Characterization and sources assignation of PM2.5 organic aerosol in a rural area of Spain

The results from a year-long study of the organic composition of PM2.5 aerosol collected in a rural area influenced by a highway of Spain are reported. The lack of prior information related to the organic composition of PM2.5 aerosol in Spain, concretely in rural areas, led definition of the goals of this study. As a result, this work has been able to characterize the main organic components of atmospheric aerosols, including several compounds of SOA, and has conducted a multivariate analysis in order to assign sources of particulate matter. A total of 89 samples were taken between April 2004 and April 2005 using a high-volume sampler. Features and abundance of n-alkanes, polycyclic aromatic hydrocarbons (PAHs), alcohols and acids were separately determined using gas chromatography/mass spectrometry and high performance liquid chromatography analysis. The Σn-alkane and ΣPAHs ranged from 3 to 81 ng m^?3 and 0.1 to 6 ng m^?3 respectively, with higher concentrations during colder months. Ambient concentrations of Σalcohols and Σacids ranged from 21 to 184 ng m^?3 and 39 to 733 ng m^?3, respectively. Also, several components of secondary organic aerosol have been quantified, confirming the biogenic contribution to ambient aerosol. In addition, factor analysis was used to reveal origin of organic compounds associated to particulate matter. Eight factors were extracted accounting more than 83% of the variability in the original data. These factors were assigned to a typical high pollution episode by anthropogenic particles, crustal material, plant waxes, fossil fuel combustion, temperature, microbiological emissions, SOA and dispersion of pollutants by wind action. Finally, a cluster analysis was used to compare the organic composition between the four seasons.     

Estimates of the contributions of biogenic and anthropogenic hydrocarbons to secondary organic aerosol at a southeastern US location

An organic tracer-based method containing laboratory and field study components was used to estimate the secondary organic aerosol (SOA) contributions of biogenic and anthropogenic hydrocarbons to ambient organic carbon (OC) concentrations in PM_2.5 during 2003 in Research Triangle Park, NC. In the laboratory, smog chamber experiments were conducted where isoprene, α-pinene, β-caryophyllene, and toluene were individually irradiated in the presence of NO_X. In each experiment, SOA was collected and analyzed for potential tracer compounds, whose concentrations were used to calculate a mass fraction of tracer compounds for each hydrocarbon. In the field, 33 PM_2.5 samples were collected and analyzed for (1) tracer compounds observed in the laboratory irradiations, (2) levoglucosan, a biomass burning tracer, and (3) total OC. For each of the four hydrocarbons, the SOA contributions to ambient OC concentrations were estimated using the tracer concentrations and the laboratory-derived mass fractions. The estimates show SOA formation from isoprene, α-pinene, β-caryophyllene, and toluene contributed significantly to the ambient OC concentrations. The relative contributions were highly seasonal with biomass burning in the winter accounting for more than 50% of the OC concentrations, while SOA contributions remained low. However, during the 6-month period between May and October, SOA from the precursor hydrocarbons contributed more than 40% of the measured OC concentration. Although the tracer-based method is subject to considerable uncertainty due to the simplification of replacing the complex set of chemical reactions responsible for SOA with a laboratory-derived single-valued mass fraction, the results suggest this approach can be used to identify major sources of SOA which can assist in the development of air quality models.     

Can secondary organic aerosol formed in an atmospheric simulation chamber continuously age?

This work investigates the oxidative aging process of SOA derived from select aromatic (m-xylene) and biogenic (α-pinene) precursors within an environmental chamber. Simultaneous measurements of SOA hygroscopicity, volatility, particle density, and elemental chemical composition (C:O:H) reveal only slight particle aging for up to the first 16 h of formation. The chemical aging observed is consistent with SOA that is decreasing in volatility and increasing in O/C and hydrophilicity. Even after aging, the O/C (0.25 and 0.40 for α-pinene and m-xylene oxidation, respectively) was below the OOAI and OOAII ambient fractions measured by high-resolution aerosol mass spectra coupled with Positive Matrix Factorization (PMF). The rate of increase in O/C does not appear to be sufficient to achieve OOAI or OOAII levels of oxygenation within regular chamber experiment duration. No chemical aging was observed for SOA during dark α-pinene ozonolysis with a hydroxyl radical scavenger present. This finding is consistent with observations by other groups that SOA from this system is comprised of first generation products.     

Model evidence for a significant source of secondary organic aerosol from isoprene

We investigate how a recently suggested pathway for production of secondary organic aerosol (SOA) affects the consistency of simulated organic aerosol (OA) mass in a global three-dimensional model of oxidant-aerosol chemistry (GEOS-Chem) versus surface measurements from the interagency monitoring of protected visual environments (IMPROVE) network. Simulations in which isoprene oxidation products contribute to SOA formation, with a yield of 2.0% by mass reduce a model bias versus measured OA surface mass concentrations. The resultant increase in simulated OA mass concentrations during summer of 0.6–1.0 μg m⁻³ in the southeastern United States reduces the regional RMSE to 0.88 μg m⁻³ from 1.26 μg m⁻³. Spring and fall biases are also reduced, with little change in winter when isoprene emissions are negligible.     

Heterogeneous SOA yield from ozonolysis of monoterpenes in the presence of inorganic acid

The secondary organic aerosol (SOA) yield of a series of montoerpenes was investigated to determine the relative amounts of organic mass, which can be attributed to mass produced by heterogeneous acid-catalyzed reactions. Five monoterpenes (α-pinene, terpinolene, d-limonene, Δ²-carene, β-pinene) were studied using a 2 m³ indoor Teflon chamber and SOA was created in the presence of both acidic and neutral inorganic seed aerosol. The relative humidity was varied to create differing acidic seed environments. The heterogeneous aerosol production was influenced by the seed mass concentration, the acidity of the inorganic seed aerosol, and also molecular structure of the monoterpene ozonolysis products. This study also can be incorporated with our previously presented model of the kinetic expression for SOA mass production from heterogeneous acid-catalyzed reactions.     

Secondary organic aerosol formation from the oxidation of aromatic hydrocarbons in the presence of dry submicron ammonium sulfate aerosol

A laboratory study was conducted to examine formation of secondary organic aerosols. A smog chamber system was developed for studying gas–aerosol interactions in a dynamic flow reactor. These experiments were conducted to investigate the fate of gas and aerosol phase compounds generated from hydrocarbon–nitrogen oxide (HC/NO_x) mixtures irradiated in the presence of fine (<2.5 μm) particulate matter. The goal was to determine to what extent photochemical oxidation products of aromatic hydrocarbons contribute to secondary organic aerosol formation through uptake on pre-existing inorganic aerosols in the absence of liquid water films. Irradiations were conducted with toluene, p-xylene, and 1,3,5-trimethylbenzene in the presence of NO_x and ammonium sulfate aerosol, with propylene added to enhance the production of radicals in the system. The secondary organic aerosol yields were determined by dividing the mass concentration of organic fraction of the aerosol collected on quartz filters by the mass concentration of the aromatic hydrocarbon removed by reaction. The mass concentration of the organic fraction was obtained by multiplying the measured organic carbon concentration by 2.0, a correction factor that takes into account the presence of hydrogen, nitrogen, and oxygen atoms in the organic species. The mass concentrations of ammonium, nitrate, and sulfate concentrations as well as the total mass of the aerosols were measured. A reasonable mass balance was found for each of the aerosols. The largest secondary organic aerosol yield of 1.59±0.40% was found for toluene at an organic aerosol concentration of 8.2 μm⁻³, followed by 1.09±0.27% for p-xylene at 6.4 μg m⁻³, and 0.41±0.10% for 1,3,5-trimethylbenzene at 2.0 μg m⁻³. In general, these results agree with those reported by Odum et al. and appear to be consistent with the gas–aerosol partitioning theory developed by Pankow. The presence of organic in the aerosol did not affect significantly the hygroscopic properties of the aerosol.     

Source apportionment of primary and secondary organic aerosols using positive matrix factorization (PMF) of molecular markers

Monthly average ambient concentrations of more than eighty particle-phase organic compounds, as well as total organic carbon (OC) and elemental carbon (EC), were measured from March 2004 through February 2005 in five cities in the Midwestern United States. A multi-variant source apportionment receptor model, positive matrix factorization (PMF), was applied to explore the average source contributions to the five sampling sites using molecular markers for primary and secondary organic aerosols (POA, SOA). Using the molecular makers in the model, POA and SOA were estimated for each month at each site. Three POA factors were derived, which were dominated by primary molecular markers such as EC, hopanes, steranes, and polycyclic aromatic hydrocarbons (PAHs), and which represented the following POA sources: urban primary sources, mobile sources, and other combustion sources. The three POA sources accounted for 57% of total average ambient OC. Three factors, characterized by the presence of reaction products of isoprene, α-pinene and β-caryophyllene, and displaying distinct seasonal trends, were consistent with the characteristics of SOA. The SOA factors made up 43% of the total average measured OC. The PMF-derived results are in good agreement with estimated SOA concentrations obtained from SOA to tracer yield estimates obtained from smog chamber experiments. A linear regression comparing the smog chamber yield estimates and the PMF SOA contributions had a regression slope of 1.01 ± 0.07 and an intercept of 0.19 ± 0.10 μg OC m^?3 (adjusted R² of 0.763, n = 58).     

Insights into the primary–secondary and regional–local contributions to organic aerosol and PM2.5 mass in Pittsburgh,Pennsylvania

This paper presents chemical mass balance (CMB) analysis of organic molecular marker data to investigate the sources of organic aerosol and PM_2.5 mass in Pittsburgh, Pennsylvania. The model accounts for emissions from eight primary source classes, including major anthropogenic sources such as motor vehicles, cooking, and biomass combustion as well as some primary biogenic emissions (leaf abrasion products). We consider uncertainty associated with selection of source profiles, selection of fitting species, sampling artifacts, photochemical aging, and unknown sources. In the context of the overall organic carbon (OC) mass balance, the contributions of diesel, wood-smoke, vegetative detritus, road dust, and coke-oven emissions are all small and well constrained; however, estimates for the contributions of gasoline-vehicle and cooking emissions can vary by an order of magnitude. A best-estimate solution is presented that represents the vast majority of our CMB results; it indicates that primary OC only contributes 27±8% and 50±14% (average±standard deviation of daily estimates) of the ambient OC in the summer and winter, respectively. Approximately two-thirds of the primary OC is transported into Pittsburgh as part of the regional air mass. The ambient OC that is not apportioned by the CMB model is well correlated with secondary organic aerosol (SOA) estimates based on the EC-tracer method and ambient concentrations of organic species associated with SOA. Therefore, SOA appears to be the major component of OC, not only in summer, but potentially in all seasons. Primary OC dominates the OC mass balance on a small number of nonsummer days with high OC concentrations; these events are associated with specific meteorological conditions such as local inversions. Primary particulate emissions only contribute a small fraction of the ambient fine-particle mass, especially in the summer.     

Gas phase precursors to anthropogenic secondary organic aerosol: Using the Master Chemical Mechanism to probe detailed observations of 1,3,5-trimethylbenzene photo-oxidation

A detailed gas-phase photochemical chamber box model, incorporating the Master Chemical Mechanism (MCMv3.1) degradation scheme for the model anthropogenic aromatic compound 1,3,5-trimethylbenzene, has been used to simulate data measured during a series of aerosol chamber experiments in order to evaluate the mechanism under a variety of VOC/NO_x conditions.The chamber model was used in the interpretation of comprehensive high (mass and time) resolution measurements of 1,3,5-trimethylbenzene and its photo-oxidation products recorded by a Chemical Ionisation Reaction Time-of-Flight Mass Spectrometer (CIR-TOF-MS). Supporting gas and aerosol measurements have also enabled us to explore the ‘missing link’ between the gas and aerosol phases. Model-measurement comparisons have been used to gain insight into the complex array of oxygenated products formed, including the peroxide bicyclic ring opening products (α,β-unsaturated-γ-dicarbonyls and furanones) and the O₂-bridged peroxide bicyclic ring-retaining products. To our knowledge this is the first time such high molecular weight species, corresponding to various peroxide bicyclic products represented in the MCMv3.1, have been observed in the gas-phase. The model was also used to give insight into which gas-phase species were participating in SOA formation, with the primary and secondary peroxide products, formed primarily under low NO_x conditions, identified as likely candidates.     

Secondary organic aerosol formation from the irradiation of simulated automobile exhaust

A laboratory study was conducted to evaluate the potential for secondary organic aerosol formation from emissions from automotive exhaust. The goal was to determine to what extent photochemical oxidation products of these hydrocarbons contribute to secondary organic aerosol (SOA) and how well their formation is described by recently developed models for SOA formation. The quality of a surrogate was tested by comparing its reactivity with that from irradiations of authentic automobile exhaust. Experiments for secondary particle formation using the surrogate were conducted in a fixed volume reactor operated in a dynamic mode. The mass concentration of the aerosol was determined from measurements of organic carbon collected on quartz filters and was corrected for the presence of hydrogen, nitrogen, and oxygen atoms in the organic species. A functional group analysis of the aerosol made by Fourier transform infrared (FTIR) spectroscopy indicated     

Estimates of heterogeneous formation of secondary organic aerosol during a wood smoke episode in Houston,Texas

Observational data, collected during a wood smoke episode in Houston, Texas, were used to assess the extent to which acid-catalyzed reactions of carbonyls might contribute to secondary organic aerosol (SOA) formation. The wood smoke episode was chosen for this analysis because of relatively high concentrations of acidic aerosol, coupled with high concentrations of SOA precursors during the episode. Photochemical modeling, coupled with ambient measurements, indicated that acid aerosol-mediated organic aerosol formation reactions, not accounted for in most current photochemical models, may have led to SOA formation of up to a few μg m⁻³. In photochemical simulations, acid-mediated organic aerosol formation was modeled by calculating the rate of impingement of aldehyde molecules on acidic particles, and then assuming that a fraction of the impingements resulted in reaction. For reaction probabilities on the order of 0.005–0.0005, the model predicted SOA concentrations were consistent with estimates of SOA based on observations. In addition, observed concentrations of particulate phase ammonium during the episode were consistent with high concentrations of the types of organic acids that would be formed through acid-catalyzed reactions of carbonyls. Although there are substantial uncertainties in the estimates of heterogeneous SOA formation, collectively, these data and modeling analyses provide evidence for the importance of acid-catalyzed SOA formation reactions.     

A kinetic mechanism for predicting secondary organic aerosol formation from toluene oxidation in the presence of NOx and natural sunlight

A kinetic mechanism to predict secondary organic aerosol (SOA) formation from the photo-oxidation of toluene was developed. Aerosol phase chemistry that includes nucleation, gas–particle partitioning and particle-phase reactions as well as the gas-phase chemistry of toluene and its degradation products were represented. The mechanism was evaluated against experimental data obtained from the University of North Carolina (UNC) 270 m³ dual outdoor aerosol smog chamber facility. The model adequately simulates the decay of toluene, the nitric oxide (NO) to nitrogen dioxide (NO₂) conversion and ozone formation. It also provides a reasonable prediction of SOA production under different conditions that range from 15 to 300 μg m⁻³. Speciation of simulated aerosol material shows that up to 70% of the aerosol mass comes from oligomers and polymers depending on initial reactant concentrations. The dominant particle-phase species predicted by the mechanism are glyoxal oligomers, ketene oligomers from the photolysis of the toluene OH reaction product 2-methyl-2,4-hexadienedial, organic nitrates, methyl nitro-phenol analogues, C7 organic peroxides, acylperoxy nitrates and for the low-concentration experiments, unsaturated hydroxy nitro acids.     

Chemical characteristics of organic aerosols in Algiers city area: influence of a fat manufacture plant

Total concentrations and homologue distributions of organic fraction constituents have been determined in particulate matter emitted from different units of a fat manufacturer (i.e. oils refining and conditioning plants, and production and conditioning units of a soap industry) located in Algiers area, as well as in atmospheric aerosols. In particular n-alkanes, n-alkanoic and n-alkenoic acids, n-alkan-2-ones and polycyclic aromatic hydrocarbons (PAH) were investigated. Organic aerosol contents varied broadly among the plant units, depending upon nature of the manufactured products. The percent composition of all classes of compounds investigated in ambient atmosphere was similar to those observed indoor at industrial plant units. Organic acids, n-alkanoic as well as n-alkenoic, appeared by far the most abundant organic constituents of aerosols, both indoor and outdoor, ranging from 7.7 to 19.8 and from 12.7 to 17.1 μg m⁻³, respectively. The huge occurrence of acids and n-alkanes in ambient aerosols was consistent with their high levels present in oil and fat materials. Among minor components of aerosols, n-alkan-2-ones and PAH, seemed to be related to thermally induced ageing and direct combustion of raw organic material used for oil and soap production.     

Temperature dependence of secondary organic aerosol

Temperature was found to have a dramatic effect on secondary organic aerosol formation from two ozonolysis systems, cyclohexene and α-pinene. Isothermal experiments were conducted for both systems where the lowest temperature, 278 K, formed approximately 2.5–3 times and 5–6 times the SOA formed at 300 K and 318 K, respectively. Changing the cyclohexene system temperature to a different isothermal experimental set point after completion of SOA formation did not lead to sufficient condensation/evaporation to reproduce the SOA formation at other temperature set points. When the system temperature was cycled between two set points at the end of an experiment, the α-pinene system showed reversibility between the initial temperature 318 K and 300 K. For temperature cycles between the initial temperature of 300 K–318 K, an irreversible loss of mass is observed after the first heating cycle with reversibility observed between subsequent temperature cycles. The SOA formed at 278 K was reversible over a 22 K range but was unable to evaporate sufficiently to match the SOA mass formed at 300 K. Hygroscopicity measurements, taken after the completion of SOA formation, indicate that hygroscopicity of the aerosol is also a function of temperature and that the aerosol does not continue to be oxidized after initial growth is complete. The differing hygroscopicity of the semi-volatile component of the aerosol is evident during system temperature changes after completion of the experiment.     

Evaluation of the UNC toluene-SOA mechanism with respect to other chamber studies and key model parameters

In a companion paper by Hu et al. [2007. A kinetic mechanism for predicting secondary organic aerosol formation from toluene oxidation in the presence of NO_x and natural sunlight. Atmospheric Environment, doi:10.1016/j.atmosenv.2007.04.025], a kinetic mechanism was developed from data generated in the University of North Carolina's (UNC) 270 m³ dual outdoor aerosol smog chamber, to predict secondary organic aerosol (SOA) formation from toluene oxidation in the atmosphere. In this paper, experimental data sets from European Photoreactor (EUPHORE), smog chambers at the California Institute of Technology (Caltech), and the UNC 300 m³ dual-outdoor gas phase chamber were used to evaluate the toluene mechanism. The model simulates SOA formation for the ‘low-NO_x’ and ‘mid-NO_x’ experiments from EUPHORE chambers reasonably well, but over-predicts SOA mass concentrations for the ‘high-NO_x’ run. The model well simulates the SOA mass concentrations observed from the Caltech chambers. Experiments with the three key toluene products, 1,4-butenedial, 4-oxo-2-pentenal and o-cresol in the presence of oxides of nitrogen (NO_x) are also simulated by the developed mechanism. The model well predicts the NO_x time–concentration profiles and the decay of these two carbonyls, but underestimates ozone (O₃) formation for 4-oxo-2-pentenal. It well simulates SOA formation from 1,4-butenedial but overestimates (possibly due to experimental problems) the measured aerosol mass concentrations from 4-oxo-2-pentenal. The model underestimates SOA production from o-cresol, mostly due to its under-prediction of o-cresol decay. The effects of varying temperature, relative humidity, glyoxal uptake, organic nitrate yields, and background seed aerosol concentrations, were also investigated.     

Oxidation of ketone groups in transported biomass burning aerosol from the 2008 Northern California Lightning Series fires

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.     

Investigating the chemical nature of humic-like substances (HULIS) in North American atmospheric aerosols by liquid chromatography tandem mass spectrometry

The high-molecular weight water-soluble organic compounds present in atmospheric aerosols underwent functional-group characterization using liquid chromatography tandem mass spectrometry (LC-MS/MS), with a focus on understanding the chemical structure and origins of humic-like substances (HULIS) in the atmosphere. Aerosol samples were obtained from several locations in North America at times when primary sources contributing to organic aerosol were well-characterized: Riverside, CA, Fresno, CA, urban and peripheral Mexico City, Atlanta, GA, and Bondville, IL. Chemical analysis targeted identification and quantification of functional groups, such as aliphatic, aromatic, and bulk carboxylic acids, organosulfates, and carbohydrate-like substances that comprise species with molecular weights (MW) 200–600 amu. Measured high-MW functional groups were compared to modeled primary sources with the purpose of identifying associations between aerosol sources, high-MW aerosol species, and HULIS. Mobile source emissions were linked to high-molecular weight carboxylic acids, especially aromatic acids, biomass burning was associated with carboxylic acids and carbohydrate-like substances, and secondary organic aerosol (SOA) correlated well with the total amount of HULIS measured, whereas organosulfates showed no correlation with aerosol sources and exhibited unique spatial trends. These results suggested the importance of motor vehicles, biomass burning, and SOA as important sources of precursors to HULIS. Structural characteristics of atmospheric HULIS were compared to terrestrial humic and fulvic acids and revealed striking similarities in chemical structure, with the exception of organosulfates which were unique to atmospheric HULIS.     

Characterization and source presumption of wintertime submicron organic aerosols at Saitama,Japan, using the Aerodyne aerosol mass spectrometer

The chemical composition and size distribution of submicron aerosols were analyzed at a suburban site at Saitama, Japan, in the winter of 2004/2005, using an Aerodyne aerosol mass spectrometer. Although organics and nitrate were the dominant species during the sampling period, a large fraction of sulfate was observed at the accumulation mode when mass loading was low and wind speed was high. The size distributions of m/z 44 (mostly CO₂⁺) and sulfate aerosols during periods of high wind speed showed remarkable similarities in the accumulation mode, indicating that oxygenated organics were aged aerosols and internally mixed with sulfate. Ozone concentrations were also increased during these high wind speed periods although nighttime (e.g., 12/17 2004), indicating that the oxygenated compounds were strongly influenced by transported and aged air masses. The diurnal profiles of ultrafine-mode organics and hydrocarbon-like organic aerosols (HOA) were similar to NO_X derived from traffic and other combustion sources. The temporal variation of oxygenated organic aerosols (OOA) agreed well with that of nitrate as a secondary aerosol tracer, and the diurnal profile of the OOA fraction of organics increased during the day associated with higher UV light intensity. The result of time and size-resolved chemical composition of submicron particles indicated that the OOA is associated with both photochemical activity and transboundary pollution, and ultrafine-mode organic and HOA aerosols are mainly associated with combustion sources.     

Carbonyl atmospheric reaction products of aromatic hydrocarbons in ambient air

To convert gaseous carbonyls to oximes during sampling, an XAD-4 resin denuder system pre-coated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine and followed by analysis with methane positive chemical ionization gas chromatography/mass spectrometry was used to measure carbonyls in ambient air samples in Riverside, CA. In conjunction with similar analyses of environmental chamber OH radical-initiated reactions of o- and p-xylene, 1,2,4-trimethylbenzene, ethylbenzene, 4-hydroxy-2-butanone and 1,4-butanediol, we identified benzaldehyde, o-, m- and p-tolualdehyde and acetophenone and the dicarbonyls glyoxal, methylglyoxal, biacetyl, ethylglyoxal, 1,4-butenedial, 3-hexene-2,5-dione, 3-oxo-butanal, 1,4-butanedial and malonaldehyde in the ambient air samples. As discussed, these carbonyls and dicarbonyls can be formed from the OH radical-initiated reactions of aromatic hydrocarbons and other volatile organic compounds emitted into the atmosphere, and we conclude that in situ atmospheric formation is a major source of these carbonyls in our Riverside, CA, ambient air samples.     

Aerosol speciation and mass prediction from toluene oxidation under high NOx conditions

A kinetically based gas-particle partitioning box model is used to highlight the importance of parameter representation in the prediction of secondary organic aerosol (SOA) formation following the photo-oxidation of toluene. The model is initialized using experimental data from York University's indoor smog chamber and provides a prediction of the total aerosol yield and speciation. A series of model sensitivity experiments were performed to study the aerosol speciation and mass prediction under high NO_x conditions (VOC/NO_x = 0.2). Sensitivity experiments indicate vapour pressure estimation to be a large area of weakness in predicting aerosol mass, creating an average total error range of 70 μg m^?3 (range of 5–145 μg m^?3), using two different estimation methods. Aerosol speciation proved relatively insensitive to changes in vapour pressure. One species, 3-methyl-6-nitro-catechol, dominated the aerosol phase regardless of the vapour pressure parameterization used and comprised 73–88% of the aerosol by mass. The dominance is associated with the large concentration of 3-methyl-6-nitro-catechol in the gas-phase. The high NO_x initial conditions of this study suggests that the predominance of 3-methyl-6-nitro-catechol likely results from the cresol-forming branch in the Master Chemical Mechanism taking a significant role in secondary organic aerosol formation under high NO_x conditions. Further research into the yields and speciation leading to this reaction product is recommended.