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ABSTRACT: We used both a conventional transmission electron microscope and an environmental transmission electron microscope (ETEM)
to determine morphology, composition, and water uptake of 80 individual aerosol particles collected from the young smoke of
flaming and smoldering fires during SAFARI-2000, a comprehensive air quality campaign in southern Africa. Six representative
carbonaceous particle types are described, including soot, tar balls, and heterogeneously internally mixed particles containing
C with S-, K-, Mg- or Na-rich inorganic phases. The hygroscopic behavior of these particles over the range 0–100% relative
humidity (RH) was studied in detail. Soot and tar balls did not take up water, whereas the mixed organic–inorganic particles
took up water between 55 and 100% RH, the exact value depending on the composition of their water-soluble phases. The inorganic
phase appeared to determine the hygroscopic properties of all mixed organic–inorganic particles. Thus, incorporation of inorganic
plant material or reactions with inorganic atmospheric components can dramatically alter the hygroscopic properties of carbonaceous
particles in smoke plumes. The fraction of these mixed organic–inorganic particles plausibly increases with time, which will
modulate the effects of smoke on radiative budgets.
Journal of Atmospheric Chemistry 04/2012; 56(3):259-273. · 0.99 Impact Factor
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ABSTRACT: Measurements of submicron particles by Fourier transform infrared spectroscopy in 14 campaigns in North America, Asia, South America, and Europe were used to identify characteristic organic functional group compositions of fuel combustion, terrestrial vegetation, and ocean bubble bursting sources, each of which often accounts for more than a third of organic mass (OM), and some of which is secondary organic aerosol (SOA) from gas-phase precursors. The majority of the OM consists of alkane, carboxylic acid, hydroxyl, and carbonyl groups. The organic functional groups formed from combustion and vegetation emissions are similar to the secondary products identified in chamber studies. The near absence of carbonyl groups in the observed SOA associated with combustion is consistent with alkane rather than aromatic precursors, and the absence of organonitrate groups can be explained by their hydrolysis in humid ambient conditions. The remote forest observations have ratios of carboxylic acid, organic hydroxyl, and nonacid carbonyl groups similar to those observed for isoprene and monoterpene chamber studies, but in biogenic aerosols transported downwind of urban areas the formation of esters replaces the acid and hydroxyl groups and leaves only nonacid carbonyl groups. The carbonyl groups in SOA associated with vegetation emissions provides striking evidence for the mechanism of esterification as the pathway for possible oligomerization reactions in the atmosphere. Forest fires include biogenic emissions that produce SOA with organic components similar to isoprene and monoterpene chamber studies, also resulting in nonacid carbonyl groups in SOA.
Proceedings of the National Academy of Sciences 02/2011; 108(9):3516-21. · 9.68 Impact Factor
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ABSTRACT: Oceans cover over two-thirds of the Earth's surface, and the particles emitted to the atmosphere by waves breaking on sea surfaces provide an important contribution to the planetary albedo. During the International Chemistry Experiment in the Arctic LOwer Troposphere (ICEALOT) cruise on the R/V Knorr in March and April of 2008, organic mass accounted for 15-47% of the submicron particle mass in the air masses sampled over the North Atlantic and Arctic Oceans. A majority of this organic component (0.1-0.4 microm(-3)) consisted of organic hydroxyl (including polyol and other alcohol) groups characteristic of saccharides, similar to biogenic carbohydrates found in seawater. The large fraction of organic hydroxyl groups measured during ICEALOT in submicron atmospheric aerosol exceeded those measured in most previous campaigns but were similar to particles in marine air masses in the open ocean (Southeast Pacific Ocean) and coastal sites at northern Alaska (Barrow) and northeastern North America (Appledore Island and Chebogue Point). The ocean-derived organic hydroxyl mass concentration during ICEALOT correlated strongly to submicron Na concentration and wind speed. The observed submicron particle ratios of marine organic mass to Na were enriched by factors of approximately 10(2)-approximately 10(3) over reported sea surface organic to Na ratios, suggesting that the surface-controlled process of film bursting is influenced by the dissolved organic components present in the sea surface microlayer. Both marine organic components and Na increased with increasing number mean diameter of the accumulation mode, suggesting a possible link between organic components in the ocean surface and aerosol-cloud interactions.
Proceedings of the National Academy of Sciences 04/2010; 107(15):6652-7. · 9.68 Impact Factor
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ABSTRACT: Atmospheric particles collected during the ICARTT 2004 field experiment at ground based sites at Appledore Island (AI), New Hampshire, Chebogue Point (CP), Nova Scotia, and aboard the R/V Ronald Brown (RB) were analyzed using Fourier transform infrared (FTIR) spectroscopy to quantify organic mass (OM) and organic functional groups. Several of these spectra contain a unique absorbance peak at 3500 cm(-1). Laboratory calibrations identify this peak with phenol functional groups. The phenol groups are associated with seawater-derived emissions based on correlations with tracer volatile organic compounds (VOCs) and ions, and potential source contribution function (PSCF) analysis. On the basis of the measured absorptivities, the project average phenol group concentrations are 0.24 +/- 0.18 microg m(-3) (4% of the total OM) at AI, 0.10 +/- 0.6 microg m(-3) (5% of the total OM) at CP, and 0.08 +/- 0.09 microg m(-3) (2% of the total OM) on board the RB, with detection limits typically between 0.06 and 0.11 microg m(-3). The spectra were partitioned into three primary factors using positive matrix factorization (PMF) sufficient to explain more than 95% of the measured OM. The fossil fuel combustion factor contributed 40% (AI), 34% (CP), and 43% (RB) of the total OM; the terrestrial biogenic factor contributed 20% (AI), 30% (CP), and 27% (RB). The seawater-derived factor contributed 40% (AI), 36% (CP) and 29% (RB) of the OM and showed similar correlations to tracers as the phenol group.
Environmental Science and Technology 03/2010; 44(7):2542-8. · 5.23 Impact Factor
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ABSTRACT: Carbon-containing aerosol particles collected in the Arctic and southeastern Pacific marine boundary layers show distinct chemical signatures of proteins, calcareous phytoplankton, and two types of polysaccharides in Near-Edge Absorption X-ray Fine Structure (NEXAFS) spectromicroscopy. Arctic samples contained mostly supermicron sea salt cuboids with a polysaccharide-like organic coating. Southeastern Pacific samples contained both continental and marine aerosol types; of the 28 analyzed marine particles, 19 were characterized by sharp alkane and inorganic carbonate peaks in NEXAFS spectra and are identified as fragments of calcareous phytoplankton. Submicron spherical particles with spectral similarities to carbohydrate-like marine sediments were also observed in Pacific samples. In both regions, supermicron amide and alkane-containing particles resembling marine proteinaceous material were observed. These four chemical types provide a framework that incorporates several independent reports of previous marine aerosol observations, showing the diversity of the composition and morphology of ocean-derived primary particles.
Advances in Meteorology. 01/2010;
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ABSTRACT: Deliquescence properties of sodium chloride are size dependent for particles smaller than 100 nm. Molecular dynamics (MD) simulations are used to determine deliquescence relative humidity (DRH) for particles in this size range by modeling idealized particles in contact with humid air. Constant humidity conditions are simulated by inclusion of a liquid reservoir of NaCl solution in contact with the vapor phase, which acts as a source of water molecules as uptake by the nanoparticle proceeds. DRH is bounded between the minimum humidity at which sustained water accumulation is observed at the particle surface and the maximum humidity at which water accumulation is not observed. Complete formation of a liquid layer is not observed due to computational limitations. The DRH determined increases with decreasing particle diameter, rising to between 91% and 93% for a 2.2 nm particle and between 81% and 85% for an 11 nm particle, higher than the 75% expected for particles larger than 100 nm. The simulated size dependence of DRH agrees well with predictions from bulk thermodynamic models and appears to converge with measurements for sizes larger than 10 nm. Complete deliquescence of nanoparticles in the 2-11 nm size range requires between 1 and 100 mus, exceeding the available computational resources for this study. Water uptake coefficients are near 0.1 with a negligible contribution from diffusion effects. Planar uptake coefficients decrease from 0.41 to 0.09 with increasing fractional water coverage from 0.002 to 1, showing a linear dependence on the logarithm of the coverage fraction with a slope of -0.08+/-0.01 (representing the effect of solvation). Particle uptake coefficients increase from 0.13 at 11 nm to 0.65 at 2.2 nm, showing a linear dependence on the logarithm of the edge fraction (which is a function of diameter) with a slope of 0.74+/-0.04 (representing larger edge effects in smaller particles).
The Journal of chemical physics 10/2008; 129(9):094508. · 3.09 Impact Factor
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ABSTRACT: Surface tensions for liquid-vapor (lv), solid-liquid (sl), and solid-vapor (sv) interfaces are calculated from molecular dynamics simulations of the NaCl-water-air system. Three distinct calculation techniques based on thermodynamic properties are used to describe the multicomponent mixtures. Simulations of each bulk phase (including a liquid saturated solution) and various interfaces are carried out at both NPT and NVT conditions. The thermodynamic relation for energy difference between interface and bulk phases provides an upper bound to the surface tension, while the energy-integral and test area methods provide direct estimates. At 1 atm and 300 K, the best predictions for surface tensions are sigmasv (NaCl-air) of 114 mN m(-1), sigmasl (NaCl- soln) of 63 mN m(-1), sigmalv (soln-air) of 82 mN m(-1), and sigmalv (water-air) of 66 mN m(-1). The calculated surface tensions from simulations have uncertainties between 5 and 10%, which are higher than measurements for the liquid interfaces and lower than the measurement uncertainty for the solid interfaces. The calculated upper bounds for surface tensions of liquid interfaces compare well with experimental results but provide no improvement over existing measurements. However, the bounding values for solid interfaces lower uncertainty by as much as a factor of 10 as compared to the indirect experimental measurements currently available. The energy-integral and test area methods appear to underestimate the surface tension of water by 10%, which is consistent with previous studies using similar model potentials. The calculated upper bounds of surface tension show a weakly positive correlation with pressure in the 0.1-100 atm range for liquid-solid, liquid-vapor, and solid-vapor interfaces.
The Journal of Physical Chemistry B 11/2007; 111(41):11989-96. · 3.70 Impact Factor
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ABSTRACT: 1] We used conventional and environmental transmission electron microscopes to determine morphology, composition, and water uptake of individual natural inorganic aerosol particles collected from industrial pollution plumes and from clean and polluted marine environments. Five particle types are described in detail. They range from relatively insoluble mineral grains to internally mixed particles containing NaCl with other soluble or relatively insoluble material. We studied the hygroscopic behavior of these particles from 0 to 100% relative humidity (RH). Relatively insoluble materials do not take up water over the experimental RH range. Single crystals of NaCl from both natural and laboratory sources have a well-defined deliquescence point of approximately 76% RH at 291 K. NaCl-bearing aggregate particles appear to deliquesce between 74 and 76% RH (same RH within error) when NaCl is internally mixed with relatively insoluble phases, but the particles deliquesce at lower RH when aggregated with other soluble phases such as NaNO 3 . For all NaCl-bearing particles studied, hygroscopic growth is pronounced above 76% RH, and water uptake by the particles is dominated by the soluble phase. Furthermore, the soluble phase initiating deliquescence controls the locus of further hygroscopic growth of the aggregate particle. Our results demonstrate that composition and mixing state affect water uptake of natural aerosol particles. Furthermore, internally mixed particles are confirmed to deliquesce at lower RH values than predicted from the individual components.
J. Geophys. Res. 01/2007; 112.
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ABSTRACT: To gain a better understanding of the interaction of water and NaCl at the surface during dissolution, we have used molecular dynamics to simulate the interface with two equal-sized slabs of solid NaCl and liquid water in contact. The introduction of voids in the bulk of the salt, as well as steps or pits on the surface of the NaCl slab results in a qualitative change of system structure, as defined by radial distribution functions (RDFs). As an example, the characteristic Na-Na RDF for the system changes from regularly spaced narrow peaks (corresponding to an ordered crystalline structure), to a broad primary and smaller secondary peak (corresponding to a disordered structure). The change is observed at computationally short time scales of 100 ps, in contrast with a much longer time scale of 1 mus expected for complete mixing in the absence of defects. The void fraction (which combines both bulk and surface defects) required to trigger dissolution varies between 15%-20% at 300 K and 1 atm, and has distinct characteristics for the physical breakdown of the crystal lattice. The void fraction required decreases with temperature. Sensitivity studies show a strong dependence of the critical void fraction on the quantity and distribution of voids on the surface, with systems containing a balanced number of surface defects and a rough surface showing a maximum tendency to dissolve. There is a moderate dependence on temperature, with a 5% decrease in required void fraction with a 100 K increase in temperature, and a weak dependence on water potential model used, with the SPC, SPC/E, TIP4P, and RPOL models giving qualitatively identical results. The results were insensitive to the total quantity of water available for dissolution and the duration of the simulation.
The Journal of Chemical Physics 05/2006; 124(15):154713. · 3.33 Impact Factor
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John H. Seinfeld,
Gregory R Carmichael,
Richard Arimoto,
William C. Conant,
Frederick J. Brechtel,
Timothy S. Bates,
Thomas A. Cahill,
Antony D. Clarke,
Sarah J Doherty,
Piotr J. Flatau, [......],
Philip B. Russell,
Atsushi Shimizu,
Yohei Shinozuka,
Chul H. Song,
Youhua Tang,
Itsushi Uno,
Andrew M. Vogelmann,
Rodney J Weber,
Jung-Hun Woo,
Xiao Y. Zhang
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ABSTRACT: Although continental-scale plumes of Asian dust and pollution reduce the amount of solar radiation reaching the earth's surface and perturb the chemistry of the atmosphere, our ability to quantify these effects has been limited by a lack of critical observations, particularly of layers above the surface. Comprehensive surface, airborne, shipboard, and satellite measurements of Asian aerosol chemical composition, size, optical properties, and radiative impacts were performed during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) study. Measurements within a massive Chinese dust storm at numerous widely spaced sampling locations revealed the highly complex structure of the atmosphere, in which layers of dust, urban pollution, and biomass-burning smoke may be transported long distances as distinct entities or mixed together. The data allow a first-time assessment of the regional climatic and atmospheric chemical effects of a continental-scale mixture of dust and pollution. Our results show that radiative flux reductions during such episodes are sufficient to cause regional climate change.
02/2004;
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Doug W. Johnson,
Simon Osborne,
Robert Wood,
Karsten Suhre,
Randy Johnson,
Steven Businger,
Patricia K. Quinn,
Alfred Wiedensohler,
Philip A. Durkee, Lynn M. Russell,
Meinrat O. Andreae,
Colin O'Dowd,
Kevin J. Noone,
Brian Bandy,
J. Rudolph,
Spyros Rapsomanikis
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ABSTRACT: One of the primary aims of the North Atlantic regional Aerosol Characterisation Experiment (ACE-2) was to quantify the physical and chemical processes affecting the evolution of the major aerosol types over the North Atlantic. The best, practical way of doing this is in a Lagrangian framework where a parcel of air is sampled over several tens of hours and its physical and chemical properties are intensively measured. During the intensive observational phase of ACE-2, between 15 June 1997 and 24 July 1997, 3 cloudy Lagrangian experiments and 3 cloud-free, Lagrangian experiments were undertaken between the south west tip of the Iberian Peninsula and the Canary Islands. This paper gives an overview of the aims and logistics of all of the Lagrangian experiments and compares and contrasts them to provide a framework for the more focused Lagrangian papers in this issue and future process modelling studies and parametrisation development. The characteristics of the cloudy Lagrangian experiments were remarkably different, enabling a wide range of different physical and chemical processes to be studied. In the 1st Lagrangian, a clean maritime air mass was sampled in which salt particle production, due to increased wind speed, dominated the change in the accumulation mode concentrations. In the 2nd Lagrangian, extensive cloud cover resulted in cloud processing of the aerosol in a polluted air mass, and entrainment of air from the free troposphere influenced the overall decrease in aerosol concentrations in the marine boundary layer (MBL). Very little change in aerosol characteristics was measured in the 3rd Lagrangian, where the pollution in the MBL was continually being topped up by entraining air from a residual continental boundary layer (CBL) above. From the analysis of all the Lagrangian experiments, it has been possible to formulate, and present here, a generalised description of a European continental outbreak of pollution over the sub-tropical North Atlantic.
Tellus B 02/2003; 52(2):290 - 320. · 4.38 Impact Factor
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ABSTRACT: A differential mobility analyzer in which particles are classified in a radial flow toward the center of parallel disk electrodes, termed the Radial Differential Mobility Analyzer (RDMA), has been developed. Its classifying characteristics have been determined experimentally using both standard polystyrene latex spheres and mobility-classified aerosol particles over the size range of 3–200 nm. The idealized transfer function for the RDMA differs from that of the conventional cylindrical DMA only by a group of geometrical constants. The RDMA is designed specifically for the measurement of ultrafine aerosol particles, achieving a transmission efficiency of 0.85 to 0.90 in the 3–10-nm size range and having a short residence time to limit diffusional broadening of the transfer function. The simple design of the RDMA reduces the number of precision parts that must be fabricated below that for cylindrical DMAs, resulting in a compact, relatively lightweight, and low-cost instrument.
Aerosol Science and Technology 01/1995; 23(3):357-372. · 2.67 Impact Factor
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ABSTRACT: In this work we have compared ground-based measurements of organic carbon (OC) in the fine aerosol (PM2.5) fraction that are reported in peer-reviewed publications as part of both short campaigns and continuous monitoring networks. The comparison provides a quantitative review of global OC measurements for the purpose of establishing the extent to which organic aerosol concentrations are known with sufficient geographic and historical resolution to constrain global climate models. Only North America has sufficient measurements to provide meaningful spatial and temporal trends, although available measurements from China and Japan indicate that the Asian region is the most polluted with OC concentrations of approximately 10 μg m−3. These measurements have a low spatial resolution, with most sites located in highly urban areas within a small geographic region. OC concentrations in North America are approximately 1 μg m−3 and are better characterized spatially, temporally, and historically by continuous monitoring networks established decades ago. OC concentration shows a weakly increasing trend in some regions from 1997 to 2006, although in most regions it has remained effectively constant over the last ten years. Eastern U.S. sites show maximum OC in the winter and western U.S. sites show maximum OC in the summer. There is no correlation at U.S. sites between OC concentration and sulfate, nitrate, or ammonium ions, with R2 < 0.1 in each case.
Atmospheric Environment.
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ABSTRACT: We used an environmental transmission electron microscope to observe deliquescence and hygroscopic growth of atmospheric particles with hygroscopic coatings over the range 0–100% relative humidity (RH). The particles were collected from polluted and clean environments. Types included a sulfate-coated NaCl/silicate aggregate particle, a sulfate-coated sea-salt particle, and a Mg-rich, chloride-coated sea-salt particle. They all exhibited initial water uptake between 50% and 60% RH, although the first major morphological changes occurred at 70% RH. A deliquescence sphere, adjacent to the core particle, formed between 70% and 76% RH when deliquescence occurred or when the liquid phase was able to break out of the solid exterior coating. The deliquescence sphere grew to engulf the particle with increasing RH. Some particles developed a splatter zone associated with a particle coating. Efflorescence occurred over the range 49–44% RH. Our results indicate that some coated particles undergo a multi-step deliquescence process and that composition of the different phases within the coating affects deliquescence and hygroscopic growth below 76% RH. Above 76% RH, the dominant hygroscopic growth was due to water uptake by NaCl. Efflorescence of these particles also was strongly linked to NaCl, although the presence of other phases inhibited formation of a single NaCl crystal. Our results show that the observed coatings can both enhance particle solubility and lower the effective deliquescence RH of the particle. Thus, these coatings cause important phase and size changes for aerosol particles that could feed back into many other chemical and physical processes that contribute to radiative forcing within the atmosphere.
Atmospheric Environment.
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ABSTRACT: Submicron particles were collected from June to September 2008 in La Jolla, California to investigate the composition and sources of atmospheric aerosol in an anthropogenically-influenced coastal site. Factor analysis of aerosol mass spectrometry (AMS) and Fourier transform infrared (FTIR) spectroscopy measurements revealed that the two largest sources of submicron organic mass (OM) at the sampling site were (1) fossil fuel combustion associated with ship and diesel truck emissions near the ports of Los Angeles and Long Beach and (2) aged smoke from large wildfires burning in central and northern California. During non-fire periods, fossil fuel combustion contributed up to 95% of FTIR OM, correlated to sulfur, and consisted mostly of alkane (86%) and carboxylic acid groups (9%). During fire periods, biomass burning contributed up to 74% of FTIR OM, consisted mostly of alkane (48%), ketone (25%), and carboxylic acid groups (17%), and correlated to AMS-derived factors resembling brush fire smoke, wood smoldering and flaming particles, and biogenic secondary organic aerosol. The two AMS-derived biomass burning factors were identified as oxygenated and hydrocarbon biomass burning aerosol on the basis of spectral similarities to smoldering and flaming smoke particles, respectively. In addition, the ratio of oxygenated to hydrocarbon biomass burning OM shows a clear diurnal trend with an afternoon peak, consistent with photochemical oxidation. Back trajectory analysis indicates that 2–4-day old forest fire emissions include substantial ketone groups, which have both lower O/C and lower m/z 44/OM fraction than carboxylic acid groups. Air masses with more than 4-day old emissions have higher carboxylic acid/ketone group ratios, showing that atmospheric processing of these ketone-containing organic aerosol particles results in increased m/z 44 and O/C. These observations may provide functionally-specific evidence for the type of chemical processing that is responsible for biomass burning particle composition in the atmosphere.
Atmospheric Environment.
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ABSTRACT: Organic aerosol chemical markers from normalized concentrations of independent measurements of mass fragments (using Aerosol Mass Spectrometry, AMS) are compared to bond-based functional groups (from Fourier Transform Infrared spectroscopy, FTIR) during eight field projects in the western hemisphere. Several field projects show weak correlations between alcohol group fractions and m/z 60 fractions, consistent with the organic hydroxyl groups and the fragmentation of saccharides, but the weakness of the correlations indicate chemical differences among the relationships for ambient aerosols in different regions. Carboxylic acid group fractions and m/z 44 fractions are correlated weakly for three projects, with correlations expected for aerosols dominated by di-acid compounds since their fragmentation is typically dominated by m/z 44. Despite differences for three projects with ratios of m/z 44 to m/z 57 fragments less than 10, five projects showed a linear trend between the project-average m/z 44 to m/z 57 ratio and the ratio of acid and alkane functional groups. While this correlation explains only a fraction of the fragment and bond variability measured, the consistency of this relationship at multiple sites indicates a general agreement with the interpretation of the relative amount of m/z 44 as a carboxylic acid group marker and m/z 57 as an alkane group marker.
Atmospheric Environment.
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ABSTRACT: During wintertime measurements in coastal southern California, organonitrate groups accounted for up to 10% of organic mass (OM) in submicron particles. In this study, we report the calibrated absorptivity, the uncertainties in the calibrations, the detection limits for 12 and 24 h ambient sampling, and the multipeak retrieval algorithm for the method developed. Organonitrate groups were observed when both submicron particle-phase nitrate and OM concentrations exceeded 1 μg m−3. These high concentrations were associated with a mixed urban fossil fuel combustion source type that had potential source regions near Riverside and the South Coast Air Basin. The high frequency of these organonitrate observations contrasts with a number of studies of aerosol particles in other regions with more humid conditions, in which organonitrate groups were not detected and submicron sulfate concentrations exceeded those of nitrate. Our results suggest both that organonitrates form and/or exist in significant concentrations during polluted urban conditions and that their lifetime may be limited by hydrolysis in the particle phase.
Atmospheric Environment.
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ABSTRACT: Aircraft measurements of organic carbon (OC) aerosol by two independent methods over the NW Pacific during the ACE-Asia campaign reveal unexpectedly high concentrations in the free troposphere (FT). Concentrations average 4 μg sm−3 in the 2–6.5 km column with little vertical gradient. These values are 10–100 times higher than computed with a global chemical transport model (CTM) including a standard 2-product simulation of secondary organic aerosol (SOA) formation based on empirical fits to smog chamber data. The same CTM reproduces the observed vertical profiles of sulfate and elemental carbon aerosols, which indicate sharp decreases from the boundary layer to the FT due to wet scavenging. Our results suggest a large, sustained source of SOA in the FT from oxidation of long-lived volatile organic compounds. We find that this SOA is the dominant component of aerosol mass in the FT, with implications for intercontinental pollution transport and radiative forcing of climate. Earth and Planetary Sciences Engineering and Applied Sciences Version of Record
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ABSTRACT: a b s t r a c t We examine the temporal and the spatial trends in the concentrations of black carbon (BC) e recorded by the IMPROVE monitoring network for the past 20 years e in California. Annual average BC concen-trations in California have decreased by about 50% from 0.46 mg m À3 in 1989 to 0.24 m gm À3 in 2008 compared to the corresponding reductions in diesel BC emissions (also about 50%) from a peak of 0.013 Tg Yr À1 in 1990 to 0.006 Tg Yr À1 by 2008. We attribute the observed negative trends to the reduction in vehicular emissions due to stringent statewide regulations. Our conclusion that the reduction in diesel emissions is a primary cause of the observed BC reduction is also substantiated by a significant decrease in the ratio of BC to non-BC aerosols. The absorption efficiency of aerosols at visible wavelengths e determined from the observed scattering coefficient and the observed BC e also decreased by about 50% leading to a model-inferred negative direct radiative forcing (a cooling effect) of À1.4 W m À2 (AE60%) over California.
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ABSTRACT: The deliquescence of sodium chloride is size dependent for par-ticles smaller than 100 nm, with some discrepancies between mea-sured and predicted deliquescence relative humidity as a function of size. Two sources of uncertainty in current models are the solid– liquid/solid–vapor surface tensions and the curvature dependence of surface tension. Molecular Dynamics simulations are used to calculate surface tensions and their corresponding upper bounds, which compare well with measured values of liquid–vapor (LV) interfaces and significantly reduce uncertainty compared to ex-perimental estimates of solid–liquid (SL) and solid–vapor (SV) in-terfaces. Surface tensions calculated for nanoparticles in the 2–10 nm size range are related to the corresponding flat interface val-ues using the first-order Tolman length (δ). At 1 atm and 300 K, the Tolman length determined from the test-area method is of the order of 0.1 nm with a precision between 5% and 10%. The δ LV (water–air) is 0.15 nm, δ LV (soln–air) is 0.10 nm, δ SL (NaCl–soln:) is 0.13 nm, and δ SV (NaCl–air) is 0.14 nm, with positive values corre-sponding to a decrease in surface tension for smaller particles. The size-dependent deliquescence relative humidity is calculated using a thermodynamic model of water uptake with these new surface ten-sion estimates and with Tolman length corrections. The reduced uncertainties in surface tension significantly improve agreement with measured deliquescence relative humidity of sodium chloride nanoparticles with 5–150 nm diameters. The size-dependent cor-rections to surface tension produce a minor improvement in the comparison of predicted and measured deliquescence relative hu-midity of particles smaller than 3 nm.
