F. Rohrer

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (128)408.63 Total impact

  • D.H. Ehhalt · F. Rohrer · A. Wahner
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    ABSTRACT: The oxidizing capacity of the troposphere relies on oxidants such as O3, NO3, and OH. Of these, OH is by far the most important. The supply of all the oxidants is limited and thus the oxidizing capacity is finite. The factors that control OH, O3, and NO3 are reviewed and examples of their tropospheric distributions are presented. The resulting lifetimes of the most important tropospheric trace gases are listed. The possible change of the oxidant distributions under the impact of anthropogenic emissions is indicated. A quantitative measure of the oxidation capacity is derived from the total loss rate of all trace gases due to OH.
    No preview · Chapter · Dec 2015
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    ABSTRACT: The analysis of the individual composition of hydrocarbon (VOC) mixtures enables us to transform observed VOC-concentrations into their respective total VOC-reactivity versus OH radicals (RVOC =Σ(kOH+VOCi*[VOCi])). This is particularly useful because local ozone production essentially depends on this single parameter rather than on the details of the underlying hydrocarbon mixture [Klemp et al., Schriften des Forschungszentrums Jülich, Energy & Environment, 2012, 21]. The VOC composition also enables us to pin down the major emission source of hydrocarbons in urban areas to be petrol cars with temporarily reduced catalyst efficiency (the so-called cold-start situation) whereas the source of nitrogen oxides (NOx=NO+NO2) is expected to be nowadays dominated by diesel cars. The observations in the vicinity of main roads in German cities show a decrease in the ratio of OH reactivities of VOC and NO2 (RVOC/RNO2) by a factor 7.5 over the time period 1994-2014. This is larger than the expected decrease of a factor 2.9 taking estimated trends of VOC and NOx traffic emissions in Germany [Umweltbundesamt Deutschland, National Trend Tables for the German Atmospheric Emission Reporting, 2015] during this time period. The observed reduction in the RVOC/RNO2 ratio leads to a drastic decrease in local ozone production driven by the reduction in hydrocarbons. The analysis reveals that the overall reduction of ozone production benefits from the low decrease of NOx emissions from road traffic which is a consequence of the eventual absence of catalytic converters for nitrogen oxide removal in diesel cars up to now.
    No preview · Article · Dec 2015 · Faraday Discussions
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    ABSTRACT: Direct detection of highly reactive, atmospheric hydroxyl radicals (OH) is widely accomplished by laser-induced fluorescence (LIF) instruments. The technique is also suitable for the indirect measurement of HO2 and RO2 peroxy radicals by chemical conversion to OH. It requires sampling of ambient air into a low pressure cell, where OH fluorescence is detected after excitation by 308 nm laser radiation. Although the residence time of air inside the fluorescence cell is typically only on the order of milliseconds, there is potential that additional OH is internally produced, which would artificially increase the measured OH concentration. Here, we present experimental studies investigating potential interferences in the detection of OH and peroxy radicals for the LIF instruments of Forschungszentrum Jülich for nighttime conditions. For laboratory experiments, the inlet of the instrument was overflown by excess synthetic air containing one or more reactants. In order to distinguish between OH produced by reactions upstream of the inlet and artificial signals produced inside the instrument, a chemical titration for OH was applied. Additional experiments were performed in the simulation chamber SAPHIR where simultaneous measurements by an open-path differential optical absorption spectrometer (DOAS) served as reference for OH to quantify potential artifacts in the LIF instrument. Experiments included the investigation of potential interferences related to the nitrate radical (NO3, N2O5), related to the ozonolysis of alkenes (ethene, propene, 1-butene, 2,3-dimethyl-2-butene, α-pinene, limonene, isoprene), and the laser photolysis of acetone. Experiments studying the laser photolysis of acetone yield OH signals in the fluorescence cell, which are equivalent to 0.05 × 106 cm−3 OH for a mixing ratio of 5 ppbv acetone. Under most atmospheric conditions, this interference is negligible. No significant interferences were found for atmospheric concentrations of reactants during ozonolysis experiments. Only for α-pinene, limonene, and isoprene at reactant concentrations which are orders of magnitude higher than in the atmosphere artificial OH could be detected. The value of the interference depends on the turnover rate of the ozonolysis reaction. For example, an apparent OH concentration of approximately 1 × 106 cm−3 is observed, if 5.8 ppbv limonene reacts with 600 ppbv ozone. Experiments with the nitrate radical NO3 reveal a small interference signal in the OH, HO2 and RO2 detection. Dependencies on experimental parameters point to artificial OH formation by surface reactions at the chamber walls or in molecular clusters in the gas expansion. The signal scales with the presence of NO3 giving equivalent radical concentrations of 1.1 × 105 cm−3 OH, 1 × 107 cm−3 HO2, and 1.7 × 107 cm−3 RO2 per 10 pptv NO3.
    No preview · Article · Nov 2015
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    ABSTRACT: A new PLant chamber Unit for Simulation (PLUS) for use with the atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) has been build and characterized at the Forschungszentrum Jülich GmbH, Germany. The PLUS chamber is an environmentally controlled flow through plant chamber. Inside PLUS the natural blend of biogenic emissions of trees are mixed with synthetic air and are transferred to the SAPHIR chamber where the atmospheric chemistry and the impact of biogenic volatile organic compounds (BVOC) can be studied in detail. In PLUS all important enviromental parameters (e.g. temperature, PAR, soil RH etc.) are well-controlled. The gas exchange volume of 9.32 m3 which encloses the stem and the leafes of the plants is constructed such that gases are exposed to FEP Teflon film and other Teflon surfaces only to minimize any potential losses of BVOCs in the chamber. Solar radiation is simulated using 15 LED panels which have an emission strength up to 800 μmol m−2 s−1. Results of the initial characterization experiments are presented in detail. Background concentrations, mixing inside the gas exchange volume, and transfer rate of volatile organic compounds (VOC) through PLUS under different humidity conditions are explored. Typical plant characteristics such as light and temperature dependent BVOC emissions are studied using six Quercus Ilex trees and compared to previous studies. Results of an initial ozonolysis experiment of BVOC emissions from Quercus Ilex at typical atmospheric concentrations inside SAPHIR are presented to demonstrate a typical experimental set up and the utility of the newly added plant chamber.
    No preview · Article · Nov 2015
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    ABSTRACT: We describe a two-channel broadband cavity enhanced absorption spectrometer (BBCEAS) for aircraft measurements of glyoxal (CHOCHO), methylglyoxal (CH3COCHO), nitrous acid (HONO), nitrogen dioxide (NO2), and water (H2O). The instrument spans 361–389 and 438–468 nm, using two light emitting diodes (LEDs) and a grating spectrometer with a charge-coupled device (CCD) detector. Robust performance is achieved using a custom optical mounting system, high power LEDs with electronic on/off modulation, state-of-the-art cavity mirrors, and materials that minimize analyte surface losses. We have successfully deployed this instrument during two aircraft and two ground-based field campaigns to date. The demonstrated precision (2σ) for retrievals of CHOCHO, HONO and NO2 are 34, 350 and 80 pptv in 5 s. The accuracy is 5.8, 9.0 and 5.0 % limited mainly by the available absorption cross sections.
    No preview · Article · Oct 2015
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    ABSTRACT: Ozone concentrations in the Po Valley of northern Italy often exceed international regulations. As both a source of radicals and an intermediate in the oxidation of most volatile organic compounds (VOCs), formaldehyde (HCHO) is a useful tracer for the oxidative processing of hydrocarbons that leads to ozone production. We investigate the sources of HCHO in the Po Valley using vertical profile measurements acquired from the airship Zeppelin NT over an agricultural region during the PEGASOS 2012 campaign. Using a 1-D model, the total VOC oxidation rate is examined and discussed in the context of formaldehyde and ozone production in the early morning. While model and measurement discrepancies in OH reactivity are small (on average 3.4 +/- 13 %), HCHO concentrations are underestimated by as much as 1.5 ppb (45 %) in the convective mixed layer. A similar underestimate in HCHO was seen in the 2002-2003 FORMAT Po Valley measurements, though the additional source of HCHO was not identified. Oxidation of unmeasured VOC precursors cannot explain the missing HCHO source, as measured OH reactivity is explained by measured VOCs and their calculated oxidation products. We conclude that local direct emissions from agricultural land are the most likely source of missing HCHO. Model calculations demonstrate that radicals from degradation of this non-photochemical HCHO source increase model ozone production rates by as much as 0.6 ppb h(-1) (12 %) before noon.
    No preview · Article · Feb 2015 · Atmospheric Chemistry and Physics
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    ABSTRACT: Diurnal and seasonal variations of gaseous sulfuric acid (H2SO4) and methane sulfonic acid (MSA) were measured in NE Atlantic air at the Mace Head atmospheric research station during the years 2010 and 2011. The measurements utilized selected-ion chemical ionization mass spectrometry (SI/CIMS) with a detection limit for both compounds of 4.3 × 104 cm−3 at 5 min signal integration. The H2SO4 and MSA gas-phase concentrations were analyzed in conjunction with the condensational sink for both compounds derived from 3 nm to 10 μm (aerodynamic diameter) aerosol size distributions. Accommodation coefficients of 1.0 for H2SO4 and 0.12 for MSA were assumed, leading to estimated atmospheric lifetimes on the order of 7 and 25 min, respectively. With the SI/CIMS instrument in OH measurement mode alternating between OH signal and background (non-OH) signal, evidence was obtained for the presence of one or more unknown oxidants of SO2 in addition to OH. Depending on the nature of the oxidant(s), its ambient concentration may be enhanced in the CIMS inlet system by additional production. The apparent unknown SO2 oxidant was additionally confirmed by direct measurements of SO2 in conjunction with calculated H2SO4 concentrations. The calculated H2SO4 concentrations were consistently lower than the measured concentrations by a factor of 4.7 ± 2.4 when considering the oxidation of SO2 by OH as the only source of H2SO4. Both the OH and the background signal were also observed to increase significantly during daytime aerosol nucleation events, independent of the ozone photolysis frequency, J(O1D), and were followed by peaks in both H2SO4 and MSA concentrations. This suggests a strong relation between the unknown oxidant(s), OH chemistry, and the atmospheric photolysis and photooxidation of biogenic iodine compounds. As to the identity of the atmospheric SO2 oxidant(s), we have been able to exclude ClO, BrO, IO, and OIO as possible candidates based on {ab initio} calculations. Never\-theless, IO could contribute significantly to the observed CIMS background signal. A detailed analysis of this CIMS background signal in context with recently published kinetic data currently suggests that Criegee intermediates (CIs) produced from ozonolysis of alkenes play no significant role for SO2 oxidation in the marine atmosphere at Mace Head. On the other hand, SO2 oxidation by small CIs such as CH2OO produced photolytically or possibly in the photochemical degradation of methane is consistent with our observations. In addition, H2SO4 formation from dimethyl sulfide oxidation via SO3 as an intermediate instead of SO2 also appears to be a viable explanation. Both pathways need to be further explored.
    No preview · Article · Nov 2014 · Atmospheric Chemistry and Physics
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    ABSTRACT: During recent field campaigns, measured hydroxyl radical (OH) concentrations were up to a factor of ten larger than predicted by current chemical models for conditions of high OH reactivity and low nitrogen monoxide (NO) concentrations. These discrepancies were most often observed in forests, where concentrations of biogenic volatile organic compounds (BVOCs) were large. We investigated the radical budget during oxidation of VOCs with OH including a full set of accurate and precise radical measurements in the atmosphere simulation chamber SAPHIR in Juelich, Germany. The conditions during the chamber experiments were comparable to those during field campaigns with respect to radical and trace gas concentrations. In particular, OH reactivity was high and NO mixing ratios were as low as 200pptv. VOC species included the most important single compound isoprene, and its major oxidation products methacrolein and methyl vinyl ketone. Significant gaps between measured OH destruction and production pathways were found for isoprene and methacrolein. The additional OH needed to close the OH budget is consistent with recently proposed reaction mechanisms suggesting OH production from isomerization and decomposition reactions of organic peroxy radicals produced in the reaction of OH with VOCs.
    No preview · Conference Paper · Sep 2014
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    ABSTRACT: The hydroxyl radical (OH) is the main oxidation agent in the atmosphere during daytime. Recent field campaigns studying the radical chemistry in forests showed large differences between measured and modeled OH concentrations at low NOx concentration and when the OH reactivity was dominated by large concentrations of volatile organic compounds (VOC). These findings were only partially explained by the introduction of new efficient hydroxyl radical regeneration pathways in the isoprene oxidation mechanism. The question arises if other reactive VOCs with high global emission rates are also capable of additional OH recycling processes. In addition to isoprene, monoterpenes and 2-methyl-3-buten-2-ol (MBO) are the VOCs with the highest global emission rates. Due to their high reactivity towards OH they can dominate the radical chemistry in forested areas under certain conditions. The photochemical degradation of α-pinene, β-pinene, limonene, myrcene and MBO was investigated in the Jülich atmosphere simulation chamber SAPHIR in a dedicated series of experiments in 2012 and 2013. The chamber was equipped with instrumentation to measure radical concentrations (OH, HO2, RO2), the total OH reactivity, concentrations of all important OH precursors (O3, HONO, HCHO), of the parent VOC, its main oxidation products and photolysis frequencies to investigate the radical budget. All experiments were carried out under low NOx conditions (< 2ppb) and atmospheric terpenoid concentrations (< 5ppb) with and without addition of ozone into the SAPHIR chamber. For the investigation of the OH budget all measured OH production terms were compared to the measured OH destruction. Within the limits of accuracy of the instruments the OH budget was balanced in all cases. Consequently unaccounted OH recycling or primary OH production processes did not play a role for conditions of these experiments. In contrast to the analysis of the measured OH budget, numerical simulations of the conducted experiments using the Master Chemical Mechanism v3.2 showed an underestimation of the OH production in the α-pinene, β-pinene and limonene experiments. At the same time the measured OH destruction was overestimated by the numerical simulation. First sensitivity studies showed that these discrepancies are most likely related to a missing source of HO2 in the model.
    No preview · Conference Paper · Sep 2014
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    ABSTRACT: The removal of trace gases from the troposphere is, in most cases, initialized by reactions with hydroxyl radicals, and the products of these reactions are eventually deposited on the Earth's surface. The concentration of these hydroxyl radicals is therefore a measure of atmospheric self-cleansing. In theory, hydroxyl-radical concentrations can be enhanced by the recycling of some of the reaction products. The only known efficient recycling process involves nitrogen oxide and leads to production of ozone, yet observations in regions with high hydrocarbon and low nitrogen oxide concentrations show substantially elevated hydroxyl-radical concentrations, up to ten times higher than expected. If we normalize observed hydroxyl-radical concentrations to the maximum achievable in model calculations with variable nitrogen oxide concentrations, this photochemical coordinate system uncovers a common feature in almost all of these observations: even in the presence of inadequate amounts of nitrogen oxides, hydroxyl-radical concentrations are enhanced to the theoretical maximum obtainable at very much higher nitrogen oxide concentrations. This means that this important part of the self-cleansing capability of the atmosphere is working at maximum efficiency even in regions with a high burden of biogenic hydrocarbons and low nitrogen oxide concentration. Since these processes do not involve nitrogen oxides, tropospheric ozone production is greatly reduced compared with the expectation from current theory.
    No preview · Article · Jul 2014 · Nature Geoscience
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    ABSTRACT: UV radiation drives the processes of O 1 D formation from O 3 and NO 2 photolysis which are very important for atmospheric chemistry. Measurements of photolysis frequencies by filter radiometry and by actinic flux density measurements are rare worldwide, due to limited instrumentation. More recently, techniques have been developed for the retrieval of the photolysis frequencies from surface irradiance data. The method proposed includes the determination of J(O 1 D) and J(NO 2) as a function of solar zenith angle, by the use of global irradiance and empirical relationships, instead of the direct way using actinic flux densities. Here we present a method using UV irradiance products from satellite data, in order to calculate the photolysis frequencies in global scale. A second-order polynomial was used to convert solar global irradiance at certain wavelengths (305, 380nm) to photolysis frequencies. Results were validated using ground measurements from two sites (Greece, Ireland) where times series of measured photolysis frequencies of J(NO2) and J(O1D) were available. The validation process shows relatively good agreement when meteorological parameters agreed in both data sets and larger deviations in case of altered estimation of clouds.
    Full-text · Conference Paper · May 2014
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    ABSTRACT: 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.
    Full-text · Article · May 2014 · Atmospheric Chemistry and Physics
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    ABSTRACT: Formation and evolution of secondary organic aerosols (SOA) from biogenic VOCs influences the Earth's radiative balance. We have examined the photo-oxidation and aging of boreal terpene mixtures in the SAPHIR simulation chamber. Changes in thermal properties and chemical composition, deduced from mass spectrometric measurements, were providing information on the aging of biogenic SOA produced under ambient solar conditions. Effects of precursor mixture, concentration and photochemical oxidation levels (OH exposure) were evaluated. OH exposure was found to be the major driver in the long term photochemical transformations, i.e. reaction times of several hours up to days, of SOA and its thermal properties, whereas the initial concentrations and terpenoid mixtures had only minor influence. The volatility distributions were parameterized using a sigmoidal function to determine TVFR0.5 (the temperature yielding a 50% particle volume fraction remaining) and the steepness of the volatility distribution. TVFR0.5 increased by 0.3±0.1% (ca. 1 K), while the steepness increased by 0.9±0.3% per hour of 1 × 10(6) cm(-3) OH exposure. Thus, aging reduces volatility and increases homogeneity of the vapor pressure distribution, presumably because highly volatile fractions become increasingly susceptible to gas phase oxidation, while less volatile fractions are less reactive with gas phase OH.
    Full-text · Article · May 2014 · Environmental Science & Technology
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    ABSTRACT: The hydroxyl radical (OH) is the main oxidation agent in the atmosphere during daytime. Recent field campaigns studying the radical chemistry in forested areas showed large discrepancies between measured and modeled OH concentration at low NOx conditions and when OH reactivity was dominated by VOC. These observations were only partially explained by the evidence for new efficient hydroxyl radical regeneration pathways in the isoprene oxidation mechanism. The question arises if also other reactive VOCs with high global emission rates are also capable of additional OH recycling. Beside isoprene, monoterpenes and 2-methyl-3-buten-2-ol (MBO) are the volatile organic compounds (VOC) with the highest global emission rates. Due to their high reactivity towards OH monoterpenes and MBO can dominate the radical chemistry of the atmosphere in forested areas under certain conditions. In the present study the photochemical degradation mechanism of α-pinene, β-pinene, limonene, myrcene and MBO was investigated in the Jülich atmosphere simulation chamber SAPHIR. The focus of this study was in particular on the investigation of the OH budget in the degradation process. The photochemical degradation of these terpenoids was studied in a dedicated series of experiments in the years 2012 and 2013. The SAPHIR chamber was equipped with instrumentation to measure radicals (OH, HO2, RO2), the total OH reactivity, all important OH precursors (O3, HONO, HCHO), the parent VOC and its main oxidation products and photolysis frequencies to investigate the radical budget in the SAPHIR chamber. All experiments were carried out under low NOx conditions (≤ 2ppb) and atmospheric terpene concentrations (≤ 5ppb) with and without addition of ozone into the SAPHIR chamber. For the investigation of the OH budget all measured OH production terms were compared to the measured OH destruction. Within the limits of accuracy of the instruments the OH budget was balanced in all cases. Conse- quently unaccounted OH recycling or primary OH production processes did not play a role for conditions of these experiments. Despite the OH budget was closed in these experiments simulation results from the Master Chemical Mechanism v3.2 showed that the OH production was underestimated by the model in the α-pinene, β-pinene and limonene experiments. The measured total OH reactivity was overestimated by the numerical simulation.
    No preview · Conference Paper · Apr 2014
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    ABSTRACT: Gaseous nitrous acid (HONO) is an important precursor of tropospheric hydroxyl radicals (OH). OH is responsible for atmospheric self-cleansing and controls the concentrations of greenhouse gases like methane and ozone. Due to lack of measurements, vertical distributions of HONO and its sources in the troposphere remain unclear. Here, we present a set of observations of HONO and its budget made onboard a Zeppelin airship. In a sunlit layer separated from Earth’s surface processes by temperature inversion, we found high HONO concentrations providing evidence for a strong gas-phase source of HONO consuming nitrogen oxides and potentially hydrogen oxide radicals. The observed properties of this production process suggest that the generally assumed impact of HONO on the abundance of OH in the troposphere is substantially overestimated.
    Preview · Article · Apr 2014 · Science
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    ABSTRACT: Current photochemical models developed to simulate the atmospheric degradation of aromatic hydrocarbons tend to underestimate OH radical concentrations. In order to analyse OH budgets, we performed experiments with benzene, toluene, p-xylene, and 1,3,5-trimethylbenzene in the atmosphere simulation chamber SAPHIR. Experiments were conducted under low-NO conditions (typically 0.1–0.2 ppb) and high-NO conditions (typically 7–8 ppb), and starting concentrations of 6–250 ppb of aromatics, dependent on OH rate constants. For the OH budget analysis a steady-state approach was applied where OH production and destruction rates (POH and DOH) have to be equal. The POH were determined from measurements of HO2, NO, HONO, and O3 concentrations, considering OH formation by photolysis and recycling from HO2. The DOH were calculated from measurements of the OH concentrations and total OH reactivities. The OH budgets were determined from DOH / POH ratios. The accuracy and reproducibility of the approach were assessed in several experiments using CO as a reference compound where an average ratio DOH / POH = 1.13 ± 0.19 was obtained. In experiments with aromatics, these ratios ranged within 1.1–1.6 under low-NO conditions and 0.9–1.2 under high-NO conditions. The results indicate that OH budgets during photo-oxidation experiments with aromatics are balanced within experimental accuracies. Inclusion of a further, recently proposed OH production via HO2 + RO2 reactions led to improvements under low-NO conditions but the differences were small and insignificant within the experimental errors.
    No preview · Article · Mar 2014 · Atmospheric Chemistry and Physics
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    ABSTRACT: Hydroxyl radicals (OH) are the most important reagent for the oxidation of trace gases in the atmosphere. OH concentrations measured during recent field campaigns in isoprene rich environments were unexpectedly large. A number of studies showed that unimolecular reactions of organic peroxy radicals (RO2) formed in the initial reaction step of isoprene with OH play an important role for the OH budget in the atmosphere at low mixing ratios of nitrogen monoxide (NO) of less than 100 pptv. It has also been suggested that similar reactions potentially play an important role for RO2 from other compounds. Here, we investigate the oxidation of methacrolein (MACR), one major oxidation product of isoprene, by OH in experiments in the simulation chamber SAPHIR under controlled atmospheric conditions. The experiments show that measured OH concentrations are approximately 50% larger than calculated by current chemical models for conditions of the experiments (NO mixing ratio of 90 pptv). The analysis of the OH budget reveals a so far unaccounted OH source, which is correlated with the production rate of RO2 radicals from MACR. In order to balance the measured OH destruction rate, (0.77±0.3) OH radicals need to be additionally reformed from each OH that has reacted with MACR. The strong correlation of the missing OH source with the production of RO2 radicals is consistent with the concept of OH formation from unimolecular isomerization and decomposition reactions of RO2. The comparison of observations with model calculations gives a lower limit of 0.03 s−1 for the reaction rate constant, if the OH source is attributed to an isomerization reaction of one RO2 species formed in the MACR+OH reaction as suggested in literature. This fast isomerization reaction would be competitive to the reaction of this RO2 species with minimum 150 pptv NO.
    No preview · Article · Feb 2014 · Atmospheric Chemistry and Physics
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    ABSTRACT: The Zeppelin NT airship is an airborne platform capable of flying at low speed throughout the entire planetary boundary layer (PBL) and an ideal platform for the study of chemical processes in the lowest atmospheric layers with high spatial resolution. Secondary atmospheric aerosols have a long lifetime, up to several days, so their observation at a location can result not only from local production but also from transport processes. The aim of this project is to characterize the origin (local production or transported) of the observed aerosols as well as their precursors. We analyze: ● one North-South transect flight (between the Apennin and San Pietro Capofiume), and ● one vertical profiling flight (near the super-site San Pietro Capofiume) to determine the regions which contributed to the air-mass under observation. The analysis is done using the EURopean Air pollution Dispersion and Inverse Modeling (EURAD-IM) system, driven by meteorology given by the Weather Research and Forecasting model (WRF). The method applied follows: 1. A forward run of the chemistry transport model is performed, until a time after the observations. 2. Then, a backward (adjoint) run is performed, in which, the evolution of artificial perturbations (discrepancy between model and observation at the time/location of the observations) are tracked backwards in time. Methodology Introduction Back-plume schematic Comparison: model-observations Observations During the height profiling flights of the south-bound PEGASOS campaign, increased mass concentration of nitrate and ammonium was observed at narrow height levels. Back-plume examples for 20-21.06.2012 The model performance is being checked with respect to vertical profiling performed in 20.06.2012: ● In mixed atmospheric conditions, flight F028 (8:44-12:29UTC), no variance is observed with height, NOX and O3 appear slightly overestimated and underestimated respectively. ● In not mixed atmospheric conditions, flight F027 (4:30-8:25UTC), NOX is underestimated (possibly weak ground emissions taken from emission inventories), while the modeled O3 meets the correct mean value but shows smaller amplitude with respect to height variations. Summary and Acknowledgment 12-hour back-plumes were performed for the 20 and 21.06.2012. The adjoint concentration represents the area that influences the zeppelin observations. The F027 and F029 flight tracks have been used. ● An effort is done to identify the sources of observed trace species. ● A modified version of the chemistry transport model EURAD-IM is developed accounting for back-plumes (identical to adjoint model calculations). ● Using the method of back-plumes one can follow the adjoint concentration along the wind path and take into account gas and aerosol chemistry ● In order to succeed, first the model-observation discrepancies, during the planetary boundary layer evolution, should be explained. We acknowledge the financial support by the EU-FP7 project PEGASOS (project no.
    Full-text · Conference Paper · Jan 2014
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    ABSTRACT: Two gas-phase formaldehyde (HCHO) measurement techniques, a modified commercial wet-chemical instrument based on Hantzsch Fluorimetry and a custom-built instrument based on Fiber-Laser Induced Fluorescence (FILIF), were deployed at the atmospheric simulation chamber SAPHIR to compare the instruments' performances under a range of conditions. Thermolysis of para-HCHO and ozonolysis of 1-butene were used as HCHO sources, allowing for calculations of theoretical HCHO mixing ratios. Calculated HCHO mixing ratios are compared to measurements, and the two measurements are also compared. Experiments were repeated under dry and humid conditions (RH < 2% and RH > 60%) to investigate the possibility of a water artifact in the FILIF measurements. The ozonolysis of 1-butene also allowed for the investigation of an ozone artifact seen in some Hantzsch measurements in previous intercomparisons. Results show that under all conditions the two techniques are well correlated (R2 ≥ 0.997), and linear regression statistics show measurements agree with within stated uncertainty (15% FILIF + 5% Hantzsch). No water or ozone artifacts are identified.
    No preview · Article · Dec 2013 · Atmospheric Measurement Techniques
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    ABSTRACT: Diurnal and seasonal variations of gaseous sulfuric acid (H2SO4) and methane sulfonic acid (MSA) were measured in N.E. Atlantic air at the Mace Head atmospheric research station during the years 2010 and 2011. The measurements utilized selected ion/chemical ionization mass spectrometry (SI/CIMS) with a detection limit for both compounds of 4.3 × 10 4 cm-3 at 5 min signal integration. The H2SO4 and MSA gas-phase concentrations were analysed in conjunction with the condensational sink for both compounds derived from 3 nm-10 μm (diameter) aerosol size distributions. Accommodation coefficients of 1.0 for H2SO4 and 0.12 for MSA were assumed leading to estimated atmospheric lifetimes of the order of 7 min and 25 min, respectively. With the SI/CIMS instrument in OH measurement mode alternating between OH signal and background (non-OH) signal evidence was obtained for the presence of one or more unknown oxidants of SO2 in addition to OH. Depending on the nature of the oxidant(s) their ambient concentration may be enhanced in the CIMS inlet system by additional production. The apparent unknown SO2 oxidant was additionally confirmed by direct measurements of SO2 in conjunction with calculated H2SO4 concentrations. The calculated concentrations were consistently lower than the measured concentrations by a factor 4.8 ± 3.4 when considering the oxidation of SO2 by OH as the only source of H2SO4. Both the OH and the background signal were also observed to increase significantly during daytime aerosol nucleation events, independent of the ozone photolysis frequency, J(O1D), and were followed by peaks in both H2SO4 and MSA concentrations. This suggests a strong relation between the unknown oxidant(s), OH chemistry, and the atmospheric photo-oxidation of biogenic iodine compounds. As to the identity of the oxidant(s), we have been able to exclude ClO, BrO, IO, and OIO as possible candidates based on ab initio calculations. Stabilized Criegee intermediates (sCI) produced from ozonolysis of alkenes potentially contribute to the oxidation efficiency of the coastal and marine atmosphere. However, analysis of the CIMS background signal in context with recently published kinetic data currently suggests that larger Criegee intermediates produced from ozonolysis play no significant role for SO2 oxidation in the marine atmosphere. The possibility of H2SO4 formation without SO2 as precursor or from SO2 oxidation by small sCI produced photolytically should be explored.
    Full-text · Article · Dec 2013 · Atmospheric Chemistry and Physics