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Final Report: Secondary Aerosol Formation from Gas and Particle Phase Reactions of Aromatic Hydrocarbons

EPA Grant Number: R831084
Title: Secondary Aerosol Formation from Gas and Particle Phase Reactions of Aromatic Hydrocarbons
Investigators: Kamens, Richard M.
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Chung, Serena
Project Period: July 28, 2003 through July 27, 2006 (Extended to July 27, 2008)
Project Amount: $400,000
RFA: Measurement, Modeling, and Analysis Methods for Airborne Carbonaceous Fine Particulate Matter (PM2.5) (2003) RFA Text |  Recipients Lists
Research Category: Particulate Matter , Air Quality and Air Toxics , Air

Objective:

This project focuses on the elucidation of the fundamental chemistry that brings about the secondary organic aerosol formation (SOA) from aromatic reactions in the atmosphere. The overall goal is to develop a “new generation” aromatic chemical mechanism that can integrate newly discovered particle phase heterogeneous processes with the known gas phase chemistry, as a unified, multi-phase, chemical reaction mechanism, which will ultimately permit the prediction of SOA formation in the aromatic system.

Summary/Accomplishments (Outputs/Outcomes):

Model description, key assumptions: The toluene mechanism developed under this project was constructed to explicitly represent the formation of first generation gas-phase products from the photooxidation of toluene and the further reactions of these products with atmospheric oxidants. We followed the general approach outlined by Calvert et al. (2002) in their review of aromatic atmospheric chemistry. First generation products are defined as those resulting from initial oxidation of toluene by the hydroxyl (OH) or nitrate (NO3) radical, and include 1,4-butenedial, 4-oxo-2-pentenal, 2-methylbutenedial, 2-methyl-2,4-hexadienedial, methyl glyoxal, glyoxal, cresol and benzaldehyde. A simple illustration for the formation of 1st generation toluene gas phase products is shown in Figure 1. Semi-explicit mechanisms for these 1st generation products were developed and evaluated against outdoor chamber data for 1,4-butenedial, 4-oxo-2-pentenal and cresol (Liu et al., 1999; Johnson et al., 2004).  Many of the second generation products from the photo-degradation of these initial products have relatively low vapor pressures (10-2 to 10-7 torr), and thus can partition between the gas and aerosol phases. A kinetic partitioning approach suggested by Kamens et al. (1999; 2001) was used to describe the time-dependent phase distribution of about 70 compounds during the reaction. Of significance is that selected heterogeneous reactions of semi-volatile carbonyl compounds that lead to the formation of large molecules have also been represented either via calculations of uptake coefficients or by reductions in desorption rate coefficients. This will be discussed in more details later. Rate constants were developed by reference to the available literature (Calvert et al., 2002; Liggio et al., 2005b) and experiment data. Where mechanistic or rate constant information were not available, a structure activity relationship (SAR) technique was applied to calculate the gas-phase kinetic parameters (Kwok and Atkinson, 1995), or the mechanistic pathways and rate coefficients from similar compound structures were used (Jenkin et al., 1997; Saunders et al., 2003). The detailed mechanism is available in the supplemental material in Hu and Kamens et al (2007)  . In addition to the toluene mechanism, a Carbon Bond 4 (2002) mechanism is also included which incorporates the reactions of inorganic species (Voicu, 2003). A chamber dependent auxiliary wall mechanism that describes the background radical sources and sinks of NOx was also used (Jeffries et al., 1999; Voicu, 2003).

Figure 1. Illustration for the formation of 1st generation toluene gas phase products

Figure 1. Illustration for the formation of 1st generation toluene gas phase products
Performance criteria for the model and results: The toluene mechanism developed under this funding was evaluated against experimental data obtained from the University of North Carolina (UNC) 270 m3 dual outdoor aerosol smog chambers. 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 mg m-3. The experiments used to evaluate the mechanism are shown in Table 1. Example comparisons between experimental data and model simulations are shown in Figure 2. Additional simulations are given in Hu et al., 2007a and b. Fits that were within +5% of the data are called “very good” fits (or “fits very well”); fits that were within 15% are called “good” (or “tracks well”); and fits within  30 % are called “reasonable” (or “reasonable fits or tracks”).

Table 1. Experiment conditions used to develop and evaluate the toluene mechanism
 
 
Initial Concentration (ppm)
 
 
 
Experiment Date
[Tol]0
[NO]0
[NO2]0
[Tol]0/[NO]0
Temp (K)
Dew Point  (K)
052204Sa
0.992
0.445
0.088
2.23
303-314
294-295
092304Nb
0.500
0.204
0.014
2.45
296-307
292-294
092304S
0.980
0.280
0.026
3.50
296-307
291-295
101604N
1.023
0.096
0.014
10.66
288-300
277-282
101604S
0.540
0.370
0.027
1.46
288-300
277-282
111504N
0.864
0.272
0.055
3.18
284-294
274-276
072705N
0.186
0.091
0.039
2.04
300-310
276-281
072705S
0.095
0.091
0.042
1.04
300-310
278-283
a One ppmV of propylene was also injected into the chamber; bdate of the experiment in mmddyy and S and N refer to the South or North Chamber of the 270m3 dual outdoor chamber.
 
Generally the model predicts the decay of toluene very well for most conditions in Table 1, except for the experiments on the following dates: given in mmddyy 052204S, 092304N and 111504N (N and S denote either the North or South chamber of the 270m3 dual outdoor chamber.) In these experiments the model overestimates the loss of toluene by about 17%. The model also tracks the NO to NO2 conversion very well and it fits the ozone concentration profiles for all high toluene concentration runs reasonably well.  The model tends to over-predict afternoon ozone concentrations by about 30% for the low concentration systems (i.e. 072705N and 072705S).  It should be noted that the SOA masses formed in these two systems are low compared to those in the high concentration systems, where the maximum aerosol mass concentrations are 16 mg m-3 for 072705S and 33 mg m-3 for 072705N.  Under these low aerosol mass conditions the amount of water in the particle phase would be too low to convert all the excess NO2 in the system into particle phase HNO3. This Text Box: Figure 2. Comparison of model simulated and measured concentrations of toluene, NOx, ozone and aerosol for two toluene/NOx experiments. The solid lines (¾) represent model simulations. The pluses (+) are measured NO; asterisks (*) are measured NO2; triangles (D) are measured O3; diamonds (à) are measured toluene; solid diamonds (• ) are measured front filter mass; solid circles (•) are filter masses corrected by backup filter subtraction; and squares (–) are calculated SMPS particle mass concentrations by assuming density of 1.4g cm-3. suggests that there may be other reaction pathways that can also remove excess NO2 in the afternoon in the toluene/NOx system.
 
With respect to particle formation, our new UNC toluene mechanism reasonably tracks experimental SOA formation except for 101604N, where the model does not do a good job of simulating the rapid initial particle growth and hence underestimates the aerosol mass concentrations. The model also tends to under-predict the initial particle burst to different extents for almost all high concentration experiments, whereas it over-predicts the initial particle generation for the low concentration runs. In the mechanism, ketene oligomers (SEED1) from the photolysis of 2-methyl-2,4-hexadienedial, together with the background chamber aerosol, SEED, provide the only available initial particle surfaces for semi-volatile species to partition between the gas and aerosol phases. As will be discussed later, the mass fraction of SEED1 is higher in the low concentration systems than that in the high concentration systems. This indicates that there must be some other unknown reaction pathways that can also contribute to the initial particle formation.
  
The dominant particle phase species predicted by the mechanism are glyoxal oligomers (GLYPOLY), SEED1, organic nitrates, methyl nitro-phenol analogues, C7 organic peroxides, acylperoxy nitrates and, for the low concentration experiments, unsaturated hydroxyl nitro-acids (C6OHNO2ACID). Their molecular structures are shown in Table 1.
 
The relative amounts of these products vary depending on initial experimental conditions.  Offenberg et al. (2006) showed that the heat of vaporization, , for SOA formed in the photo-oxidation of toluene at steady state conditions (average of 6 hours of reaction) in their smog chamber was similar to that formed from nebulized glyoxal.  It is important to note, however, that the relative amount of different SOA species dramatically changes with time.  For example, in the 092304S experiment of this study, which started with 0.980 ppmV toluene and 0.280 ppm NO, the mass fraction of GLYPOLY in the aerosol phase was 10 % at the time particle growth begins and it increased to 61% at the end of the experiment. The mass fraction of total organic nitrates and acylperoxy nitrates was as high as 47% when particles started growing and it decreased to 7% at the end of the experiment. At the maximum aerosol concentration, the oligomers formed from glyoxal, methyl glyoxal and C14KETENE contributed 75% of the total particle mass, while organic nitrates, acylperoxy nitrates and methyl nitro-phenol analogues accounted for the other 18%.
 
Table 1. Chemical structures of major model simulated aerosol phase species.
 
Namea
Structure
Namea
Structure
GLYPOLY
RgDOHNO3
 
 
 
RgTOHNO3
    
MGLYPOLY
ROHALDNO3
   
 
 
   
RgOHDINO2
 
  
RgDIOHNO2
      
ROHALDACIDNO3
   
RgOHNO2
 
  
CROHOOH
     
 
  
RgTOHNO2OOH
  
BUDACIDONO3
  
TOLOOH
     
MBUDACIDONO3
  
SEED1
C6OHNO2ACID
 
a The species’ name in the mechanism
  
 Theory behind the model, expressed in non-mathematical terms: In this work a predictive kinetic chemical mechanism for the photo-oxidation of toluene was developed and applied to predict the secondary organic aerosol (SOA) and ozone formation observed in UNC dual outdoor aerosol smog chambers. This model is based on atmospheric processes that bring about SOA formation. Particle phase reactions of some major carbonyl products and gas-particle partitioning processes have been integrated with fundamental gas phase chemistry.
 
Mathematics used, including formulas and calculation methods: This approach incorporates gas-particle partitioning with gas phase kinetics by explicitly expressing absorption and desorption of each semi-volatile partitioning species. An example of the C5OHACID partitioning sequence is illustrated below in Rxns 1 and 2.
 
C5OHACID + SEED  C5OHACIDP + SEED         kon            Rxn. 1
C5OHACIDP  C5OHACID                                      koff           Rxn. 1
Where kon is the rate of absorption, koff is the rate of desorption and C5OHACID and C5OHACIDP are the gas and particle phases of a C5 hydroxycarboxylic acid. To reduce the number of gas-particle partitioning cross-reaction steps, a scalar parameter TSP was introduced into the mechanism, which is the sum of all particle phase products and is computed at each time step. Instead of letting the gas phase semi-volatile compound explicitly partition onto each particle phase species, one pseudo 1st order reaction was used to represent all these reactions with a rate coefficient of konTSP.  If there are 20 partition species, this reduces the number of partitioning reaction steps by 202-20, or 380 steps.
 
    C5OHACIDG   C5OHACIDP                          konTSP          Rxn. 3
Semi-volatile products generated in the gas phase are assumed to be governed by equilibrium between the liquid particle phase and surrounding gas phase. At equilibrium, the gas-particle partitioning equilibrium constant iKp (m3 mg-1 of particle mass) equals to the ratio of kon over koff.  The equilibrium constant (m3 mg-1) may be theoretically estimated from the absorption model of Pankow (1994).
 
                                                                 Eq. 1   
where T is the temperature (K), fom is the mass fraction of organic material (om) in particulate matter, MWom is the average molecular weight of a given liquid medium (g mol-1), igom is the activity coefficient of a given organic compound, i, in a given organic mixture, ipoL<

Conclusions:

Accomplishments:
Over the course of this project we developed a chemical mechanism for toluene which reasonably predicts the gas phase chemistry and also predicts the SOA formation from toluene oxidation (Figure 1).  Two papers have  appeared in the peer reviewed literature on this mechanism. Our model includes 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 are represented.  A series of experiments that cover a wide range of temperature, solar condition and initial reactant concentrations, were carried out in the UNC 270 m3 dual outdoor aerosol smog chambers. Data obtained from these experiments were used to develop and test the mechanism. The model adequately simulates the decay of toluene, the NO to NO2 conversion, and ozone formation.  Although it provides a reasonable prediction of SOA production under different conditions that range from 15 to 300 mg m-3, the model tends to underestimate the initial particle burst for almost all high toluene concentration experiments. 

The main contribution that this study makes to science is that describes an atmospheric aromatic mechanism that: 1. simultaneously considers gas phase reactions, trace gas phase-particle phase partitioning, and subsequent particle phase reactions; 2. it proposes a simple chemical mechanism for particle phase nucleation;  3. permits one to distinguish between gas-particle partitioning of SOA, and heterogeneous SOA formation;  4. demonstrates the relative importance of organic nitrate formation at high and low toluene concentrations; and 5. successfully simulates gas phase toluene oxidation in smog chamber systems.  To date, this has not been accomplished by any of the existing aromatic mechanisms.

The dominant particle phase species predicted by the mechanism are glyoxal oligomers (organic nitrates, methyl nitro-phenol analogues, C7 organic peroxides, acylperoxy nitrates and, for the low concentration experiments, unsaturated hydroxyl nitro-acids. The relative amounts of these products vary depending on initial experimental conditions. In general, with decreasing toluene/NO ratios, the relative amount of total organic nitrates and acylperoxy nitrates in the particle phase increases, the mass fraction of total oligomers and organic peroxides decreases. It is also important to note that the relative amount of different SOA species dramatically changes with time. The model also well predicts the SOA mass concentrations observed from the European Photoreactor (EUPHORE) and smog chambers at the California Institute of Technology (Caltech). But to implement the developed mechanism into the regional airshed model, it would be desirable to reduce the reaction steps and number of represented species. It is recommended that future studies should focus on: 1. reaction mechanisms that contribute to  rapid particle formation in toluene/NOx system, 2. identification of particle phase oligomers, 3. measurement of organic nitrates and acylperoxy nitrates, 4. chamber experiments with reactant concentrations at ambient levels, and 5. combining aromatics and monoterpenes mechanisms into one unified mechanism.

References

Calvert, J.G., Atkinson, R., Becker, K.H., Kamens, R.M., Seinfeld, J.H., Wallington, T.J., and Yarwood, G., 2002. The Mechanisms of Atmospheric Oxidation of Aromatic Hydrocarbons. Oxford University Press, Inc., New York .

Glasstone, S., Laidler, K. J., and Eyring, H., 1941. The Theory of Rate Processes: The Kinetics of Chemical reactions, Viscosity, Diffusion and Electrochemical Phenomena. McGraw-Hill, New York . 

 

Hamilton , J.F., Webb, P.J., Lewis, A.C., and Reviejo, M.M., 2005. Quantifying small molecules in secondary organic aerosol formed during the photo-oxidation of toluene with hydroxyl radicals. Atmospheric Environment 39, 7263-7275.

Jang, M. and Kamens, R. M., 1998. A Thermodynamic Approach for Modeling Partitioning of Semivolatile Organic Compounds on Atmospheric Particulate Matter: Humidity Effects. Environmental Science and Technology 32(9), 1237-1243.

Jang, M. and Kamens, R. M., 2001a. Atmospheric Secondary Aerosol Formation by Heterogeneous Reactions of Aldehydes in the Presence of a Sulfuric Acid Aerosol Catalyst. Environmental Science and Technology 35(24), 4758-4766.

Jang, M. and Kamens, R. M., 2001b. Characterization of Secondary Aerosol from the Photooxidation of Toluene in the Presence of NOx and 1-Propene. Environmental Science and Technology 35(18), 3626-3639.

Jang, M., Czoschke, N. M., Lee, S., and Kamens, R. M., 2002. Heterogeneous Atmospheric Aerosol Production by Acid- Catalyzed Particle-Phase Reactions. Science ( Washington, DC, United States ) 298(5594), 814-817.

Jang, M., Lee, S., and Kamens, R. M., 2003. Organic aerosol growth by acid-catalyzed heterogeneous reactions of octanal in a flow reactor. Atmospheric Environment 37(15), 2125-2138.

Jeffries, H., Sexton, K., and Adelman, Z., 1999. Auxiliary mechanisms (wall models) for UNC outdoor chamber. EPA/600/R-00/076.

Jeffries, H., Kessler, M., and Gery, M., 2003. MComp/MEval: The Morphecule Photochemical Reaction Mechanism.

Jenkin, M.E. , Saunders, S.M., and Pilling, M.J., 1997. The tropospheric degradation of volatile organic compounds: a protocol for mechanism development. Atmospheric Environment 33(1), 81-104.

Joback, K.G. and Reid, R.C., 1987.  Estimation of pure-component properties from group contributions.  Chemical engineering Communications 57, 233-243. 

Johnson, D., Jenkin, M.E. , Wirtz, K., and Martin-Reviejo, M., 2004. Simulating the formation of secondary organic aerosol from the photooxidation of toluene. Environmental Chemistry 1, 150-165.

Kalberer, M., Paulsen, D., Sax, M., Steinbacher, M., dommen, J., Prevot, A.S.H., Fisseha, R., Weingartner, E., Frankevich, V., Zenobi, R., and Baltensperger, U., 2004. Identification of polymers as major components of atmospheric organic aerosols. Science 303(5664), 1659-1662.

Kamens, R. M. and Jaoui, M., 2001. Modeling aerosol formation from -Pinene + NOx in the presence of natural sunlight using gas-phase kinetics and gas-particle partitioning theory. Environmental Science and Technology 35(7), 1394-1405.

Kamens, R., Jang, M., Chien, C.-J., and Leach, K., 1999. Aerosol Formation from the Reaction of -Pinene and Ozone Using a Gas-Phase Kinetics-Aerosol Partitioning Model. Environmental Science and Technology 33(9), 1430-1438.

Klotz, B., Barnes, I. , and Becker, K.H., 1999. Kinetic study of the gas-phase photolysis and OH radical reaction of E,Z- and E,E-2,4-hexadienedial. International Journal of Chemical Kinetics 31, 689-697.

Kroll, J.H., Ng, N.L., Murphy, S.M., Varutbangkul, V., Flagan, R.C., and Seinfeld, J.H., 2005. Chamber studies of secondary organic aerosol growth by reactive uptake of simple carbonyl compounds. Journal of Geophysics Research 110, D23207.

Kwok, E.S.C. and Atkinson, R., 1995. Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structure-reactivity relationship: an update. Atmospheric Environment 29, 1685-1695.

Kwok, E.S.C., Aschmann, S.M., Atkinson, R., and Arey, J., 1997. Products of the gas-phase reactions of o-, m-, and p-xylene with the OH radical in the presence and absence of NOx. Journal of the Chemical Society, Faraday Transactions 93, 2847-2854.

Leungsakul, S., Jeffries, H. E., and Kamens, R. M., 2005a. A kinetic mechanism for predicting secondary aerosol formation from the reactions of d-limonene in the presence of oxides of nitrogen and natural sunlight. Atmospheric Environment 39(37), 7063-7082.

Leungsakul, S., Jaoui, M., and Kamens, R. M. , 2005b. Kinetic Mechanism for Predicting Secondary Organic Aerosol Formation from the Reaction of d-Limonene with Ozone. Environmental Science and Technology 39(24), 9583-9594.

Liggio, J., Li, S.-M., and McLaren, R., 2005a. Heterogeneous Reactions of Glyoxal on Particulate Matter: Identification of Acetals and Sulfate Esters. Environmental Science and Technology 39(6), 1532-1541.

Liggio, J., Li, S.-M., and McLaren, R., 2005b. Reactive uptake of glyoxal by particulate matter. Journal of Geophysical Research 110, D10304.

Liu, X., Jeffries, H. E., and Sexton, K. G., 1999. Atmospheric Photochemical Degradation of 1,4-Unsaturated Dicarbonyls. Environmental Science and Technology 33(23), 4212-4220.

Mackay, D.B.A., Chan, D.W., and Shiu, W.Y., 1982. Vapor pressure correlations for low-volatility environmental chemicals. Environmental Science and Technology 16, 645-649.

Matsumoto, M., Yasuoka, K., and kataoka, Y., 1994. Evaporation and condensation at a liquid surface. II. Methanol.

Nielsen, T., Platz, J., Granby , K., Hansen, A.B., Skov, H., and Egelov, A.H., 1998. Particulate organic nitrates: sampling and night/day variation. Atmospheric Environment  32(14/15), 2601-2608.

Offenberg, J.H., Kleindienst, T.E., Jaoui, M., Lewandowski, M., and Edney, E.O., 2006. Thermal Properties of Secondary Organic Aerosols. Geophysical Research Letters 33, L03816.

Pankow, J.F., 1994. An absorption model of gas/particle partitioning of organic comounds in the atmosphere. Atmospheric Environment 28, 185-188.

Saunders, S.M., Jenkin, M.E. , Derwent, R.G., and Pilling, M.J., 2003. Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part A): tropospheric degradation of non-aromatic volatile organic compounds. Atmospheric Chemistry and Physics 3, 161-180.

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Stroud, C.A. , Makar, P.A., Michelangeli, D.V., Mozurkewich, M., Hastie, D.R., Bardu, A., and Humble, J., 2004. Simulating organic aerosol formation during the photooxidation of toluene/NOx mixtures: comparing the equilibrium and kinetic assumption. Environmental Science and Technology 38(5), 1471-1479.

Tolocka, M. P., Jang, M., Ginter, J. M., Cox, F. J., Kamens, R. M., and Johnston, M. V., 2004. Formation of Oligomers in Secondary Organic Aerosol. Environmental Science and Technology 38(5), 1428-1434.

Voicu, I. , 2003. A revised carbon bond mechanism. the University of North Carolina at Chapel Hill , Department of Environmental Sciences and Engineering.

Yasuoka, K., Matsumoto, M., and Kataoka, Y., 1994. Evaporation and condensation at a liquid surface. I. Argon. The Journal of Chemical Physics 101, 7904-7911.

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Journal Articles on this Report : 5 Displayed | Download in RIS Format

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Other project views: All 8 publications 6 publications in selected types All 6 journal articles
Publications
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Journal Article Hu D, Tolocka M, Li Q, Kamens RM. A kinetic mechanism for predicting secondary organic aerosol formation from toluene oxidation in the presence of NOx and natural sunlight. Atmospheric Environment 2007;41(31):6478-6496. R831084 (2006)
R831084 (2007)
R831084 (Final)
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    • Journal Article Hu D, Kamens RM. Evaluation of the UNC toluene-SOA mechanism with respect to other chamber studies and key model parameters. Atmospheric Environment 2007;41(31):6465-6477. R831084 (Final)
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    • Journal Article Lee S, Kamens RM. Particle nucleation from the reaction of α-pinene and O3. Atmospheric Environment 2005;39(36):6822-6832. R831084 (2005)
    R831084 (2006)
    R831084 (2007)
    R831084 (Final)
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    • Journal Article Leungsakul S, Jeffries HE, Kamens RM. A kinetic mechanism for predicting secondary aerosol formation from the reactions of d-limonene in the presence of oxides of nitrogen and natural sunlight. Atmospheric Environment 2005;39(37):7063-7082. R831084 (2005)
    R831084 (2006)
    R831084 (2007)
    R831084 (Final)
    R828176 (Final)
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    • Journal Article Li Q, Hu D, Leungsakul S, Kamens RM. Large outdoor chamber experiments and computer simulations: (I) secondary organic aerosol formation from the oxidation of a mixture of d-limonene and α-pinene. Atmospheric Environment 2007;41(40):9341-9352. R831084 (2007)
    R831084 (Final)
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    • Supplemental Keywords:

    Secondary organic aerosol formation, aromatics, modeling, organic particle formation, RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Environmental Chemistry, Monitoring/Modeling, Atmospheric Sciences, Environmental Monitoring, air quality, secondary organic aerosol, particle phase reactions, gas phase chemistry, environmental measurement, Toluene, organic chemistry, aerosol analyzers

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