Effect of oxidant concentration, exposure time, and seed particles on secondary organic aerosol chemical composition and yield

Abstract. We performed a systematic intercomparison study of the chemistry and yields of secondary organic aerosol (SOA) generated from OH oxidation of a common set of gas-phase precursors in a Potential Aerosol Mass (PAM) continuous flow reactor and several environmental chambers. In the flow reactor, SOA precursors were oxidized using OH concentrations ranging from 2.0 × 108 to 2.2 × 1010 molec cm−3 over exposure times of 100 s. In the environmental chambers, precursors were oxidized using OH concentrations ranging from 2 × 106 to 2 × 107 molec cm−3 over exposure times of several hours. The OH concentration in the chamber experiments is close to that found in the atmosphere, but the integrated OH exposure in the flow reactor can simulate atmospheric exposure times of multiple days compared to chamber exposure times of only a day or so. In most cases, for a specific SOA type the most-oxidized chamber SOA and the least-oxidized flow reactor SOA have similar mass spectra, oxygen-to-carbon and hydrogen-to-carbon ratios, and carbon oxidation states at integrated OH exposures between approximately 1 × 1011 and 2 × 1011 molec cm−3 s, or about 1–2 days of equivalent atmospheric oxidation. This observation suggests that in the range of available OH exposure overlap for the flow reactor and chambers, SOA elemental composition as measured by an aerosol mass spectrometer is similar whether the precursor is exposed to low OH concentrations over long exposure times or high OH concentrations over short exposure times. This similarity in turn suggests that both in the flow reactor and in chambers, SOA chemical composition at low OH exposure is governed primarily by gas-phase OH oxidation of the precursors rather than heterogeneous oxidation of the condensed particles. In general, SOA yields measured in the flow reactor are lower than measured in chambers for the range of equivalent OH exposures that can be measured in both the flow reactor and chambers. The influence of sulfate seed particles on isoprene SOA yield measurements was examined in the flow reactor. The studies show that seed particles increase the yield of SOA produced in flow reactors by a factor of 3 to 5 and may also account in part for higher SOA yields obtained in the chambers, where seed particles are routinely used.

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Comparison of secondary organic aerosol formed with an aerosol flow reactor and environmental reaction chambers: effect of oxidant concentration, exposure time and seed particles on chemical composition and yield

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-14-30575-2014 ◽

2014 ◽

Vol 14 (22) ◽

pp. 30575-30609 ◽

Cited By ~ 1

Author(s):

A. T. Lambe ◽

P. S. Chhabra ◽

T. B. Onasch ◽

W. H. Brune ◽

J. F. Hunter ◽

...

Keyword(s):

Chemical Composition ◽

Gas Phase ◽

Flow Reactor ◽

Heterogeneous Oxidation ◽

Flow Reactors ◽

Seed Particles ◽

Aerosol Mass ◽

Reaction Chambers ◽

Environmental Chambers ◽

Intercomparison Study

Abstract. We performed a systematic intercomparison study of the chemistry and yields of SOA generated from OH oxidation of a common set of gas-phase precursors in a Potential Aerosol Mass (PAM) continuous flow reactor and several environmental chambers. In the flow reactor, SOA precursors were oxidized using OH concentrations ranging from 2.0×108 to 2.2&times1010 molec cm−3 over exposure times of 100 s. In the environmental chambers, precursors were oxidized using OH concentrations ranging from 2×106 to 2×107 molec cm−3 over exposure times of several hours. The OH concentration in the chamber experiments is close to that found in the atmosphere, but the integrated OH exposure in the flow reactor can simulate atmospheric exposure times of multiple days compared to chamber exposure times of only a day or so. A linear correlation analysis of the mass spectra (m=0.91–0.92, r2=0.93–0.94) and carbon oxidation state (m=1.1, r2=0.58) of SOA produced in the flow reactor and environmental chambers for OH exposures of approximately 1011 molec cm−3 s suggests that the composition of SOA produced in the flow reactor and chambers is the same within experimental accuracy as measured with an aerosol mass spectrometer. This similarity in turn suggests that both in the flow reactor and in chambers, SOA chemical composition at low OH exposure is governed primarily by gas-phase OH oxidation of the precursors, rather than heterogeneous oxidation of the condensed particles. In general, SOA yields measured in the flow reactor are lower than measured in chambers for the range of equivalent OH exposures that can be measured in both the flow reactor and chambers. The influence of sulfate seed particles on isoprene SOA yield measurements was examined in the flow reactor. The studies show that seed particles increase the yield of SOA produced in flow reactors by a factor of 3 to 5 and may also account in part for higher SOA yields obtained in the chambers, where seed particles are routinely used.

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Investigating the use of secondary organic aerosol as seed particles in simulation chamber experiments

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-5917-2011 ◽

2011 ◽

Vol 11 (12) ◽

pp. 5917-5929 ◽

Cited By ~ 34

Author(s):

J. F. Hamilton ◽

M. Rami Alfarra ◽

K. P. Wyche ◽

M. W. Ward ◽

A. C. Lewis ◽

...

Keyword(s):

Mass Spectrometry ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Oxidation Products ◽

Ion Cyclotron Resonance ◽

Photo Oxidation ◽

Seed Particles ◽

Aerosol Mass ◽

Seed Aerosol ◽

Limonene Oxidation

Abstract. The use of β-caryophyllene secondary organic aerosol particles as seeds for smog chamber simulations has been investigated. A series of experiments were carried out in the Manchester photochemical chamber as part of the Aerosol Coupling in the Earth System (ACES) project to study the effect of seed particles on the formation of secondary organic aerosol (SOA) from limonene photo-oxidation. Rather than use a conventional seed aerosol containing ammonium sulfate or diesel particles, a method was developed to use in-situ chamber generated seed particles from β-caryophyllene photo-oxidation, which were then diluted to a desired mass loading (in this case 4–13 μg m−3). Limonene was then introduced into the chamber and oxidised, with the formation of SOA seen as a growth in the size of oxidised organic seed particles from 150 to 325 nm mean diameter. The effect of the partitioning of limonene oxidation products onto the seed aerosol was assessed using aerosol mass spectrometry during the experiment and the percentage of m/z 44, an indicator of degree of oxidation, increased from around 5 to 8 %. The hygroscopicity of the aerosol also changed, with the growth factor for 200 nm particles increasing from less than 1.05 to 1.25 at 90 % RH. The detailed chemical composition of the limonene SOA could be extracted from the complex β-caryophyllene matrix using two-dimensional gas chromatography (GC × GC) and liquid chromatography coupled to mass spectrometry. High resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS) was used to determine exact molecular formulae of the seed and the limonene modified aerosol. The average O:C ratio was seen to increase from 0.32 to 0.37 after limonene oxidation products had condensed onto the organic seed.

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Secondary organic aerosol formation from photooxidation of naphthalene and alkylnaphthalenes: implications for oxidation of intermediate volatility organic compounds (IVOCs)

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-9-1873-2009 ◽

2009 ◽

Vol 9 (1) ◽

pp. 1873-1905

Author(s):

A. W. H. Chan ◽

K. E. Kautzman ◽

P. S. Chhabra ◽

J. D. Surratt ◽

M. N. Chan ◽

...

Keyword(s):

Polycyclic Aromatic Hydrocarbons ◽

Gas Phase ◽

Aromatic Hydrocarbons ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Aerosol Formation ◽

Atmospheric Models ◽

Gas Phase Reactions ◽

Aerosol Mass ◽

Polycyclic Aromatic

Abstract. Current atmospheric models do not include secondary organic aerosol (SOA) production from gas-phase reactions of polycyclic aromatic hydrocarbons (PAHs). Recent studies have shown that primary semivolatile emissions, previously assumed to be inert, undergo oxidation in the gas phase, leading to SOA formation. This opens the possibility that low-volatility gas-phase precursors are a potentially large source of SOA. In this work, SOA formation from gas-phase photooxidation of naphthalene, 1-methylnaphthalene (1-MN), 2-methylnaphthalene (2-MN), and 1,2-dimethylnaphthalene (1,2-DMN) is studied in the Caltech dual 28-m3 chambers. Under high-NOx conditions and aerosol mass loadings between 10 and 40 μg m

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Investigating the use of secondary organic aerosol as seed particles in simulation chamber experiments

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-25117-2010 ◽

2010 ◽

Vol 10 (10) ◽

pp. 25117-25151

Author(s):

J. F. Hamilton ◽

M. Rami Alfarra ◽

K. P. Wyche ◽

M. W. Ward ◽

A. C. Lewis ◽

...

Keyword(s):

Mass Spectrometry ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Oxidation Products ◽

Ion Cyclotron Resonance ◽

Photo Oxidation ◽

Seed Particles ◽

Aerosol Mass ◽

Seed Aerosol ◽

Limonene Oxidation

Abstract. The use of β-caryophyllene secondary organic aerosol particles as seeds for smog chamber simulations has been investigated. A series of experiments were carried out in the Manchester photochemical chamber as part of the Aerosol Coupling in the Earth System (ACES) project to study the effect of seed particles on the formation of secondary organic aerosol (SOA) from limonene photo-oxidation. Rather than use a conventional seed aerosol containing ammonium sulphate or diesel particles, a method was developed to use in situ chamber generated seed particles from β-caryophyllene photo-oxidation, which were then diluted to a desired mass loading (in this case 4–13 μg m-3). Limonene was then introduced into the chamber and oxidised, with the formation of SOA seen as a growth in the size of oxidised organic seed particles from 150 to 325 nm mean diameter. The effect of the partitioning of limonene oxidation products onto the seed aerosol was assessed using aerosol mass spectrometry during the experiment and the percentage of m/z 44, an indicator of degree of oxidation, increased from around 5 to 8%. The hygroscopicity of the aerosol also changed, with the growth factor for 200 nm particles increasing from less than 1.05 to 1.25 at 90% RH. The detailed chemical composition of the limonene SOA could be extracted from the complex β-caryophyllene matrix using two-dimensional gas chromatography (GC×GC) and liquid chromatography coupled to mass spectrometry. High resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS) was used to determine exact molecular formulae of the seed and the limonene modified aerosol. The average O:C ratio was seen to increase from 0.32 to 0.37 after limonene oxidation products had condensed onto the organic seed.

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Temperature dependence of secondary organic aerosol yield from the ozonolysis of β-pinene

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-6-10275-2006 ◽

2006 ◽

Vol 6 (5) ◽

pp. 10275-10297 ◽

Cited By ~ 2

Author(s):

C. Stenby ◽

U. Pöschl ◽

P. von Hessberg ◽

M. Bilde ◽

O. J. Nielsen ◽

...

Keyword(s):

Temperature Dependence ◽

Gas Phase ◽

Flow Reactor ◽

Secondary Organic Aerosol ◽

Measurement Data ◽

Organic Aerosol ◽

Reaction Products ◽

Aerosol Formation ◽

Product Model ◽

High Particle

Abstract. The temperature dependence of secondary organic aerosol (SOA) formation from ozonolysis of β-pinene was studied in a flow reactor at 263–303 K and 1007 hPa. The observed SOA yields were of similar magnitude as predicted by a two-product model based on detailed gas phase chemistry (Jenkin, 2004), reaching maximum values of 0.22–0.39 at high particle mass concentrations. However, the measurement data exhibited significant deviations (up to 50%) from the predicted linear dependence on inverse temperature. When fitting the measurement data with a two-product model, we found that both the partitioning coefficients (Kom,i) and the stoichiometric yields (αi) of the low-volatile and semi-volatile species vary with temperature. The results indicate that not only the reaction product vapour pressures but also the relative contributions of different gas-phase or multiphase reaction channels are dependent on temperature. We suggest that the modelling of secondary organic aerosol formation in the atmosphere needs to take into account the effects of temperature on the pathways and kinetics of the involved chemical reactions as well as on the gas-particle partitioning of the reaction products.

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Comparison of secondary organic aerosol formation from toluene on initially wet and dry ammonium sulfate particles at moderate relative humidity

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-5677-2018 ◽

2018 ◽

Vol 18 (8) ◽

pp. 5677-5689 ◽

Cited By ~ 10

Author(s):

Tengyu Liu ◽

Dan Dan Huang ◽

Zijun Li ◽

Qianyun Liu ◽

ManNin Chan ◽

...

Keyword(s):

Relative Humidity ◽

Ammonium Sulfate ◽

Functional Groups ◽

Flow Reactor ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Aerosol Formation ◽

Seed Particles ◽

The Difference ◽

Wet Particles

Abstract. The formation of secondary organic aerosol (SOA) has been widely studied in the presence of dry seed particles at low relative humidity (RH). At higher RH, initially dry seed particles can exist as wet particles due to water uptake by the seeds as well as the SOA. Here, we investigated the formation of SOA from the photooxidation of toluene using an oxidation flow reactor in the absence of NOx under a range of OH exposures on initially wet or dry ammonium sulfate (AS) seed particles at an RH of 68 %. The ratio of the SOA yield on wet AS seeds to that on dry AS seeds, the relative SOA yield, decreased from 1.31 ± 0.02 at an OH exposure of 4.66 × 1010 molecules cm−3 s to 1.01 ± 0.01 at an OH exposure of 5.28 × 1011 molecules cm−3 s. This decrease may be due to the early deliquescence of initially dry AS seeds after being coated by highly oxidized toluene-derived SOA. SOA formation lowered the deliquescence RH of AS and resulted in the uptake of water by both AS and SOA. Hence the initially dry AS seeds contained aerosol liquid water (ALW) soon after SOA formed, and the SOA yield and ALW approached those of the initially wet AS seeds as OH exposure and ALW increased, especially at high OH exposure. However, a higher oxidation state of the SOA on initially wet AS seeds than that on dry AS seeds was observed at all levels of OH exposure. The difference in mass fractions of m ∕ z 29, 43 and 44 of SOA mass spectra, obtained using an aerosol mass spectrometer (AMS), indicated that SOA formed on initially wet seeds may be enriched in earlier-generation products containing carbonyl functional groups at low OH exposures and later-generation products containing acidic functional groups at high exposures. Our results suggest that inorganic dry seeds become at least partially deliquesced particles during SOA formation and hence that ALW is inevitably involved in the SOA formation at moderate RH. More laboratory experiments conducted with a wide variety of SOA precursors and inorganic seeds under different NOx and RH conditions are warranted.

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Gas-phase composition and secondary organic aerosol formation from standard and particle filter-retrofitted gasoline direct injection vehicles investigated in a batch and flow reactor

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-9929-2018 ◽

2018 ◽

Vol 18 (13) ◽

pp. 9929-9954 ◽

Cited By ~ 20

Author(s):

Simone M. Pieber ◽

Nivedita K. Kumar ◽

Felix Klein ◽

Pierre Comte ◽

Deepika Bhattu ◽

...

Keyword(s):

Mass Spectrometry ◽

Flow Reactor ◽

Secondary Organic Aerosol ◽

Matrix Effects ◽

Direct Injection ◽

Organic Aerosol ◽

Gasoline Direct Injection ◽

Particulate Filter ◽

Aerosol Formation ◽

Aerosol Mass

Abstract. Gasoline direct injection (GDI) vehicles have recently been identified as a significant source of carbonaceous aerosol, of both primary and secondary origin. Here we investigated primary emissions and secondary organic aerosol (SOA) from four GDI vehicles, two of which were also retrofitted with a prototype gasoline particulate filter (GPF). We studied two driving test cycles under cold- and hot-engine conditions. Emissions were characterized by proton transfer reaction time-of-flight mass spectrometry (gaseous non-methane organic compounds, NMOCs), aerosol mass spectrometry (sub-micron non-refractory particles) and light attenuation measurements (equivalent black carbon (eBC) determination using Aethalometers) together with supporting instrumentation. Atmospheric processing was simulated using the PSI mobile smog chamber (SC) and the potential aerosol mass oxidation flow reactor (OFR). Overall, primary and secondary particulate matter (PM) and NMOC emissions were dominated by the engine cold start, i.e., before thermal activation of the catalytic after-treatment system. Trends in the SOA oxygen to carbon ratio (O : C) for OFR and SC were related to different OH exposures, but divergences in the H : C remained unexplained. SOA yields agreed within experimental variability between the two systems, with a tendency for higher values in the OFR than in the SC (or, vice versa, lower values in the SC). A few aromatic compounds dominated the NMOC emissions, primarily benzene, toluene, xylene isomers/ethylbenzene and C3-benzene. A significant fraction of the SOA was explained by those compounds, based on comparison of effective SOA yield curves with those of toluene, o-xylene and 1,2,4-trimethylbenzene determined in our OFR, as well as others from literature. Remaining discrepancies, which were smaller in the SC and larger in the OFR, were up to a factor of 2 and may have resulted from diverse reasons including unaccounted precursors and matrix effects. GPF retrofitting significantly reduced primary PM through removal of refractory eBC and partially removed the minor POA fraction. At cold-started conditions it did not affect hydrocarbon emission factors, relative chemical composition of NMOCs or SOA formation, and likewise SOA yields and bulk composition remained unaffected. GPF-induced effects at hot-engine conditions deserve attention in further studies.

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Effect of NO<sub>x</sub> level on secondary organic aerosol (SOA) formation from the photooxidation of terpenes

Atmospheric Chemistry and Physics ◽

10.5194/acp-7-5159-2007 ◽

2007 ◽

Vol 7 (19) ◽

pp. 5159-5174 ◽

Cited By ~ 297

Author(s):

N. L. Ng ◽

P. S. Chhabra ◽

A. W. H. Chan ◽

J. D. Surratt ◽

J. H. Kroll ◽

...

Keyword(s):

Chemical Composition ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Organic Nitrates ◽

Biogenic Hydrocarbons ◽

Alkoxy Radicals ◽

Volatile Products ◽

Products Analysis ◽

Environmental Chambers ◽

Aerosol Chemical Composition

Abstract. Secondary organic aerosol (SOA) formation from the photooxidation of one monoterpene (α-pinene) and two sesquiterpenes (longifolene and aromadendrene) is investigated in the Caltech environmental chambers. The effect of NOx on SOA formation for these biogenic hydrocarbons is evaluated by performing photooxidation experiments under varying NOx conditions. The NOx dependence of α-pinene SOA formation follows the same trend as that observed previously for a number of SOA precursors, including isoprene, in which SOA yield (defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) decreases as NOx level increases. The NOx dependence of SOA yield for the sesquiterpenes, longifolene and aromadendrene, however, differs from that determined for isoprene and α-pinene; the aerosol yield under high-NOx conditions substantially exceeds that under low-NOx conditions. The reversal of the NOx dependence of SOA formation for the sesquiterpenes is consistent with formation of relatively low-volatility organic nitrates, and/or the isomerization of large alkoxy radicals leading to less volatile products. Analysis of the aerosol chemical composition for longifolene confirms the presence of organic nitrates under high-NOx conditions. Consequently the formation of SOA from certain biogenic hydrocarbons such as sesquiterpenes (and possibly large anthropogenic hydrocarbons as well) may be more efficient in polluted air.

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Laboratory studies of the chemical composition and cloud condensation nuclei (CCN) activity of secondary organic aerosol (SOA) and oxidized primary organic aerosol (OPOA)

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-8913-2011 ◽

2011 ◽

Vol 11 (17) ◽

pp. 8913-8928 ◽

Cited By ~ 203

Author(s):

A. T. Lambe ◽

T. B. Onasch ◽

P. Massoli ◽

D. R. Croasdale ◽

J. P. Wright ◽

...

Keyword(s):

Flow Reactor ◽

Secondary Organic Aerosol ◽

Cloud Condensation Nuclei ◽

Oxidation Mechanism ◽

Organic Aerosol ◽

Anthropogenic Sources ◽

Oh Radicals ◽

Condensation Nuclei ◽

Aerosol Mass ◽

Ccn Activity

Abstract. Secondary organic aerosol (SOA) and oxidized primary organic aerosol (OPOA) were produced in laboratory experiments from the oxidation of fourteen precursors representing atmospherically relevant biogenic and anthropogenic sources. The SOA and OPOA particles were generated via controlled exposure of precursors to OH radicals and/or O3 in a Potential Aerosol Mass (PAM) flow reactor over timescales equivalent to 1–20 days of atmospheric aging. Aerosol mass spectra of SOA and OPOA were measured with an Aerodyne aerosol mass spectrometer (AMS). The fraction of AMS signal at m/z = 43 and m/z = 44 (f43, f44), the hydrogen-to-carbon (H/C) ratio, and the oxygen-to-carbon (O/C) ratio of the SOA and OPOA were obtained, which are commonly used to characterize the level of oxidation of oxygenated organic aerosol (OOA). The results show that PAM-generated SOA and OPOA can reproduce and extend the observed f44–f43 composition beyond that of ambient OOA as measured by an AMS. Van Krevelen diagrams showing H/C ratio as a function of O/C ratio suggest an oxidation mechanism involving formation of carboxylic acids concurrent with fragmentation of carbon-carbon bonds. Cloud condensation nuclei (CCN) activity of PAM-generated SOA and OPOA was measured as a function of OH exposure and characterized as a function of O/C ratio. CCN activity of the SOA and OPOA, which was characterized in the form of the hygroscopicity parameter κorg, ranged from 8.4×10−4 to 0.28 over measured O/C ratios ranging from 0.05 to 1.42. This range of κorg and O/C ratio is significantly wider than has been previously obtained. To first order, the κorg-to-O/C relationship is well represented by a linear function of the form κorg = (0.18±0.04) ×O/C + 0.03, suggesting that a simple, semi-empirical parameterization of OOA hygroscopicity and oxidation level can be defined for use in chemistry and climate models.

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Secondary organic aerosol formation from photooxidation of naphthalene and alkylnaphthalenes: implications for oxidation of intermediate volatility organic compounds (IVOCs)

Atmospheric Chemistry and Physics ◽

10.5194/acp-9-3049-2009 ◽

2009 ◽

Vol 9 (9) ◽

pp. 3049-3060 ◽

Cited By ~ 202

Author(s):

A. W. H. Chan ◽

K. E. Kautzman ◽

P. S. Chhabra ◽

J. D. Surratt ◽

M. N. Chan ◽

...

Keyword(s):

Gas Phase ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Diesel Emissions ◽

Aerosol Formation ◽

Gas Phase Reactions ◽

Large Source ◽

Aerosol Mass ◽

Polycyclic Aromatic ◽

Primary Emissions

Abstract. Current atmospheric models do not include secondary organic aerosol (SOA) production from gas-phase reactions of polycyclic aromatic hydrocarbons (PAHs). Recent studies have shown that primary emissions undergo oxidation in the gas phase, leading to SOA formation. This opens the possibility that low-volatility gas-phase precursors are a potentially large source of SOA. In this work, SOA formation from gas-phase photooxidation of naphthalene, 1-methylnaphthalene (1-MN), 2-methylnaphthalene (2-MN), and 1,2-dimethylnaphthalene (1,2-DMN) is studied in the Caltech dual 28-m3 chambers. Under high-NOx conditions and aerosol mass loadings between 10 and 40 μg m−3, the SOA yields (mass of SOA per mass of hydrocarbon reacted) ranged from 0.19 to 0.30 for naphthalene, 0.19 to 0.39 for 1-MN, 0.26 to 0.45 for 2-MN, and constant at 0.31 for 1,2-DMN. Under low-NOx conditions, the SOA yields were measured to be 0.73, 0.68, and 0.58, for naphthalene, 1-MN, and 2-MN, respectively. The SOA was observed to be semivolatile under high-NOx conditions and essentially nonvolatile under low-NOx conditions, owing to the higher fraction of ring-retaining products formed under low-NOx conditions. When applying these measured yields to estimate SOA formation from primary emissions of diesel engines and wood burning, PAHs are estimated to yield 3–5 times more SOA than light aromatic compounds over photooxidation timescales of less than 12 h. PAHs can also account for up to 54% of the total SOA from oxidation of diesel emissions, representing a potentially large source of urban SOA.

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