ketones, aldehydes, and acids (Chacon-Madrid et al., 2010. While these studies have provided useful insights we are far from constraining this parameter. ...
A lot of effort has been made to understand and constrain the
atmospheric aging of the organic aerosol (OA). Different parameterizations of
the organic aerosol formation and evolution in the two-dimensional volatility
basis set (2D-VBS) framework are evaluated using ground and airborne
measurements collected in the 2012 Pan-European Gas
AeroSOls-climate interaction Study (PEGASOS) field campaign in the Po
Valley (Italy). A number of chemical aging schemes are examined, taking into
account various functionalization and fragmentation pathways for biogenic and
anthropogenic OA components. Model predictions and measurements, both at the
ground and aloft, indicate a relatively oxidized OA with little average
diurnal variation. Total OA concentration and O : C ratios
are reproduced within experimental error by a number of chemical aging
schemes. Anthropogenic secondary OA (SOA) is predicted to contribute
15–25 % of the total OA, while SOA from intermediate volatility
compound oxidation contributes another 20–35 %. Biogenic SOA (bSOA) contributions varied
from 15 to 45 % depending on the modeling scheme. Primary OA contributed
around 5 % for all schemes and was comparable to the hydrocarbon-like
OA (HOA) concentrations derived from the positive matrix factorization of the
aerosol mass spectrometer (PMF-AMS) ground measurements. The average OA and
O : C diurnal variation and their vertical profiles showed
a surprisingly modest sensitivity to the assumed vaporization enthalpy for
all aging schemes. This can be explained by the interplay between the
partitioning of the semi-volatile compounds and their gas-phase chemical
aging reactions.
... CC BY 4.0 License. fragmentation reactions as organic vapors age and become more functionalized (Jimenez et al., 2009;Kroll et al., 2009Kroll et al., , 2011Chacon-Madrid et al., 2010;Chacon-Madrid and Donahue, 2011;Lambe et al., 2012;Wilson et al., 2012). Reduced organic vapors generally functionalize without fragmentation upon oxidation, decreasing their volatility. ...
Oxidation flow reactors (OFRs) allow the concentration of a given atmospheric oxidant to be increased beyond ambient levels in order to study secondary organic aerosol (SOA) formation and aging over varying periods of equivalent aging by that oxidant. Previous studies have used these reactors to determine the bulk OA mass and chemical evolution. To our knowledge, no OFR study has focused on the interpretation of the evolving aerosol size distributions. In this study, we use size distribution measurements of the OFR and an aerosol microphysics model to learn about size-dependent processes in the OFR. Specifically, we use OFR exposures between 0.09–0.9 equivalent days of OH aging from the 2011 BEACHON-RoMBAS and the GoAmazon2014/5 field campaigns. We use simulations in the TOMAS (TwO-Moment Aerosol Sectional) microphysics box model to constrain the following parameters in the OFR: (1) the rate constant of gas-phase functionalization reactions of organic compounds with OH, (2) the rate constant of gas-phase fragmentation reactions of organic compounds with OH, (3) the reactive uptake coefficient for heterogeneous fragmentation reactions with OH, (4) the nucleation rate constants for three different nucleation schemes, and (5) an effective accommodation coefficient that accounts for possible particle diffusion limitations of particles larger than 60 nm in diameter. We find the best model-to-measurement agreement when the accommodation coefficient of the larger particles (Dp > 60 nm) was 0.1 or lower (with an accommodation coefficient of 1 for smaller particles), which suggests a diffusion limitation in the larger particles. When using these low accommodation-coefficient values, the model agrees with measurements when using a published H2SO4-organics nucleation mechanism and previously published values of rate constants for gas-phase oxidation reactions. Further, gas-phase fragmentation was found to have a significant impact upon the size distribution, and including fragmentation was necessary for accurately simulating the distributions in the OFR. The model was insensitive to the value of the reactive uptake coefficient on these aging timescales. Monoterpenes and isoprene could explain 24–95 % of the observed change in total volume of aerosol in the OFR, with ambient semivolatile and intermediate-volatility organic compounds (S/IVOCs) appearing to explain the remainder of the change in total volume. These results provide support to the mass-based findings of previous OFR studies, give insight to important size-distribution dynamics in the OFR, and enable the design of future OFR studies focused on new particle formation and/or microphysical processes.
... Smog-chamber experiments at Carnegie Mellon have shown that pinonaldehyde is a mod- est but significant source of SOA at both high NO (ChaconMadrid and Donahue, 2011) and low NO (Chacon-Madrid et al., 2013) conditions. Aldehyde chemistry is dominated by OH radical attack on the terminal -CHO moiety, caus- ing fragmentation (Chacon-Madrid et al., 2010), but OH attack along the carbon backbone leads to functionalized products that condense to enhance SOA formation from the first-generation parent α-pinene, with mass yields of roughly 10 % under atmospherically relevant conditions. If the most volatile α-pinene product can enhance SOA production, it stands to reason that less volatile organic compound prod- ucts would have an even greater effect. ...
We have investigated the production of secondary organic aerosol (SOA) from
pinanediol (PD), a precursor chosen as a semi-volatile surrogate for
first-generation oxidation products of monoterpenes. Observations at the
CLOUD facility at CERN have shown that oxidation of organic compounds such
as PD can be an important contributor to new-particle formation. Here we
focus on SOA mass yields and chemical composition from PD photo-oxidation in
the CMU smog chamber. To determine the SOA mass yields from this
semi-volatile precursor, we had to address partitioning of both the PD and
its oxidation products to the chamber walls. After correcting for these
losses, we found OA loading dependent SOA mass yields from PD oxidation that
ranged between 0.1 and 0.9 for SOA concentrations between 0.02 and 20 µg m−3, these mass yields are 2–3 times larger than typical of
much more volatile monoterpenes. The average carbon oxidation state measured
with an aerosol mass spectrometer was around −0.7. We modeled the chamber
data using a dynamical two-dimensional volatility basis set and found that a
significant fraction of the SOA comprises low-volatility organic compounds
that could drive new-particle formation and growth, which is consistent with
the CLOUD observations.
... This latter route is generally the predominant one to form OA. Continuous oxidation of VOCs and their oxidation products yields a broad range of products, including those that have intermediate and low volatility. The importance of such multi-generational oxidation on SOA production has been widely established in laboratory chamber experiments (Chacon-Madrid et al., 2010Yee et al., 2013;Donahue et al., 2012;Chhabra et al., 2011;Henry and Donahue, 2012). Multi-generational oxidation includes the initial formation of oxidized products of lower volatility as well as the loss of SOA mass after initial formation owing to fragmentation reactions. ...
Multi-generational oxidation of volatile organic compound (VOC) oxidation
products can significantly alter the mass, chemical composition and
properties of secondary organic aerosol (SOA) compared to calculations that
consider only the first few generations of oxidation reactions. However, the
most commonly used state-of-the-science schemes in 3-D regional or global
models that account for multi-generational oxidation (1) consider only
functionalization reactions but do not consider fragmentation reactions,
(2) have not been constrained to experimental data and (3) are added on top of
existing parameterizations. The incomplete description of multi-generational
oxidation in these models has the potential to bias source apportionment and
control calculations for SOA. In this work, we used the statistical oxidation model (SOM) of Cappa and Wilson (2012), constrained by
experimental laboratory chamber data, to evaluate the regional implications
of multi-generational oxidation considering both functionalization and
fragmentation reactions. SOM was implemented into the regional University of California at Davis / California Institute of Technology (UCD/CIT) air quality model and applied to air quality episodes in California and the
eastern USA. The mass, composition and properties of SOA predicted using SOM
were compared to SOA predictions generated by a traditional two-product
model to fully investigate the impact of explicit and self-consistent
accounting of multi-generational oxidation.Results show that SOA mass concentrations predicted by the UCD/CIT-SOM model
are very similar to those predicted by a two-product model when both models
use parameters that are derived from the same chamber data. Since the
two-product model does not explicitly resolve multi-generational oxidation
reactions, this finding suggests that the chamber data used to parameterize
the models captures the majority of the SOA mass formation from
multi-generational oxidation under the conditions tested. Consequently, the
use of low and high NOx yields perturbs SOA concentrations by a factor
of two and are probably a much stronger determinant in 3-D models than
multi-generational oxidation. While total predicted SOA mass is similar for
the SOM and two-product models, the SOM model predicts increased SOA
contributions from anthropogenic (alkane, aromatic) and sesquiterpenes and
decreased SOA contributions from isoprene and monoterpene relative to the
two-product model calculations. The SOA predicted by SOM has a much lower
volatility than that predicted by the traditional model, resulting in better
qualitative agreement with volatility measurements of ambient OA. On account
of its lower-volatility, the SOA mass produced by SOM does not appear to be
as strongly influenced by the inclusion of oligomerization reactions,
whereas the two-product model relies heavily on oligomerization to form low-volatility SOA products. Finally, an unconstrained contemporary hybrid
scheme to model multi-generational oxidation within the framework of a
two-product model in which ageing reactions are added on top of the
existing two-product parameterization is considered. This hybrid scheme
formed at least 3 times more SOA than the SOM during regional
simulations as a result of excessive transformation of semi-volatile vapors
into lower volatility material that strongly partitions to the particle
phase. This finding suggests that these hybrid multi-generational
schemes should be used with great caution in regional models.
... It is widely recognized that gas-phase VOC oxidation products (or more generically organic vapors) can undergo multi-generational oxidation, given sufficient time in the at-mosphere, which may substantially alter the mass and properties of SOA. For example, chamber studies using surrogate molecules -aldehydes to represent gas-phase oxidation products of alkanes (Chacon-Madrid et al., 2010) and biogenic VOCs (Chacon-Madrid et al., 2013) and phenols to represent those from aromatics -have highlighted the potential of VOC oxidation products to undergo multi-generational oxidation to form SOA. In chamber experiments conducted at four different facilities, Donahue et al. (2012b) showed that semi-volatile organic vapors, formed from the ozonolysis of α-pinene, subsequently reacted with the hydroxyl radical (OH) to enhance SOA mass concentrations. While it is likely that virtually all oxidation products from SOA precursors subsequently react, what is less clear is the relevance of multi-generational oxidation of different classes of SOA precursors to the concentrations and properties of ambient OA under typical atmospheric conditions. ...
Multi-generational gas-phase oxidation of organic vapors can influence the
abundance, composition and properties of secondary organic aerosol (SOA).
Only recently have SOA models been developed that explicitly represent
multi-generational SOA formation. In this work, we integrated the statistical
oxidation model (SOM) into SAPRC-11 to simulate the multi-generational
oxidation and gas/particle partitioning of SOA in the regional UCD/CIT (University of California, Davis/California Institute of Technology) air
quality model. In the SOM, evolution of organic vapors by reaction with the
hydroxyl radical is defined by (1) the number of oxygen atoms added per
reaction, (2) the decrease in volatility upon addition of an oxygen atom and
(3) the probability that a given reaction leads to fragmentation of the
organic molecule. These SOM parameter values were fit to laboratory smog
chamber data for each precursor/compound class. SOM was installed in the
UCD/CIT model, which simulated air quality over 2-week periods in the South
Coast Air Basin of California and the eastern United States. For the regions
and episodes tested, the two-product SOA model and SOM produce similar SOA
concentrations but a modestly different SOA chemical composition. Predictions
of the oxygen-to-carbon ratio qualitatively agree with those measured
globally using aerosol mass spectrometers. Overall, the implementation of the
SOM in a 3-D model provides a comprehensive framework to simulate the
atmospheric evolution of organic aerosol.
... For example, in many recent models [Hodzic et al., 2010;Robinson et al., 2007;Shrivastava et al., 2011;Tsimpidi et al., 2010] that use the volatility basis-set approach (VBS) to represent multigenerational aging of SOA precursors, functionalization reactions (the addition of oxygen-containing functional groups) that continuously decrease the volatility of all organic vapors are treated, but fragmentation reactions (the breaking of carbon-carbon bonds) that produce higher volatility products are mostly neglected. Several studies suggest that as a mixture of organic vapors is aged, fragmentation reactions may become increasingly important [Chacon-Madrid and Donahue, 2011;Chacon-Madrid et al., 2010;Jimenez et al., 2009;Kroll et al., 2009Kroll et al., , 2011Lambe et al., 2012;Wilson et al., 2012]. ...
We investigate the sensitivity of secondary organic aerosol (SOA) loadings simulated by a regional chemical transport model to 7 selected model parameters using a modified volatility basis-set (VBS) approach: 4 involving emissions of anthropogenic and biogenic volatile organic compounds, anthropogenic semi-volatile and intermediate volatility organics (SIVOCs), and NOx; 2 involving dry deposition of SOA precursor gases, and one involving particle-phase transformation of SOA to low volatility. We adopt a quasi-Monte Carlo sampling approach to effectively sample the high-dimensional parameter space, and perform a 250 member ensemble of simulations using a regional model, accounting for some of the latest advances in SOA treatments based on our recent work. We then conduct a variance-based sensitivity analysis using the generalized linear model method to study the responses of simulated SOA loadings to the model parameters. Analysis of SOA variance from all 250 simulations shows that the volatility transformation parameter, which controls whether or not SOA that starts as semi-volatile is rapidly transformed to non-volatile SOA by particle-phase processes such as oligomerization and/or accretion, is the dominant contributor to variance of simulated surface-level daytime SOA (65% domain average contribution). We also split the simulations into 2 subsets of 125 each, depending on whether the volatility transformation is turned on/off. For each subset, the SOA variances are dominated by the parameters involving biogenic VOC and anthropogenic SIVOC emissions. Furthermore, biogenic VOC emissions have a larger contribution to SOA variance when the SOA transformation to non-volatile is on, while anthropogenic SIVOC emissions have a larger contribution when the transformation is off. NOx contributes less than 4.3% to SOA variance, and this low contribution is mainly attributed to dominance of intermediate to high NOx conditions throughout the simulated domain. However, we note that SOA yields have a more complex non-linear dependence on NOx levels, which needs to be addressed by more integrated model-measurement approaches focused on gaining a better process-level understanding of anthropogenic-biogenic interactions. The two parameters related to dry deposition of SOA precursor gases also have very low contributions to SOA variance. This study highlights the large sensitivity of SOA loadings to the particle-phase processes such as oligomerization that rapidly cause large decrease in the volatility of SOA, which is neglected in most previous models. This article is protected by copyright. All rights reserved.
... Oxidants include ozone, hydroxyl (OH q ) radicals, and nitrate (NO 3 ) radicals (Turpin et al., 2000), and oxidation can occur in the gas phase (Pandis et al., 1991) or in the aqueous phase (Turpin et al., 2000). Oxidation can add functional groups to an organic backbone (functionalization), forming products with a lowered volatility; however, oxidation can also lead to C-C bond cleavage (fragmentation), often forming products with an elevated volatility (Kroll et al., 2011;Chacon-Madrid et al., 2010). In addition, association reactions between relatively volatile reaction products can lead to higher-molecular-weight, lowervapor-pressure products (oligomers) (Kalberer et al., 2004). ...
When NOx is introduced to organic emissions, aerosol production is
sometimes, but not always, reduced. Under certain conditions, these
interactions will instead increase aerosol concentrations. We expanded the
two-dimensional volatility basis set (2D-VBS) to include the effects of
NOx on aerosol formation. This includes the formation of organonitrates,
where the addition of a nitrate group contributes to a decrease of 2.5 orders
of magnitude in volatility. With this refinement, we model outputs from
experimental results, such as the atomic N : C ratio, organonitrate mass,
and nitrate fragments in Aerosol Mass Spectrometer (AMS) measurements. We also discuss the mathematical methods
underlying the implementation of the 2D-VBS and provide the complete code in
the Supplement. A developer version is available on Bitbucket, an online
community repository.
... This latter route is generally the predominant one to form OA. Continuous oxidation of VOCs and their oxidation products yields a broad range of products, including those that have intermediate and low volatility. The importance of such multi-generational oxidation on SOA production has been widely established in laboratory chamber experiments (Chacon-Madrid et al., 2010Yee et al., 2013;Donahue et al., 2012;Chhabra et al., 2011;Henry and Donahue, 2012). Multi-generational oxidation includes the initial formation of oxidized products of lower volatility as well as the loss of SOA mass after initial formation owing to fragmentation reactions. ...
Multi-generational oxidation of volatile organic compound (VOC) oxidation products can significantly alter the mass, chemical composition and properties of secondary organic aerosol (SOA) compared to calculations that consider only the first few generations of oxidation reactions. However, the most commonly used state-of-the-science schemes in 3-D regional or global models that account for multi-generational oxidation (1) consider only functionalization reactions but do not consider fragmentation reactions, (2) have not been constrained to experimental data and (3) are added on top of existing parameterizations. The incomplete description of multi-generational oxidation in these models has the potential to bias source apportionment and control calculations for SOA. In this work, we used the statistical oxidation model (SOM) of Cappa and Wilson (2012), constrained by experimental laboratory chamber data, to evaluate the regional implications of multi-generational oxidation considering both functionalization and fragmentation reactions. SOM was implemented into the regional University of California at Davis / California Institute of Technology (UCD/CIT) air quality model and applied to air quality episodes in California and the eastern USA. The mass, composition and properties of SOA predicted using SOM were compared to SOA predictions generated by a traditional two-product model to fully investigate the impact of explicit and self-consistent accounting of multi-generational oxidation.
Results show that SOA mass concentrations predicted by the UCD/CIT-SOM model are very similar to those predicted by a two-product model when both models use parameters that are derived from the same chamber data. Since the two-product model does not explicitly resolve multi-generational oxidation reactions, this finding suggests that the chamber data used to parameterize the models captures the majority of the SOA mass formation from multi-generational oxidation under the conditions tested. Consequently, the use of low and high NOx yields perturbs SOA concentrations by a factor of two and are probably a much stronger determinant in 3-D models than multi-generational oxidation. While total predicted SOA mass is similar for the SOM and two-product models, the SOM model predicts increased SOA contributions from anthropogenic (alkane, aromatic) and sesquiterpenes and decreased SOA contributions from isoprene and monoterpene relative to the two-product model calculations. The SOA predicted by SOM has a much lower volatility than that predicted by the traditional model, resulting in better qualitative agreement with volatility measurements of ambient OA. On account of its lower-volatility, the SOA mass produced by SOM does not appear to be as strongly influenced by the inclusion of oligomerization reactions, whereas the two-product model relies heavily on oligomerization to form low-volatility SOA products. Finally, an unconstrained contemporary hybrid scheme to model multi-generational oxidation within the framework of a two-product model in which ageing reactions are added on top of the existing two-product parameterization is considered. This hybrid scheme formed at least 3 times more SOA than the SOM during regional simulations as a result of excessive transformation of semi-volatile vapors into lower volatility material that strongly partitions to the particle phase. This finding suggests that these hybrid multi-generational schemes should be used with great caution in regional models.
... For example, many recent models [Hodzic et al., 2010;Pye and Seinfeld, 2010;Robinson et al., 2007;Shrivastava et al., 2011;Tsimpidi et al., 2010] assume that during chemical aging, functionalization reactions (the addition of oxygen-containing functional groups) continuously decrease the volatility of all organic vapors, but they have mostly neglected fragmentation reactions (the breaking of carbon-carbon bonds) that result in higher-volatility products. Several studies suggest that as a mixture of organic vapors is aged, fragmentation reactions may become increasingly important [Chacon-Madrid and Donahue, 2011;Chacon-Madrid et al., 2010;Jimenez et al., 2009;Kroll et al., 2011;Kroll et al., 2009;Lambe et al., 2012;Wilson et al., 2012]. Therefore, models need to account for both functionalization and fragmentation reactions of gas-phase organics. ...
Secondary organic aerosols (SOA) are large contributors to fine‐particle loadings and radiative forcing but are often represented crudely in global models. We have implemented three new detailed SOA treatments within the Community Atmosphere Model version 5 (CAM5) that allow us to compare the semivolatile versus nonvolatile SOA treatments (based on some of the latest experimental findings) and to investigate the effects of gas‐phase fragmentation reactions. The new treatments also track SOA from biomass burning and biofuel, fossil fuel, and biogenic sources. For semivolatile SOA treatments, fragmentation reactions decrease the simulated annual global SOA burden from 7.5 Tg to 1.8 Tg. For the nonvolatile SOA treatment with fragmentation, the burden is 3.1 Tg. Larger differences between nonvolatile and semivolatile SOA (up to a factor of 5) exist in areas of continental outflow over the oceans. According to comparisons with observations from global surface Aerosol Mass Spectrometer measurements and the U.S. Interagency Monitoring of Protected Visual Environments (IMPROVE) network measurements, the FragNVSOA treatment, which treats SOA as nonvolatile and includes gas‐phase fragmentation reactions, agrees best at rural locations. Urban SOA is underpredicted, but this may be due to the coarse model resolution. All three revised treatments show much better agreement with aircraft measurements of organic aerosols (OA) over the North American Arctic and sub‐Arctic in spring and summer, compared to the standard CAM5 formulation. This is mainly due to the oxidation of SOA precursor gases from biomass burning, not included in standard CAM5, and long‐range transport of biomass burning OA at high altitudes. The revised model configurations that include fragmentation (both semivolatile and nonvolatile SOA) show much better agreement with MODerate resolution Imaging Spectrometers (MODIS) aerosol optical depth data over regions dominated by biomass burning during the summer compared to standard CAM5, and predict biomass burning and biofuel as the largest global source of OA, followed by biogenic and fossil fuel sources. The large contribution of biomass burning OA in the revised treatments is supported by these measurements, but the emissions and aging of SOA precursors and POA are uncertain, and need further investigation. The nonvolatile and semivolatile configurations with fragmentation predict the direct radiative forcing of SOA as −0.5 W m−2 and −0.26 W m−2 respectively, at top of the atmosphere, which are higher than previously estimated by most models, but in reasonable agreement with a recent constrained modeling study. This study highlights the importance of improving process‐level representation of SOA in global models. Fragmentation is an important sink of SOABiomass burning is the largest SOA sourceSOA DRF is higher than most models
... The competition between functionalization and fragmentation shifts in favor of increasing fragmentation for molecules with lower C number for two reasons. First, the branching ratio for CO 2 elimination from peroxyacyl radicals increases with decreasing molecular length ( Arey et al., 2001;Chacon-Madrid et al., 2010), and second, longer molecules generally have lower volatility and thus partition earlier to the particle phase, where they are protected from further gas-phase reaction ). For the smaller molecules (C number = 4-9), fragmentation is the major fate, with only a few percent of the carbon in each bin becoming condensed. ...
Secondary organic aerosol (SOA) production in air masses containing either
anthropogenic or biogenic (terpene-dominated) emissions is investigated using
the explicit gas-phase chemical mechanism generator GECKO-A. Simulations show
several-fold increases in SOA mass continuing for multiple days in the urban
outflow, even as the initial air parcel is diluted into the regional
atmosphere. The SOA mass increase in the forest outflow is more modest
(~50%) and of shorter duration (1–2 days). The multiday production
in the urban outflow stems from continuing oxidation of gas-phase precursors
which persist in equilibrium with the particle phase, and can be attributed
to multigenerational reaction products of both aromatics and alkanes,
especially those with relatively low carbon numbers (C4–15). In particular
we find large contributions from substituted maleic anhydrides and
multi-substituted peroxide-bicyclic alkenes. The results show that the
predicted production is a robust feature of our model even under changing
atmospheric conditions and different vapor pressure schemes, and contradict
the notion that SOA undergoes little mass production beyond a short initial
formation period. The results imply that anthropogenic aerosol precursors
could influence the chemical and radiative characteristics of the atmosphere
over an extremely wide region, and that SOA measurements near precursor
sources may routinely underestimate this influence.
Volatile chemical products (VCP) are an increasingly important source of hydrocarbon and oxygenated volatile organic compound (OVOC) emissions to the atmosphere, and these emissions are likely to play an important role as anthropogenic precursors for secondary organic aerosol (SOA). While the SOA from VCP hydrocarbons is often accounted for in models, the formation, evolution, and properties of SOA from VCP OVOCs remain uncertain. We use environmental chamber data and a kinetic model to develop SOA parameters for 10 OVOCs representing glycols, glycol ethers, esters, oxygenated aromatics, and amines. Model simulations suggest that the SOA mass yields for these OVOCs are of the same magnitude as widely studied SOA precursors (e.g., long-chain alkanes, monoterpenes, and single-ring aromatics), and these yields exhibit a linear correlation with the carbon number of the precursor. When combined with emissions inventories for two megacities in the United States (US) and a US-wide inventory, we find that VCP VOCs react with OH to form 0.8-2.5× as much SOA, by mass, as mobile sources. Hydrocarbons (terpenes, branched and cyclic alkanes) and OVOCs (terpenoids, glycols, glycol ethers) make up 60-75 and 25-40% of the SOA arising from VCP use, respectively. This work contributes to the growing body of knowledge focused on studying VCP VOC contributions to urban air pollution.
Atmospheric peroxyacetyl nitrate (PAN), as an essential constituent in the photochemical smog, is formed from photochemical reactions between volatile organic compounds (VOCs) and NOx. However, limited regional studies on distribution, formation and sources of PAN restrict the further understanding of the atmospheric behavior and environmental significance of PAN. In this study, the variation characteristics of PAN and the influencing factors to PAN concentrations were investigated using the WRF-CMAQ model simulation in the central China during July 2019. The results showed that the monthly mean concentration of PAN in the near-surface layer was 0.4 ppbv and increased with the height rising, accompanied by strong intra-day variation. The process analysis suggested that the removal was mainly controlled by dry deposition (57 %), followed by the gas-phase chemistry (43 %) which was mainly attributed to the thermal decomposition. Based on the sensitivity simulation, PAN concentrations decreased effectively in most of the simulated regions when precursors of VOCs and NOx emissions were reduced, and PAN concentrations were more sensitive to VOCs emissions than NOx emissions. The reduction of NOx and VOCs could lead to enhanced atmospheric oxidation in east-central region, which in turn hindered the decrease of PAN concentrations. During the simulation period, we found that emissions from industry and transportation sectors had the greatest impact on PAN concentrations in the central China, with contributions of 39 %-49 % and 33 %-41 %, respectively.
Aldehydes are common constituents of natural and polluted atmospheres, and their gas-phase oxidation has recently been reported to yield highly oxygenated organic molecules (HOM) that are key players in the formation of atmospheric aerosol. However, insights into the molecular level mechanism of this oxidation reaction have been scarce. While OH initiated oxidation of small aldehydes, with two to five carbon atoms, under high NOx conditions generally leads to fragmentation products, longer chain aldehydes involving an initial non-aldehydic hydrogen abstraction can be a path to molecular functionalization and growth. In this work, we conduct a joint theoretical-experimental analysis of the autoxidation chain reaction of a common aldehyde, hexanal. We computationally study the initial steps of OH oxidation at the RHF-RCCSD(T)-F12a/VDZ-F12//ωB97X-D/aug-cc-pVTZ level, and show that both aldehydic (on C1) and non-aldehydic (on C4) H-abstraction channels contribute to HOM via autoxidation. The oxidation products predominantly form through the H-abstraction from C1 and C4, followed by fast unimolecular 1,6 H-shifts with rate coefficients 1.7 × 10−1 s−1 and 8.6 × 10−1 s−1, respectively. Experimental flow reactor measurements at variable reaction times show that hexanal oxidation products including HOM monomers up to C6H11O7 and accretion products C12H22O9−10 form within 3 seconds reaction time. Kinetic modeling simulation including atmospherically relevant precursor concentrations agrees with the experimental results and the expected timescales. Finally, we estimate the hexanal HOM yields up to seven O atoms with mechanistic details through both C1 and C4 channels.
Organic peroxides (POs) are organic molecules with one or more peroxide (-O-O-) functional groups. POs are commonly regarded as chemically labile termination products from gas-phase radical chemistry and therefore serve as temporary reservoirs for oxidative radicals (HOx and ROx) in the atmosphere. Owing to their ubiquity, active gas-particle partitioning behavior, and reactivity, POs are key reactive intermediates in atmospheric multiphase processes determining the life cycle (formation, growth, and aging), climate, and health impacts of aerosol. However, there remain substantial gaps in the origin, molecular diversity, and fate of POs due to their complex nature and dynamic behavior. Here, we summarize the current understanding on atmospheric POs, with a focus on their identification and quantification, state-of-the-art analytical developments, molecular-level formation mechanisms, multiphase chemical transformation pathways, as well as environmental and health impacts. We find that interactions with SO2 and transition metal ions are generally the fast PO transformation pathways in atmospheric liquid water, with lifetimes estimated to be minutes to hours, while hydrolysis is particularly important for α-substituted hydroperoxides. Meanwhile, photolysis and thermolysis are likely minor sinks for POs. These multiphase PO transformation pathways are distinctly different from their gas-phase fates, such as photolysis and reaction with OH radicals, which highlights the need to understand the multiphase partitioning of POs. By summarizing the current advances and remaining challenges for the investigation of POs, we propose future research priorities regarding their origin, fate, and impacts in the atmosphere.
Secondary organic aerosol (SOA) is a very important component of fine particulate matter (PM2.5) in the atmosphere. However, the simulations of SOA, which could help to elucidate the detailed mechanism of SOA formation and quantify the roles of various precursors, remains unsatisfactory, as SOA levels are frequently underestimated. It has been found that the performance of SOA formation models can be significantly improved by incorporating the emission and evolution of semivolatile and intermediate-volatility organic compounds (S/IVOCs). In order to explore the roles of S/IVOCs in SOA formation, this study reviews some simulation models which could consider S/IVOCs for SOA formation as well as the development of emission inventories of S/IVOCs and S/IVOC modules for SOA formation. In addition, the future research directions for simulations of the effect of S/IVOCs on SOA formation are suggested.
Oxidation of the monoterpene Δ3-carene (C10H16) is a potentially important and understudied source of atmospheric secondary organic aerosol (SOA). We present chamber-based measurements of speciated gas and particle phases during photochemical oxidation of Δ3-carene. We find evidence of highly oxidized organic molecules (HOMs) in the gas phase and relatively low-volatility SOA dominated by C7-C10 species. We then use computational methods to develop the first stages of a Δ3-carene photochemical oxidation mechanism and explain some of our measured compositions. We find that alkoxy bond scission of the cyclohexyl ring likely leads to efficient HOM formation, in line with previous studies. We also find a surprising role for the abstraction of primary hydrogens from methyl groups, which has been calculated to be rapid in the α-pinene system, and suggest more research is required to determine if this is more general to other systems and a feature of autoxidation. This work develops a more comprehensive view of Δ3-carene photochemical oxidation products via measurements and lays out a suggested mechanism of oxidation via computationally derived rate coefficients.
Cooking emissions contribute significantly to organic aerosol in urban areas, but the evolution of physicochemical properties and health impacts upon atmospheric aging remain poorly understood. In this study, detailed chemical composition and oxidative potential (OP) of primary organic aerosol (POA) and secondary organic aerosol (SOA) from heated cooking oils (canola, olive, peanut) were characterized. Results from acellular oxidative potential measurements indicate an enhanced OP in photochemically aged cooking SOA compared to that in POA, which is accompanied by increasing abundance of reactive oxygen species (ROS) measured using electron paramagnetic resonance (EPR) spectroscopy. In SOA from heated canola oil, enhanced total ROS production (by a factor of 26) coincides with the increased particle-phase peroxide content (from 15 to 26%) across six different photooxidation conditions (up to 2 days of atmospheric aging equivalent). Positive correlations were found among total ROS abundance, peroxide content, and average carbon oxidation state of canola oil SOA along with aging. Photooxidation of representative volatile compounds from heated cooking oils shows that unsaturated aldehydes are the dominant contributors to peroxides in cooking oil SOA during atmospheric aging, and the degree of unsaturation in the aldehyde precursor is linked with the total ROS/peroxide content in SOA. The abundant OH radicals suggest that peroxides are the major radical source in aged cooking SOA, likely initiated by the homolytic cleavage of the oxygen-oxygen single bond. Our study bridges the chemical composition and OP of cooking aerosol upon atmospheric aging, shedding light on the evolution of cooking emissions and their dynamic toxicity in the atmosphere.
Atmospheric aerosol particles are a serious health risk, especially in
regions like East Asia. We investigated the photochemical aging of ambient
aerosols using a potential aerosol mass (PAM) reactor at Baengnyeong Island
in the Yellow Sea during 4–12 August 2011. The size distributions and
chemical compositions of aerosol particles were measured alternately every
6 min from the ambient air or through the highly oxidizing environment of a
potential aerosol mass (PAM) reactor. Particle size and chemical composition
were measured by using the combination of a scanning mobility particle
sizer (SMPS) and a high-resolution time-of-flight aerosol mass
spectrometer (HR-ToF-AMS). Inside the PAM reactor, O3 and OH levels were equivalent to 4.6 days of integrated OH exposure at typical atmospheric
conditions. Two types of air masses were distinguished on the basis of the
chemical composition and the degree of aging: air transported from China,
which was more aged with a higher sulfate concentration and O : C ratio,
and the air transported across the Korean Peninsula, which was less aged with
more organics than sulfate and a lower O : C ratio. For both episodes,
the particulate sulfate mass concentration increased in the 200–400 nm size
range when sampled through the PAM reactor. A decrease in organics was
responsible for the loss of mass concentration in 100–200 nm particles when
sampled through the PAM reactor for the organics-dominated episode. This loss
was especially evident for the m∕z 43 component, which represents less
oxidized organics. The m∕z 44 component, which represents further oxidized
organics, increased with a shift toward larger sizes for both episodes. It is
not possible to quantify the maximum possible organic mass concentration for
either episode because only one OH exposure of 4.6 days was used, but it is
clear that SO2 was a primary precursor of secondary aerosol in northeast
Asia, especially during long-range transport from China. In addition,
inorganic nitrate evaporated in the PAM reactor as sulfate was added to the
particles. These results suggest that the chemical composition of aerosols
and their degree of photochemical aging, particularly for organics, are also
crucial in determining aerosol mass concentrations.
Decades of policy in developed regions has successfully reduced total
anthropogenic emissions of gas-phase organic compounds, especially volatile
organic compounds (VOCs), with an intentional, sustained focus on motor
vehicles and other combustion-related sources. We examine potential secondary
organic aerosol (SOA) and ozone formation in our case study megacity (Los
Angeles) and demonstrate that non-combustion-related sources now contribute
a major fraction of SOA and ozone precursors. Thus, they warrant greater
attention beyond indoor environments to resolve large uncertainties in their
emissions, oxidation chemistry, and outdoor air quality impacts in cities
worldwide. We constrain the magnitude and chemical composition of emissions
via several bottom-up approaches using chemical analyses of products,
emissions inventory assessments, theoretical calculations of emission
timescales, and a survey of consumer product material safety datasheets. We
demonstrate that the chemical composition of emissions from consumer
products as well as commercial and industrial products, processes, and materials is
diverse across and within source subcategories. This leads to wide ranges of SOA and
ozone formation potentials that rival other prominent sources, such as motor
vehicles. With emission timescales from minutes to years, emission rates and
source profiles need to be included, updated, and/or validated in emissions
inventories with expected regional and national variability. In particular,
intermediate-volatility and semi-volatile organic compounds (IVOCs and SVOCs)
are key precursors to SOA, but are excluded or poorly represented in emissions
inventories and exempt from emissions targets. We present an expanded
framework for classifying VOC, IVOC, and SVOC emissions from this diverse
array of sources that emphasizes a life cycle approach over longer timescales
and three emission pathways that extend beyond the short-term evaporation of
VOCs: (1) solvent evaporation, (2) solute off-gassing, and
(3) volatilization of degradation by-products. Furthermore, we find
that ambient SOA formed from these non-combustion-related emissions could be
misattributed to fossil fuel combustion due to the isotopic signature of
their petroleum-based feedstocks.
Cooking emissions have been identified as a major source of primary organic aerosol (POA) in urban environments. Cooking may also be a potential source of secondary organic aerosol (SOA) due to abundant emissions of non-methane organic gases (NMOGs). We studied SOA formation from the photooxidation of emissions from seven vegetable oils heated at 200 °C under high-NOx conditions in a smog chamber. After aging under an OH exposure of 1.0 × 10¹⁰ molecules cm⁻³ s, the SOA formation rate was generally one order of magnitude higher than the primary organic aerosol (POA) emission rate. We determined that alkenals, which are not traditional SOA precursors in chemical transport models, accounted for 5–34% of the observed SOA. The unexplained SOA may be attributed to the oxidation of primary semi-volatile and intermediate-volatility organic compounds (S/IVOCs), which were estimated to contribute to an additional 9–106% of the observed SOA assuming the same volatility distribution of heated cooking oils as vehicle exhaust. Our results suggest that cooking can potentially be an important source of SOA in urban areas and that there is a need to characterize both S/IVOCs emitted from cooking and their SOA yields.
We have investigated the production of secondary organic aerosol (SOA) from pinanediol (PD), a precursor chosen as a semi-volatile surrogate for first-generation oxidation products of monoterpenes. Observations at the CLOUD facility at CERN have shown that oxidation of organic compounds such as PD can be an important contributor to new-particle formation. Here we focus on SOA mass yields and chemical composition from PD photo-oxidation in the CMU smog chamber. To determine the SOA mass yields from this semi-volatile precursor, we had to address partitioning of both the PD and its oxidation products to the chamber walls. After correcting for these losses, we found OA loading dependent SOA mass yields from PD oxidation that ranged between 0.1 and 0.9 for SOA concentrations between 0.02 and 20 µg m−3, these mass yields are 2–3 times larger than typical of much more volatile monoterpenes. The average carbon oxidation state measured with an Aerosol Mass Spectrometer was around −0.7. We modeled the chamber data using a dynamical two-dimensional volatility basis set and found that a significant fraction of the SOA comprises low volatility organic compounds that could drive new-particle formation and growth, which is consistent with the CLOUD observations.
Decades of policy in developed regions has successfully reduced total anthropogenic emissions of gas-phase organic compounds, especially volatile organic compounds (VOCs), with an intentional, sustained focus on motor vehicles and other combustion-related sources. We examine potential secondary organic aerosol (SOA) and ozone formation in our case study megacity (Los Angeles), and demonstrate that non-combustion-related sources now contribute a major fraction of SOA and ozone precursors. Thus, they warrant greater attention beyond indoor environments to resolve large uncertainties in their emissions, oxidation chemistry, and outdoor air quality impacts in cities worldwide. We constrain the magnitude and chemical composition of emissions via several bottom-up approaches using: chemical analyses of products, emissions inventory assessments, theoretical calculations of emission timescales, and a survey of consumer product material safety datasheets. We demonstrate that the chemical composition of emissions from consumer products, and commercial/industrial products, processes, and materials is diverse across and within product/material-types with a wide range of SOA and ozone formation potentials that rivals other prominent sources, such as motor vehicles. With emission timescales from minutes to years, emission rates and source profiles need to be included, updated, and/or validated in emissions inventories, with expected regional/national variability. In particular, intermediate-volatility and semivolatile organic compounds (IVOCs and SVOCs) are key precursors to SOA but are excluded or poorly represented in emissions inventories, and exempt from emissions targets. We present an expanded framework for classifying VOC, IVOC, and SVOC emissions from this diverse array of sources that emphasizes a lifecycle approach over longer timescales and three emission pathways that extend beyond the short-term evaporation of VOCs: (1) solvent evaporation, (2) solute off-gassing, and (3) volatilization of degradation by-products. Furthermore, we find that ambient SOA formed from these non-combustion-related emissions could be misattributed to fossil fuel combustion due to the isotopic signature of their petroleum-based feedstocks.
An INdoor air Detailed Chemical Model (INDCM) was developed to investigate the impact of ozone reactions with indoor surfaces (including occupants), on indoor air chemistry in simulated apartments subject to ambient air pollution. The results are consistent with experimental studies showing that approximately 80% of ozone indoors is lost through deposition to surfaces. The human body removes ozone most effectively from indoor air per square meter of surface, but the most significant surfaces for C6-C10 aldehyde formation are soft furniture and painted walls owing to their large internal surfaces. Mixing ratios of between 8-11 ppb of C6-C10 aldehydes are predicted to form in apartments in various locations in summer, the highest values are when ozone concentrations are enhanced outdoors. The most important aldehyde formed indoors is predicted to be nonanal (5-7 ppb), driven by oxidation-derived emissions from painted walls. In addition, ozone-derived emissions from human skin were estimated for a small bedroom at nighttime with concentrations of nonanal, decanal and 4-oxopentanal predicted to be 0.5, 0.7 and 0.7 ppb respectively. A detailed chemical analysis shows that ozone-derived surface aldehyde emissions from materials and people change chemical processing indoors, through enhanced formation of nitrated organic compounds and decreased levels of oxidants. This article is protected by copyright. All rights reserved.
We investigated the photochemical aging of ambient aerosols using a potential aerosol mass (PAM) reactor at Baegryeong Island in the Yellow Sea during August 4–12, 2011. The size distributions and chemical compositions of the ambient and aged PAM aerosols were measured alternately every 6 min by Scanning Mobility Particle Sizer (SMPS) and High Resolution-Time of Flight-Aerosol Mass Spectrometer (HR-ToF-AMS), respectively. Inside the PAM reactor, the O3 and OH levels were equivalent to approximately 5 days of integrated OH exposure at typical atmospheric conditions. Two types of air masses were distinguished on the basis of the chemical composition and the degree of aging: Sulfate was predominant with higher O : C ratio for the air transported from China and organic concentration was higher than that of sulfate with lower O : C ratio when the air came through the Korean Peninsula. In PAM reactor, sulfate was constantly formed, resulting in the increase of particle mass at 200–400 nm size range. Organics were responsible for an overall loss of mass in 100–200 nm particles. This loss was especially evident for the m/z 43 component representing semi-volatile organics. Conversely, the m/z 44 component corresponding to low-volatile organics increased with a shift toward larger sizes during the organics-dominated episode. Therefore, we hypothesize that the oxidation of semi-volatile organics was facilitated by gas-phase oxidation and partitioning for re-equilibrium between the gas and particle phases. Nitrate evaporated in the PAM reactor upon the addition of sulfate to the particles. These results suggest that the chemical composition of aerosols and their degree of photochemical aging particularly for organics are also crucial in determining aerosol mass concentrations. Because sulfate in the atmosphere was stable for about a week of the nominal lifetime of aerosols, SO2 is a unquestionably primary precursor of secondary aerosol in northeast Asia. In comparison, the contribution of organics to secondary aerosols is more variable during transport in the atmosphere. Notably, an increase in low-volatility organics was associated with sulfate and evident at 200–400 nm, highlighting the role of secondary organic aerosol (SOA) in cloud condensation nuclei (CCN) formation.
Secondary organic aerosol (SOA) production in air masses containing either anthropogenic or
biogenic (terpene-dominated) emissions is investigated using the explicit gas-phase chemical
mechanism generator GECKO-A. Simulations show several-fold increases in SOA mass continuing for
several days in the urban outflow, even as the initial air parcel is diluted into the regional
atmosphere. The SOA mass increase in the forest outflow is more modest (∼50%) and of
shorter duration (1–2 days). The production in the urban outflow stems from continuing oxidation
of gas-phase precursors which persist in equilibrium with the particle phase, and can be
attributed to multigenerational reaction products of both aromatics and alkanes. In particular we
find large contributions from substituted maleic anhydrides and multi-substituted
peroxide-bicyclic alkenes. The results show that the predicted production is a robust feature of
our model even under changing atmospheric conditions, and contradict the notion that SOA undergoes
little mass production beyond a short initial formation period. The results imply that
anthropogenic aerosol precursors could influence the chemical and radiative characteristics of the
atmosphere over an extremely wide region, and that SOA measurements near precursor sources may
routinely underestimate this influence.
Multigenerational oxidation chemistry of atmospheric organic compounds and its effects on aerosol loadings and chemical composition is investigated by implementing the Two-Dimensional Volatility Basis Set (2-D-VBS) in a Lagrangian host chemical transport model. Three model formulations were chosen to explore the complex interactions between functionalization and fragmentation processes during gas-phase oxidation of organic compounds by the hydroxyl radical. The base case model employs a conservative transformation by assuming a reduction of one order of magnitude in effective saturation concentration and an increase of oxygen content by one or two oxygen atoms per oxidation generation. A second scheme simulates functionalization in more detail using group contribution theory to estimate the effects of oxygen addition to the carbon backbone on the compound volatility. Finally, a fragmentation scheme is added to the detailed functionalization scheme to create a functionalization-fragmentation parameterization. Two condensed-phase chemistry pathways are also implemented as additional sensitivity tests to simulate (1) heterogeneous oxidation via OH uptake to the particle-phase and (2) aqueous-phase chemistry of glyoxal and methylglyoxal. The model is applied to summer and winter periods at three sites where observations of organic aerosol (OA) mass and O:C were obtained during the European Integrated Project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) campaigns. The base case model reproduces observed mass concentrations and O:C well, with fractional errors (FE) lower than 55% and 25%, respectively. The detailed functionalization scheme tends to overpredict OA concentrations, especially in the summertime, and also underpredicts O:C by approximately a factor of 2. The detailed functionalization model with fragmentation agrees well with the observations for OA concentration, but still underpredicts O:C. Both heterogeneous oxidation and aqueous-phase processing have small effects on OA levels but heterogeneous oxidation, as implemented here, does enhance O:C by about 0.1. The different schemes result in very different fractional attribution for OA between anthropogenic and biogenic sources.
Yields of secondary organic aerosol (SOA) were measured for OH radical-initiated reactions of the 2- through 6-dodecanone positional isomers and also n-dodecane and n-tetradecane in the presence of NOx. Yields decreased in the order n-tetradecane > dodecanone isomer average > n-dodecane, and the dodecanone isomer yields decreased as the keto group moved toward the center of the molecule, with 6-dodecanone being an exception. Trends in the yields can be explained by the effect of carbon number and keto group presence and position on product vapor pressures, and by the isomer-specific effects of the keto group on branching ratios for keto alkoxy radical isomerization, decomposition, and reaction with O2. Most importantly, results indicate that isomerization of keto alkoxy radicals via 1,5- and 1,6-H shifts are significantly hindered by the presence of a keto group whereas decomposition is enhanced. Analysis of particle composition indicates that the SOA products are similar for all isomers, and that compared to those formed from the corresponding reactions of alkanes the presence of a pre-existing keto group opens up additional heterogeneous/multiphase reaction pathways that can lead to the formation of new products. The results demonstrate that the presence of a keto group alters gas and particle phase chemistry, and provide new insights into the potential effects of molecular structure on the products of the atmospheric oxidation of volatile organic compounds and subsequent formation of SOA.
Volatile organic compounds (VOCs) are major precursors for ozone and secondary organic aerosol (SOA), both of which greatly harm human health and significantly affect the Earth's climate. We simultaneously estimated ozone and SOA formation from anthropogenic VOCs emissions in China by employing photochemical ozone creation potential (POCP) values and SOA yields. We gave special attention to large molecular species and adopted the SOA yield curves from latest smog chamber experiments. The estimation shows that alkylbenzenes are greatest contributors to both ozone and SOA formation (36.0% and 51.6%, respectively), while toluene and xylenes are largest contributing individual VOCs. Industry solvent use, industry process and domestic combustion are three sectors with the largest contributions to both ozone (24.7%, 23.0% and 17.8%, respectively) and SOA (22.9%, 34.6% and 19.6%, respectively) formation. In terms of the formation potential per unit VOCs emission, ozone is sensitive to open biomass burning, transportation, and domestic solvent use, and SOA is sensitive to industry process, domestic solvent use, and domestic combustion. Biomass stoves, paint application in industrial protection and buildings, adhesives application are key individual sources to ozone and SOA formation, whether measured by total contribution or contribution per unit VOCs emission. The results imply that current VOCs control policies should be extended to cover most important industrial sources, and the control measures for biomass stoves should be tightened. Finally, discrepant VOCs control policies should be implemented in different regions based on their ozone/aerosol concentration levels and dominant emission sources for ozone and SOA formation potential.
A laboratory study on the heterogeneous reactions of straight-chain aldehydes was performed by exposing n-octanal, nonanal, and decanal vapors to ambient aerosol particles. The aerosol and blank filters were extracted using methanol. The extracts were nebulized and the resulting compositions were examined using a high-resolution time-of-flight aerosol mass spectrometer. The mass spectral analysis showed that the exposures of the aldehydes to aerosol samples increased the peak intensities in the high mass range. The peaks in the mass spectra of the aerosol samples after exposure to different aldehydes were characterized by a homologous series of peak shifts due to the addition of multiple CH2 units. This result is explained by the formation of high-molecular-weight (HMW) compounds that contain single or multiple aldehyde moieties. The HMW fragment peaks for the blank filters exposed to n-aldehydes were relatively weak, indicating an important contribution from the ambient aerosol components to the formation of the HMW compounds. Among the factors affecting the overall interaction of aldehydes with atmospheric aerosol components, gas phase diffusion possibly limited the reactions under the studied conditions; therefore, their occurrence to a similar degree in the atmosphere is not ruled out, at least for the reactions involving n-nonanal and decanal. The major formation pathways for the observed HMW products may be the self-reactions of n-aldehydes mediated by atmospheric aerosol components and the reactions of n-aldehydes with organic aerosol components. The observed formation of HMW compounds encourages further investigations into their effects on the aerosol properties as well as the organic aerosol mass in the atmosphere.
The linear C15 alkene, 1-pentadecene, was reacted with NO3 radicals in a Teflon environmental chamber and yields of secondary organic aerosol (SOA) and particulate β-hydroxynitrates, β-carbonylnitrates, and organic peroxides (β-nitrooxyhydroperoxides + dinitrooxyperoxides) were quantified using a variety of methods. Reaction occurs almost solely by addition of NO3 to the C=C double bond and measured yields of β-hydroxynitrate isomers indicate that 92% of addition occurs at the terminal carbon. Molar yields of reaction products determined from measurements, a proposed reaction mechanism, and mass-balance considerations were 0.065 for β-hydroxynitrates (0.060 and 0.005 for 1-nitrooxy-2-hydroxypentadecane and 1-hydroxy-2-nitrooxypentadecane isomers), 0.102 for β-carbonylnitrates, 0.017 for organic peroxides, 0.232 for β-nitrooxyalkoxy radical isomerization products, and 0.584 for tetradecanal and formaldehyde, the volatile C14 and C1 products of β-nitrooxyalkoxy radical decomposition. Branching ratios for decomposition and isomerization of β-nitrooxyalkoxy radicals were 0.716 and 0.284, and should be similar for other linear 1-alkenes ≥ C6 whose alkyl chains are long enough to allow for isomerization to occur. These branching ratios have not been measured previously and they differ significantly from those estimated using structure activity relationships, which predict >99% isomerization. It appears that the presence of a -ONO2 group adjacent to an alkoxy radical site greatly enhances the rate of decomposition relative to isomerization, which is otherwise negligible, and that the effect is similar to that of a -OH group. The results provide insight into the effects of molecular structure on mechanisms of oxidation of volatile organic compounds and should be useful for improving structure-activity relationships that are widely used to predict the fate of these compounds in the atmosphere and for modeling SOA formation and aging.
When NOx is introduced to organic emissions, aerosol production is sometimes, but not always, reduced. Under certain conditions, these interactions will instead increase aerosol concentrations. We expanded the two-dimensional volatility basis set (2-D-VBS) to include the effects of NOx on aerosol formation. This includes the formation of organonitrates, where the addition of a nitrate group contributes to a decrease of 2.5 orders of magnitude in volatility. With this refinement, we model outputs from experimental results, such as the atomic N : C ratio, organonitrate mass, and nitrate fragments in AMS measurements. We also discuss the mathematical methods underlying the implementation of the 2-D-VBS and provide the complete code in the Supplemental material. A developer version is available on Bitbucket, an online community repository.
Fine aerosols have an important impact on health, visibility and climate. Secondary Organic Aerosols (SOA) represent an important fraction of fine aerosol composition. SOA are formed by nucleation or condensation onto pre-existing particles of gaseous species formed during the oxidation of emitted volatile organic compounds (VOC). VOC oxidation implies a huge number of secondary intermediates which are potentially involved in SOA formation. In order to study SOA formation, it is necessary to develop chemical schemes describing explicitly the formation and condensation of the gaseous secondary intermediates. The LISA has thus developed in collaboration with NCAR (National Center of Atmospheric Research) a generator of explicit chemical schemes : GECKO-A (Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere). This work aims at testing (i) the reliability of GECKO-A to simulate observed SOA concentrations in Atmospheric Simulation Chamber (ASC) and (ii) exploring the SOA sensitivity to physico-chemical parameters such as saturation vapour pressures, chamber walls effects or kinetics rate constants. In order to assess GECKO-A's chemical schemes, the model has been confronted to chamber experiments performed to study SOA. Saturation vapour pressure (Pvap) is the key parameter controlling the gas/particles partitioning of organic compounds The three Pvap estimation methods considered as the more reliable in the literature have been implemented in GECKO-A. Pvap estimated by the three methods differs highly, up to several orders of magnitude. Despite of these discrepancies, simulated SOA concentration and speciation show a low sensitivity to the method used to estimate the Pvap. Moreover, none of the methods were able to make the model fit the observations. SOA concentration is systematically overestimated of a factor 2. Semi volatile organic compounds deposition on a chamber walls has been investigated. The implementation of this process in the model leads to a significant decrease of the simulated SOA concentrations, up to factor of 2. Simulated SOA yields are in good agreement with measured SOA yields. The hypothesis of a misrepresentation of some gaseous processes has then been investigated through sensitivity tests. SOA formation sensitivity to COV+ OH reactions rate constants has been explored. Results exhibited a high sensitivity to the rate constants estimations (regarding the rate constants values estimation, as well as the determination of the OH attack sites). The estimated alkoxy radicals decomposition rate constants have also been tested. This test showed however no significant impact on the simulated SOA yields
The effects of molecular structure on the products and mechanisms of SOA formation from OH radical-initiated reactions of linear, branched, and cyclic alkanes in the presence of NOx were investigated in a series of environmental chamber experiments. SOA mass spectra were obtained in real time and off line using a thermal desorption particle beam mass spectrometer and used to identify reaction products. Real-time mass spectra were used to classify products according to their temporal behavior, and off-line temperature-programmed thermal desorption analysis of collected SOA was used to separate products by volatility prior to mass spectral analysis and to gain information on compound vapor pressures. A reaction mechanism that includes gas- and particle-phase reactions was developed that explains the formation of SOA products and is consistent with the various lines of mass spectral information. Results indicate that the SOA products formed from the reactions of linear, branched, and cyclic alkanes are similar, but differ in a few important ways. Proposed first-generation SOA products include alkyl nitrates, 1,4-hydroxynitrates, 1,4-hydroxycarbonyls, and dihydroxycarbonyls. The 1,4-hydroxycarbonyls and dihydroxycarbonyls rapidly isomerize in the particle phase to cyclic hemiacetals that then dehydrate to volatile dihydrofurans. This conversion process is catalyzed by HNO3 formed in the chamber and is slowed by the presence of NH3. Volatile products can react further with OH radicals, forming multi-generation products containing various combinations of the same functional groups present in first-generation products. For linear and branched alkanes, the products are acyclic or monocyclic, whereas for cyclic alkanes they are acyclic, monocyclic, or bicyclic. Some of the products, especially those formed from ring-opening reactions of cyclic alkanes appear to be low volatility oligomers. The implications of the results for the formation of atmospheric SOA are discussed.
Terpenes are emitted in large quantities from vegetation into the troposphere, where they react readily with ozone, OH and NO3 radicals leading to a number of oxidation products. The current knowledge about gas-phase terpene oxidation products is reviewed. Their formation and decomposition pathways, their products and their relevance for the troposphere, and their chemical analysis are discussed. Data on oxidation kinetics, and product yields is presented for 23 terpenes and 65 oxidation products. A total of 84 references are quoted.
Framework for Change
Organic aerosols make up 20 to 90% of the particulate mass of the troposphere and are important factors in both climate and human heath. However, their sources and removal pathways are very uncertain, and their atmospheric evolution is poorly characterized. Jimenez et al. (p. 1525 ; see the Perspective by Andreae ) present an integrated framework of organic aerosol compositional evolution in the atmosphere, based on model results and field and laboratory data that simulate the dynamic aging behavior of organic aerosols. Particles become more oxidized, more hygroscopic, and less volatile with age, as they become oxygenated organic aerosols. These results should lead to better predictions of climate and air quality.
There is growing awareness that heterogeneous reactions may be important in the atmospheric formation of secondary organic aerosols (SOA). Here, we report on the investigation of a series of recently identified heterogeneous reactions that convert 1,4-hydroxycarbonyls, a major product of alkane oxidation, to cyclic hemiacetals and then dihydrofurans in the particle-phase. Through these reactions, saturated 1,4-hydroxycarbonyls are converted to more reactive, unsaturated dihydrofurans, which can evaporate and react rapidly with atmospheric oxidants such as OH radicals, NO3 radicals, or O3. In order to investigate the conversion process quantitatively, a model was developed based on a proposed mechanism that includes gas-phase and heterogeneous reactions, as well as gas-particle partitioning. This model was used to simulate the time profiles of products formed from OH radical-initiated reactions of C11-C17 n-alkanes in the presence of NOx, for comparison with profiles of particle-phase cyclic hemiacetals measured during environmental chamber reactions of the same alkanes using a thermal desorption particle beam mass spectrometer. Results showed that the particle-phase isomerization of 1,4-hydroxycarbonyls to cyclic hemiacetals was fast in dry air, with a reactive uptake coefficient of at least 0.5. The lifetime for the subsequent particle-phase dehydration of cyclic hemiacetals to dihydrofurans was approximately 15 min. The addition of water vapor (relative humidity approximately 50%) slowed the conversion process, apparently by neutralizing adsorbed HNO3 that is thought to catalyze the reactions. Simulations performed with model parameters obtained from the experiments indicate that for typical atmospheric aerosol mass and oxidant concentrations and sufficiently acidic particles, 1,4-hydroxycarbonyls will be almost entirely converted to dihydrofurans in less than a day in both clean and polluted areas, whereas in the presence of neutralized particles the conversion may be minor. The outcome that occurs could have a significant impact on reaction products and SOA formation.
Toluene and other aromatics have long been viewed as the dominant anthropogenic secondary organic aerosol (SOA) precursors, but the SOA mass yields from toluene reported in previous studies vary widely. Experiments conducted in the Carnegie Mellon University environmental chamber to study SOA formation from the photo-oxidation of toluene show significantly larger SOA production than parameterizations employed in current air-quality models. Aerosol mass yields depend on experimental conditions: yields are higher under higher UV intensity, under low-NOx conditions and at lower temperatures. The extent of oxidation of the aerosol also varies with experimental conditions, consistent with ongoing, progressive photochemical aging of the toluene SOA. Measurements using a thermodenuder system suggest that the aerosol formed under high- and low-NOx conditions is semi-volatile. These results suggest that SOA formation from toluene depends strongly on ambient conditions. An approximate parameterization is proposed for use in air-quality models until a more thorough treatment accounting for the dynamic nature of this system becomes available.
The SIMPOL.1 group contribution method is developed for predicting the liquid vapor pressure poL (atm) and enthalpy of vaporization Δ Hvap (kJ mol-1) of organic compounds as functions of temperature (T). For each compound i, the method assumes log10poL,i (T)=∑kνk,ibk(T) where νk,i is the number of groups of type k, and bk (T) is the contribution to log10poL,i (T) by each group of type k. A zeroeth group is included that uses b0 (T) with ν0,i=1 for all i. A total of 30 structural groups are considered: molecular carbon, alkyl hydroxyl, aromatic hydroxyl, alkyl ether, alkyl ring ether, aromatic ether, aldehyde, ketone, carboxylic acid, ester, nitrate, nitro, alkyl amine (primary, secondary, and tertiary), aromatic amine, amide (primary, secondary, and tertiary), peroxide, hydroperoxide, peroxy acid, C=C, carbonylperoxynitrate, nitro-phenol, nitro-ester, aromatic rings, non-aromatic rings, C=C–C=O in a non-aromatic ring, and carbon on the acid-side of an amide. The T dependence in each of the bk (T) is assumed to follow b(T)=B1/T+B2+B3T+B4ln T. Values of the B coefficients are fit using an initial basis set of 272 compounds for which experimentally based functions po L,i=fi (T) are available. The range of vapor pressure considered spans fourteen orders of magnitude. The ability of the initially fitted B coefficients to predict poL values is examined using a test set of 184 compounds and a T range that is as wide as 273.15 to 393.15 K for some compounds. σFIT is defined as the average over all points of the absolute value of the difference between experimental and predicted values of log10poL,i (T). After consideration of σFIT for the test set, the initial basis set and test set compounds are combined, and the B coefficients re-optimized. For all compounds and temperatures, σFIT=0.34: on average, poL,i (T) values are predicted to within a factor of 2. Because d(log10 poL,i (T))d(1/T) is related to the enthalpy of vaporization ΔHvap,i, the fitted B provide predictions of ΔHvap,i based on structure.
We describe measurements of total peroxy nitrates (ΣPNs), NO<sub>2</sub>, O<sub>3</sub> and several aldehydes at Granite Bay, California, during the Chemistry and Transport of the Sacramento Urban Plume-2001 (CATSUP 2001) campaign, from 19 July–16 September 2001. We observed a strong photochemically driven variation of ΣPNs during the day with the median of 1.2 ppb at noon. Acetaldehyde, pentanal, hexanal and methacrolein had median abundances in the daytime of 1.2 ppb, 0.093 ppb, 0.14 ppb, and 0.27 ppb, respectively. We compare steady state and time dependent calculations of the dependence of ΣPNs on aldehydes, OH, NO and NO<sub>2</sub> showing that the steady state calculations are accurate to ±30% between 10:00 and 18:00 h. We use the steady state calculation to investigate the composition of ΣPNs and the concentration of OH at Granite Bay. We find that PN molecules that have never been observed before make up an unreasonably large fraction of the ΣPNs unless we assume that there exists a PAN source that is much larger than the acetaldehyde source. We calculate that OH at the site varied between 2 and 7×10<sup>6</sup> molecule cm<sup>−3</sup> at noon during the 8 weeks of the experiment.
The effect of hydrocarbon molecular structure on the measured yield and volatility of secondary organic aerosol (SOA) formed from OH radical-initiated reactions of linear, branched, and cyclic alkanes in the presence of NOx was investigated in an environmental chamber. SOA yields from reactions of homologous series of linear and cyclic alkanes increased monotonically with increasing carbon number due to the decreasing volatility of the parent alkanes and thus the reaction products. For a given carbon number, yields followed the order cyclic > linear > branched, a trend that appears to be determined primarily by the extent to which alkoxy radical intermediates decompose and the nature ofthe resulting products, with parent alkane volatility being of secondary importance. The trend was investigated quantitatively by correlating SOA yields with the fraction of OH radical reactions that lead to alkoxy radical decomposition (the remainder isomerize), calculated using structure-reactivity relationships. For alkoxy radicals with branched or strained cyclic structures, decomposition can compete with isomerization, whereas for those with linear structures it cannot Branched alkoxy radicals fragment to form pairs of smaller, more volatile products, whereas cyclic alkoxy radicals undergo ring opening to form products similar to those formed from reactions of linear alkanes, but with an additional aldehyde group. The lower volatility of multifunctional aldehydes, and their tendency to form oligomers, appears to enhance SOA yields.
A substantial fraction of the total ultrafine particulate mass is comprised of organic compounds. Of this fraction, a significant subfraction is secondary organic aerosol (SOA), meaning that the compounds are a by-product of chemistry in the atmosphere. However, our understanding of the kinetics and mechanisms leading to and following SOA formation is in its infancy. We lack a clear description of critical phenomena; we often don't know the key, rate limiting steps in SOA formation mechanisms. We know almost nothing about aerosol yields past the first generation of oxidation products. Most importantly, we know very little about the derivatives in these mechanisms; we do not understand how changing conditions, be they precursor levels, oxidant concentrations, co-reagent concentrations (i.e., the VOC/NOx ratio) or temperature will influence the yields of SOA. In this paper we explore the connections between fundamental details of physical chemistry and the multitude of steps associated with SOA formation, including the initial gas-phase reaction mechanisms leading to condensible products, the phase partitioning itself, and the continued oxidation of the condensed-phase organic products. We show that SOA yields in the alpha-pinene + ozone are highly sensitive to NOx, and that SOA yields from beta-caryophylene + ozone appear to increase with continued ozone exposure, even as aerosol hygroscopicity increases as well. We suggest that SOA yields are likely to increase substantially through several generations of oxidative processing of the semi-volatile products.
A large body of epidemiologic literature has found an association of increased fine particulate air pollution (PM2.5) with acute and chronic mortality. The effect of improvements in particle exposure is less clear.
Earlier analysis of the Harvard Six Cities adult cohort study showed an association between long-term ambient PM2.5 and mortality between enrollment in the mid-1970s and follow-up until 1990. We extended mortality follow-up for 8 yr in a period of reduced air pollution concentrations.
Annual city-specific PM2.5 concentrations were measured between 1979 and 1988, and estimated for later years from publicly available data. Exposure was defined as (1) city-specific mean PM2.5 during the two follow-up periods, (2) mean PM2.5 in the first period and change between these periods, (3) overall mean PM2.5 across the entire follow-up, and (4) year-specific mean PM2.5. Mortality rate ratios were estimated with Cox proportional hazards regression controlling for individual risk factors.
We found an increase in overall mortality associated with each 10 microg/m3 increase in PM2.5 modeled either as the overall mean (rate ratio [RR], 1.16; 95% confidence interval [CI], 1.07-1.26) or as exposure in the year of death (RR, 1.14; 95% CI, 1.06-1.22). PM2.5 exposure was associated with lung cancer (RR, 1.27; 95% CI, 0.96-1.69) and cardiovascular deaths (RR, 1.28; 95% CI, 1.13-1.44). Improved overall mortality was associated with decreased mean PM2.5 (10 microg/m3) between periods (RR, 0.73; 95% CI, 0.57-0.95).
Total, cardiovascular, and lung cancer mortality were each positively associated with ambient PM2.5 concentrations. Reduced PM2.5 concentrations were associated with reduced mortality risk.
The 1,4-hydroxycarbonyl 5-hydroxy-2-pentanone is an important product of the gas-phase reaction of OH radicals with n-pentane in the presence of NO. We have used a relative rate method with 4-methyl-2-pentanone as the reference compound to measure the rate constant for the reaction of OH radicals with 5-hydroxy-2-pentanone at 296 ± 2 K. The carbonyls were sampled by on-fiber derivatization using a Solid Phase Micro Extraction (SPME) fiber coated with O>
-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride with subsequent thermal desorption of the oxime derivatives and quantification by gas chromatography with flame ionization detection. For comparison, the reference compound was also analyzed following sample collection onto a Tenax adsorbent cartridge. Products of the reaction were investigated using coated-fiber SPME sampling with gas chromatography-mass spectrometry analysis as well as by using in situ atmospheric pressure ionization mass spectrometry. A rate constant for the reaction of OH radicals with 5-hydroxy-2-pentanone of (1.6 ± 0.4) × 10-11 cm3 molecule-1 s-1 was obtained at 296 ± 2 K. Two dicarbonyl products, of molecular weight 86 and 100, were observed and are attributed to CH3C(O)CH2CHO and CH3C(O)CH2CH2CHO, respectively. Reaction schemes leading to these products are presented.
Secondary organic aerosol (SOA) formation from reactions of n-alkanes with OH radicals in the presence of NOx was investigated in an environmental chamber using a thermal desorption particle beam mass spectrometer for particle analysis. SOA consisted of both first- and higher-generation products,all of which were nitrates, Major first-generation products were 6-hydroxynitrates, while higher-generation products consisted of dinitrates, hydroxydinitrates, and substituted tetrahydrofurans containing nitrooxy, hydroxyl, and carbonyl groups. The substituted tetrahydrofurans are formed by a series of reactions in which delta-hydroxycarbonyis isomerize to cyclic hemiacetals, which then dehydrate to form substituted dihydrofurans (unsaturated compounds) that quickly react with OH radicals to form lower volatility products. SOA yields ranged from similar to 0.5% for C-8 to similar to 53% for C-15, with a sharp increase from similar to 8% for C-11 to similar to 50% for C-13. This was probably due to an increase in the contribution of first-generation products, as well as other factors. For example, SOA formed from the C-10 reaction contained no first-generation products, while for the C15 reaction SOA was similar to 40% first-generation and similar to 60% higher-generation products, respectively. First-generation delta-hydroxycarbonyls are especially important in SOA formation, since their subsequent reactions can rapidly form low volatility compounds. In the atmosphere, substituted dihydrofurans created from 6-hydroxycarbonyls will primarily react with O-3 or NO3 radicals, thereby opening reaction pathways not normally accessible to saturated compounds.
A dilution source sampling system was used to quantify the organic air pollutant emissions from commercial-scale meat charbroiling operations. Emission rates of gas-phase volatile organic compounds, semivolatile organic compounds, and high molecular weight particle-phase organic compounds were simultaneously quantified on a single compound basis. Fine particle mass emission rates and fine particle elemental chemical composition were measured as well. Emission rates of 120 organic compounds, spanning carbon numbers from C1 to C29 were quantified including n-alkanoic acids, n-alkenoic acids, carbonyls, lactones, alkanes, aromatics, polycyclic aromatic hydrocarbons, alkenes, and steroids. Ethylene, formaldehyde, and acetaldehyde were found to be the predominant light gas-phase organic compounds emitted from the charbroiling operations. n-Alkanoic acids, n-alkenoic acids, and carbonyls made up a significant fraction of the quantified semivolatile and particle-phase organic compound emissions. Meat charbroiling is one of the few sources identified to date that contributes to the high molecular weight aldehydes measured in the urban atmosphere. Semivolatile and particle-phase organic compounds were collected for quantification by two simultaneous sampling protocols: (1) quartz fiber filters followed by polyurethane foam (PUF) cartridges, and (2) XAD-coated annular denuders followed by quartz fiber filters and PUF cartridges. Good agreement was observed for the total mass emissions collected by the two different sampling procedures; however, the partitioning of the semivolatile organic compounds between the gas phase and particle phase, as measured by the two sampling procedures, showed significant differences for n-alkanoic acids, indicating that significant artifact adsorption of these compounds occurs to the filter in the filter/PUF sampling system.
The reactions of 1-butoxy and 1-pentoxy radicals were studied using time-resolved and simultaneous measurement of NO2 and OH concentrations in laser pulse initiated oxidation studies followed by numerical simulations of the concentration profiles. The alkoxy radicals were produced selectively by the excimer-laser photolysis of 1-butyl bromide and 1-pentyl bromide at 248 nm and subsequent reaction of the 1-alkyl radicals with O2 and NO. Whereas NO2 was detected by cw-LIF, OH was monitored by laser long-path absorption at 308 nm. All experiments were performed at 293±3 K and a total pressure of 50 mbar. The reactions with O2 and the isomerisations via a 1,5-H-shift, viz.,
It has previously been shown that in dry air 5-hydroxy-2-pentanone cyclizes and dehydrates to form 4,5-dihydro-2-methylfuran. A series of C5–C8 1,4-hydroxycarbonyls were generated in situ from the OH radical-initiated reactions of their n-alkane precursors, and their dark decays in air investigated as a function of water vapor concentration. To remove any reactive dihydrofurans formed, in some experiments O3 was added after 120–240min and the 1,4-hydroxycarbonyls monitored for a further time period. In general, at low water vapor concentrations the 1,4-hydroxycarbonyl decayed in the dark in the absence of added O3, with the concentration reaching a plateau indicating that an equilibrium between the 1,4-hydroxycarbonyl and the dihydrofuran had been attained. Addition of O3 led to further decay of the hydroxycarbonyl. At higher water vapor concentrations, no significant decay of the 1,4-hydroxcarbonyl was observed in the absence of added O3, but addition of O3 resulted in a measurable decay of the 1,4-hydroxycarbonyl. Finally, at yet higher water vapor concentrations, no decay of the 1,4-hydroxycarbonyl was observed in the absence or presence of O3. At >50% relative humidity at 296K, the C5–C8 1,4-hydroxycarbonyls examined here were stable against cyclization and dehydration.
Gas- and particle-phase tailpipe emissions from late-model medium duty diesel trucks are quantified using a two-stage dilution source sampling system. The diesel trucks are driven through the hot-start Federal Test Procedure (FTP) urban driving cycle on a transient chassis dynamometer. Emission rates of 52 gas-phase volatile hydrocarbons, 67 semivolatile and 28 particle-phase organic compounds, and 26 carbonyls are quantified along with fine particle mass and chemical composition. When all Câ--Cââ carbonyls are combined, they account for 60% of the gas-phase organic compound mass emissions. Fine particulate matter emission rates and chemical composition are quantified simultaneously by two methods: a denuder/filter/PUF sampler and a traditional filter sampler. Both sampling techniques yield the same elemental carbon emission rate of 56 mg kmâ»Â¹ driven, but the particulate organic carbon emission rate determined by the denuder-based sampling technique is found to be 35% lower than the organic carbon mass collected by the traditional filter-based sampling technique due to a positive vapor-phase sorption artifact that affects the traditional filter sampling technique. The distribution of organic compounds in the diesel fuel used in this study is compared to the distribution of these compounds in the vehicle exhaust. Significant enrichment in the ratio of unsubstituted polycyclic aromatic hydrocarbons (PAH) to their methyl- and dimethyl-substituted homologues is observed in the tailpipe emissions relative to the fuel. Isoprenoids and tricyclic terpanes are quantified in the semivolatile organics emitted from diesel vehicles. When used in conjunction with data on the hopanes, steranes, and elemental carbon emitted, the isoprenoids and the tricyclic terpanes may help trace the presence of diesel exhaust in atmospheric samples.
Transient species in the photooxidation of formaldehyde in air have been investigated by using the technique of flash photolysis kinetic spectroscopy. The absorption spectrum attributed to the HOCHâOâ radical was observed with a maximum near 230 nm. This radical is formed by the reaction HOâ + HCHO ⺠HOCHâOâ (1, -1). The rate constants were measured for the two reactions: kâ = 7.7 à 10â»Â¹âµ exp((625 {plus minus} 550)/T) cm³ moleculeâ»Â¹ sâ»Â¹ and kââ = 2.0 à 10¹² exp((-7000 {plus minus} 2000)/T) sâ»Â¹. The equilibrium constant is Kâ* = 3.85 à 10â»Â²â· exp(7625/T) cm³ moleculeâ»Â¹, which corresponds to a reaction enthalpy ÎHâ° = -16.25 {plus minus} 0.30 kcal molâ»Â¹, which is based on the K{sub p} value and quantum calculations of ÎSâ° and therefore determined accurately. Kinetic measurements performed under various experimental conditions allowed determinations of the rate constants for the reactions HOâ + HOCHâOâ â products (3) and 2HOCHâOâ â Oâ + CHâ(OH)â + HCOOH (4b); kâ = 5.6 à 10â»Â¹âµ exp((2300 {plus minus} 1100)/T); k{sub 4b} = 5.65 à 10â»Â¹â´ exp((750 {plus minus} 400)/T) cm³ moleculeâ»Â¹ sâ»Â¹. The branching ratios for kâ and kâ were determined in separate experiments described in part 2 of this work.
Products of the gas-phase reactions of OH radicals with the n-alkanes n-pentane through n-octane at 298 ± 2 K and atmospheric pressure of air have been investigated using gas chromatography with flame ionization detection (GC-FID), combined gas chromatography−mass spectrometry (GC-MS), and in situ atmospheric pressure ionization tandem mass spectrometry. The formation yields of alkyl nitrates from n-hexane, n-heptane, and n-octane were measured by GC-FID, with the sum of the isomeric alkyl nitrates being 0.141 ± 0.020, 0.178 ± 0.024, and 0.226 ± 0.032, respectively. These alkyl nitrate yields are 35% lower than previous data reported from this laboratory in the early 1980s. Using negative ion atmospheric pressure chemical ionization with the addition of pentafluorobenzyl alcohol to study the n-pentane through n-octane reactions and those of the fully deuterated n-alkanes, hydroxyalkyl nitrate products were identified from the n-pentane, n-heptane, and n-octane reactions for the first time and the presence of hydroxycarbonyl products was confirmed. Adding NO2 to the chamber reaction mixture postreaction to form [NO2·M]- adducts of the hydroxycarbonyls and hydroxynitrates, together with the use of 5-hydroxy-2-pentanone and 2-nitrooxy-3-butanol as internal standards for the hydroxycarbonyls and hydroxynitrates, respectively, enabled the yields of the hydroxycarbonyl and hydroxynitrate reaction products to be estimated.
Isomerizations of the alkoxy radicals formed from the OH radical reactions with n-butane through n-octane and n-pentane-d12 through n-octane-d18 have been studied in the presence of NO at 296 ± 2 K and 740 Torr total pressure of air. In addition to carbonyls of the same carbon number as the alkane precursors, the previously predicted δ-hydroxycarbonyls were detected by direct air sampling atmospheric pressure ionization triple quadrupole mass spectrometry. The formation yields of carbonyl compounds containing the same number of carbons as the parent n-alkane decreased with increasing carbon number in the n-alkane, while the hydroxycarbonyl/carbonyl formation yield ratios increased markedly from n-butane through n-octane, in agreement with previous theoretical predictions.
This paper presents quantum chemical studies of the unimolecular isomerization (1,5 H-shift) and decomposition (β C−C scission) reactions of a series of six oxygenated alkoxy radicals and 1-butoxy radical. The goal is to better understand the effects of ether, carbonyl, and ester functional groups on the reactivity of alkoxy radicals relevant to atmospheric chemistry. We also report the first quantum chemical study of the α-ester rearrangement: CH3C(O)OCH2O• → CH3C(O)OH + O. The six radicals are CH3OC(O)CH2O•, CH3C(O)OCH2O•, CH3CH2C(O)CH2O•, CH3C(O)CH2CH2O•, CH3OCH2CH2O•, and CH3CH2OCH2O•. All these radicals are, like 1-butoxy, primary alkoxy radicals with a methyl group δ− to the radical center. Calculations are carried out at the B3LYP/6-31G(d,p) and /6-311G(2df,2p) level of theory for all reactions. In addition, the G2(MP2,SVP) level of theory is used to study all isomerization reactions and selected decomposition reactions. Substituent effects on structure are very large and certainly significant for the fate of these radicals in the atmosphere; fates depend as much or more on the position of functional groups as their identity. We also make a preliminary examination of the effects of tunneling on the computed rate constants for the α-ester rearrangement and the 1,5 H-shift reaction of 1-butoxy. At 298 K, we find tunneling to increase the rate of the 1,5 H-shift reaction by a factor of 19−210, and the rate of the α-ester rearrangement by a factor of 1.3 to 6. The effects of tunneling have been neglected in most previous computational studies of the 1,5 H-shift reaction.
Secondary organic aerosol (SOA) formation is considered in the framework of the gas/particle partitioning absorption model outlined by Pankow (1, 2). Expressions for the fractional SOA yield (Y) are developed within this framework and shown to be a function of the organic aerosol mass concentration, Mo. These expressions are applied to over 30 individual reactive organic gas (ROG) photooxidation smog chamber experiments. Analysis of the data from these experiments clearly shows that Y is a strong function of Mo and that secondary organic aerosol formation is best described by a gas/particle partitioning absorption model. In addition to the 30 individual ROG experiments, three experiments were performed with ROG mixtures. The expressions developed for Y in terms of Mo, used in conjunction with the overall yield data from the individual ROG experiments, are able to account for the Mo generated in the ROG mixture experiments. This observation not only suggests that SOA yields for individual ROGs are additive but that smog chamber SOA yield data may be confidently extrapolated to the atmosphere in order to determine the important ambient sources of SOA in the environment.
The oxidation of isoprene (2-methyl-1,3-butadiene) is known to play a central role in the photochemistry of the troposphere, but is generally not considered to lead to the formation of secondary organic aerosol (SOA), due to the relatively high volatility of known reaction products. However, in the chamber studies described here, we measure SOA production from isoprene photooxidation under high-NOx conditions, at significantly lower isoprene concentrations than had been observed previously. Mass yields are low (0.9-3.0%), but because of large emissions, isoprene photooxidation may still contribute substantially to global SOA production. Results from photooxidation experiments of compounds structurally similar to isoprene (1,3-butadiene and 2- and 3-methyl-1-butene) suggest that SOA formation from isoprene oxidation proceeds from the further reaction of first-generation oxidation products (i.e., the oxidative attack of both double bonds). The gas-phase chemistry of such oxidation products is in general poorly characterized and warrants further study.
Recent laboratory studies show that delta-hydroxycarbonyls formed via OH-initiated reactions with alkanes cyclize and then dehydrate to form substituted dihydrofurans. These dihydrofurans are highly reactive, with estimated lifetimes in the atmosphere of 1.3 h (OH), 24 s (NO3), and 7 min (O-3). These studies also show that secondary organic aerosol (SOA) yields from alkanes increase with carbon number from 4% for C-8 to 44% for C-13 to almost 90% for C-17. The reaction mechanism proposed for these observations has been incorporated explicitly into the Caltech Atmospheric Chemistry Mechanism (CACM) to investigate the factors controlling the yield curve over the homologous series of C-8-C-17 n-alkanes. It was found that the hypothesized chemical reaction sequence was incomplete. Results from simulations indicate as yet unknown chemistry involving the carbonylester products may explain the discrepancies between observed and simulated SOA yields. Using the carbonylesters (which do not contribute directly to SOA) as proxies for their SOA-forming products, the SOA yield curve was reproduced. Prior versions of CACM did not include SOA formation from medium-chain alkanes. Laboratory data show SOA yields from these compounds range from 4% to 35% (C-8-C-12). The majority of SOA for these alkanes derives from second- and third-generation Compounds (99-88% over the C-8-C-12 interval) that had not been represented before. The long-chain alkalies in CACM previously were allowed to form aerosol, but only the first-generation products were represented. Here, the second- and third-generation products were found to constitute 78-69% of the SOA mass over the C-13-C-17 interval, indicating the importance of including this additional chemistry in simulations of SOA formation from n-alkanes. (C) 2008 Elsevier Ltd. All rights reserved.
Secondary organic aerosol (SOA), particulate matter composed of compounds formed from the atmospheric transformation of organic species, accounts for a substantial fraction of tropospheric aerosol. The formation of low-volatility (semivolatile and possibly nonvolatile) compounds that make up SOA is governed by a complex series of reactions of a large number of organic species, so the experimental characterization and theoretical description of SOA formation presents a substantial challenge. In this review we outline what is known about the chemistry of formation and continuing transformation of low-volatility species in the atmosphere. The primary focus is chemical processes that can change the volatility of organic compounds: (1) oxidation reactions in the gas phase, (2) reactions in the particle phase, and (3) continuing chemistry (in either phase) over several generations. Gas-phase oxidation reactions can reduce volatility by the addition of polar functional groups or increase it by the cleavage of carbon–carbon bonds; key branch points that control volatility are the initial attack of the oxidant, reactions of alkylperoxy (RO2) radicals, and reactions of alkoxy (RO) radicals. Reactions in the particle phase include oxidation reactions as well as accretion reactions, non-oxidative processes leading to the formation of high-molecular-weight species. Organic carbon in the atmosphere is continually subject to reactions in the gas and particle phases throughout its atmospheric lifetime (until lost by physical deposition or oxidized to CO or CO2), implying continual changes in volatility over the timescales of several days. The volatility changes arising from these chemical reactions must be parameterized and included in models in order to gain a quantitative and predictive understanding of SOA formation.
The branching fraction of H-abstraction in the elementary reactions of acetaldehyde and propionaldehyde with OH at 290 K was determined directly using a fast-flow reactor coupled to a molecular beam sampling mass spectrometry apparatus. The primary-product H2O yield of the title reactions was quantified relative to that of the isobutane+OH reaction, and found to be 89±6 and 100±10% for the reactions of acetaldehyde+OH and propionaldehyde+OH, respectively. Furthermore, an upper limit of 3% could be determined for the yield of formic acid in the hypothetical addition/elimination reaction pathway. We conclude that the reaction of OH radicals with aldehydes proceeds predominantly, if not exclusively, via H-abstraction, forming H2O and RCO.
Recent experimental studies have identified cis-pinic acid (a C9 dicarboxylic acid) as a condensed-phase product of the ozonolysis of both α- and β-pinene, and it is currently believed to be the most likely degradation product leading to the prompt formation of new aerosols by nucleation. The observed timescale of aerosol formation appears to require that cis-pinic acid is a first-generation product, and a possible mechanism for its formation has therefore been developed. The key step in the proposed mechanism requires that the isomerisation of a complex C9 acyl-oxy radical by a 1,7 H atom shift is able to compete with the alternative decomposition to CO2 and a C8 organic radical:
Alkoxy radicals are key intermediates in the atmospheric degradations of volatile organic compounds, and can typically undergo reaction with O2, unimolecular decomposition or unimolecular isomerization. Previous structure–reactivity relationships for the estimation of rate constants for these processes for alkoxy radicals [Atkinson, R., 1997. Atmospheric reactions of alkoxy and β-hydroxyalkoxy radicals. International Journal of Chemical Kinetics, 29, 99–111; Aschmann, S.M., Atkinson, R., 1999. Products of the gas-phase reactions of the OH radical with n-butyl methyl ether and 2-isopropoxyethanol: reactions of ROC(O)< radicals. International Journal of Chemical Kinetics, 31, 501–513] have been updated to incorporate recent kinetic data from absolute and relative rate studies. Temperature-dependent rate expressions are derived allowing rate constants for all three of these alkoxy radical reaction pathways to be calculated at atmospherically relevant temperatures.
The levels of semi-volatile aldehydes with 4–10 carbon atoms and a ketone identified as 6-methyl-5-hepten-2-one by mass spectrometry have b