JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D10301, doi: /2007jd008726, 2008

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007jd008726, 2008 Box model studies of the secondary organic aerosol formation under different HC/NO x conditions using the subset of the Master Chemical Mechanism for -pinene oxidation Adam G. Xia, 1,2 Diane V. Michelangeli, 1,3 and Paul A. Makar 4 Received 30 March 2007; revised 10 January 2008; accepted 8 February 2008; published 20 May [1] A subset of a near-explicit Master Chemical Mechanism (v3.1) describing -pinene oxidation (976 reactions and 331 compounds) coupled with a gas/particle absorptive partitioning model is used as a benchmark for the study of secondary organic aerosol (SOA) formation within a box model under atmospheric relevant conditions of averaged HC/NO x ratios between 0.18 and 8.43 (ppbvc/ppbv). Results from the detailed mechanism for -pinene oxidation show that total SOA mass increases as the HC/NO x ratio increases within the studied range. The mass of peroxynitrates and the nitrates in the aerosol phase increases with increasing HC/NO x ratio, despite decreases in the total (gas plus aerosol) mass of these species, because of increases in mass of organic peroxides and acids in these conditions. The fractional composition of aerosol mass indicates organic peroxides and acids dominate at high HC/NO x ratios and peroxynitrates and nitrates dominate at low HC/NO x ratios. In addition, 28 out of 149 condensable products are identified as important compounds for SOA formation. Of the organic nitrates, only two contribute consistently to organic aerosol mass. Organic peroxide and acid mass in the aerosol phase is distributed over a larger number of species. The 28 species identified here are suitable targets for future laboratory and field analysis of organic aerosols and are recommended for use in future mechanism reduction work. Citation: Xia, A. G., D. V. Michelangeli, and P. A. Makar (2008), Box model studies of the secondary organic aerosol formation under different HC/NO x conditions using the subset of the Master Chemical Mechanism for -pinene oxidation, J. Geophys. Res., 113,, doi: /2007jd Introduction [2] A great deal of research effort [Went, 1960; Rasmussen, 1972; Andreae and Crutzen, 1997; Claeys et al., 2004b] has been made toward the understanding of the mechanism for the formation of secondary organic aerosol (SOA). Particular attention has been given to the oxidation of biogenic compounds - and -pinene in the tropospheric and smog chamber studies [Calvert et al., 2000; Seinfeld and Pankow, 2003]. [3] Generally, the pathway for the SOA formation is initiated through the oxidation of gaseous precursors with three major oxidants of NO 3,O 3,OH[Seinfeld and Pandis, 1998; Kanakidou et al., 2005], and even O( 3 P) [Carter, 2000; Russell and Allen, 2005] and the chlorine atom (Cl) [Cai and Griffin, 2006]. These oxygenated reaction products in the gas phase may further react with the same 1 Department of Earth and Space Science and Engineering, York University, Toronto, Ontario, Canada. 2 Now at Air Quality Research Division, Environment Canada, Toronto, Ontario, Canada. 3 Deceased 30 August Air Quality Research Division, Environment Canada, Toronto, Ontario, Canada. Copyright 2008 by the American Geophysical Union /08/2007JD oxidants or other compounds to create higher-generation products. As a result of chemical reactions in the gas phase, some oxygenated products may have low volatility (low saturation vapor pressures), and these semi-or non-volatile organic compounds (VOCs) may condense into/onto (absorptive and adsorptive) existing particles or nucleate to form new particles [Pankow, 1994a]. Some oxygenated products may further react with other compounds within the particles through processes such as acid-catalyzed heterogeneous oligomerization to generate high molecular weight (up to 1600 g mole 1 ) compounds [Jang and Kamens, 2001; Jang et al., 2002; Gao et al., 2004a, 2004b; Iinuma et al., 2004; Tolocka et al., 2004; Kalberer et al., 2004]. Reactive uptake of H 2 O 2 and glyoxal through multiphase reactions [Claeys et al., 2004a; Knopf et al., 2005; Liggio et al., 2005a, 2005b; Gelencser and Varga, 2005] may also occur. In addition, radical-initiated chemical aging of primary organic aerosol has been shown to contribute to SOA formation in the atmosphere [Poschl et al., 2001; Decesari et al., 2002; Tsigaridis and Kanakidou, 2003; Maria et al., 2004; Molina et al., 2004; Poschl, 2005; Zuberi et al., 2005]. [4] Ultimately, gas phase chemical mechanisms for the oxidation of hydrocarbons are critical to describe SOA formation quantitatively. Over the years, some simple chemical mechanisms based on smog chamber studies were 1of19

2 proposed and applied to describe SOA formation from the oxidation of well-studied -pinene [Kamens et al., 1999], -pinene [Stockwell et al., 1997; Barthelmie and Pryor, 1999], d-limonene [Leungsakul et al., 2005a, 2005b], and cyclohexene [Keywood et al., 2004]. However, some of these mechanisms were developed to describe total SOA mass rather than individual components or functional groups. Numerous organic compounds are known to be created in even a simple chemical oxidation system. For example, more than 30 -pinene oxidation products in the aerosol phase were identified by using a liquid chromatography mass spectrometry analysis [Winterhalter et al., 2003] in a chamber investigation. The use of a detailed mechanism may therefore provide insight into the formation processes of organic aerosol. [5] A recent example of a detailed mechanism is the Master Chemical Mechanism (MCM). The new version of MCM (v3.1) was applied in a zero-dimensional box model to describe SOA formation against smog chamber experiments for ozonolysis of - and -pinene [Jenkin, 2004], photo-oxidation of a toluene/no x mixture [Stroud et al., 2004; Johnson et al., 2004], and other aromatics [Johnson et al., 2005]. [6] In this work, the subset of the gas phase MCM v3.1 describing -pinene oxidation is used to describe SOA formation under a wider range of oxidants and conditions than in the work by Jenkin [2004]. All organic condensable products generated from gas phase chemistry may partition themselves between gas and aerosol phases, on the basis of Pankow s [1994a] gas/particle absorptive partitioning theory. Modeling results for the total SOA mass and components from different functional groups are analyzed under different hydrocarbon/no x conditions. Finally, 28 condensable organic compounds are identified as important species for the SOA formation (i.e., species comprising a significant portion of the aerosol mass under different initial conditions). These species are suitable for future mechanism confirmation analysis of -pinene-generated aerosols. They are also used in another paper (A. G. Xia et al., Mechanism reduction for the formation of secondary organic aerosol for integration into a 3-dimensional regional air quality model, submitted to Atmospheric Chemistry and Physics, 2008) for mechanism reduction purposes. 2. Model Description 2.1. Master Chemical Mechanism [7] Recently published detailed chemical mechanisms include, for example, the National Center for Atmospheric Research s Master Mechanism [Madronich and Calvert, 1990], the Statewide Air Pollution Research Center mechanism SAPRC-99 [Carter, 2000] for the oxidation of about 400 VOC compounds, University of Leeds Master Chemical Mechanism version 3.1 [Jenkin et al., 1997; Saunders et al., 2003; Jenkin et al., 2003; Bloss et al., 2005b] (which covers the oxidation of 135 primary organic compounds under tropospheric conditions), and a newly developed fully explicit chemical mechanism [Aumont et al., 2005] based on a self-generating approach (similar method as the generation of MCM) to represent complete degradation scheme for acyclic compounds up to ten carbons with 350,000 compounds and 2 million reactions. [8] Specifically, MCM describes complete degradation of the 135 primary VOCs, selected on the basis of UK emission inventories, with about 6,000 compounds and 13,500 chemical reactions [Bloss et al., 2005b] down to H 2 O and CO 2. Like many other detailed mechanisms, a large number of compounds are organic radicals (peroxy, oxy, and Criegee radicals) and secondary products such as alcohols, aldehydes, ketones, nitrates, peroxides, carboxylic acids, and peracids [Atkinson, 2000; Atkinson and Arey, 2003]. [9] An early version of the MCM was compared with smog chamber data for the gas phase chemistry and tested for computer timing and accuracy with use of sparse matrix techniques [Liang and Jacobson, 2000]. The new version of MCM v3.1 has also been evaluated against controllable smog chamber experiments for gas phase chemistry of alkenes [Hynes et al., 2005], isoprene [Pinho et al., 2005], - and -pinene [Saunders et al., 2003; Pinho et al., 2007], and aromatics [Bloss et al., 2005a] SOA Formation [10] Conceptually, a degradation product from the MCM (or other detailed chemical mechanisms) will condense into the aerosol phase if that product has a very low saturation vapor pressure (SVP). In order to describe the formation of SOA in this work, Pankow s absorptive model [Pankow, 1994a], a widely used primary mechanism for gas/particle partitioning in smog chamber studies [Odum et al., 1996, 1997; Yu et al., 1999b; Cocker et al., 2001; Pankow et al., 2001; Seinfeld et al., 2001; Stroud et al., 2004] is coupled with gas phase chemistry to predict bulk SOA under a wide range of conditions. Note that no heterogeneous or multiphase reactions are included in this study for the SOA formation, although these chemical reactions [Jang and Kamens, 2001; Jang et al., 2002; Iinuma et al., 2004; Kalberer et al., 2004; Liggio et al., 2005a] help partly explain the frequently observed enigma in the literature that the observed SOA is generally much higher than the model results predicted by using gas/particle partitioning theories Gas/Particle Absorptive Partitioning Theory [11] Pankow [1994a] proposed an absorptive partitioning mechanism for the formation of organic aerosols. This mechanism describes a dynamic equilibrium between gas and aerosol phases for each condensable organic compound. The partitioning coefficient K p,i (m 3 g 1 ) is expressed as K p;i ¼ A i=tsp ¼ 760RTf om G i 10 6 MW om i PL;i o ; ð1þ where A i (g m 3 ) and G i (g m 3 ) are the aerosol phase and gas phase concentrations of organic compound i, respectively. TSP (g m 3 ) represents total suspended particulate concentration. R is the ideal gas constant ( m 3 atm K 1 mol 1 ) (1 atm = N m 2 ), T (K) is the temperature, f om is the mass fraction between total absorbing organic matter and TSP, MW om is the mean molecular weight (g mol 1 ) of the condensed organic material, i is the activity coefficient of the given compound i in the condensed organic phase, and P O L,i (torr (1 torr = mbar)) is the saturation vapor pressure (subcooled if necessary) of pure compound i at temperature T. 2of19

3 [12] Equation (1) indicates that K p,i depends directly on temperature and depends indirectly on temperature via saturation vapor pressure [Andersson-Skold and Simpson, 2001; Sheehan and Bowman, 2001; Schell et al., 2001], and it depends on the composition in the aerosol phase via mean molecular weight and activity coefficients [Seinfeld et al., 2001; Bowman and Karamalegos, 2002]. In particular, the effect of the activity coefficients on the gas/particle partitioning coefficients and mean molecular weight were investigated extensively by Seinfeld et al. [2001]. Generally, the activity coefficients of the compounds in the organic aerosols are in the range of 0.3 to 3.0 [Seinfeld and Pankow, 2003]. However, as adopted by many researchers [Pankow, 1994b; Kamens et al., 1999; Stroud et al., 2004; Jenkin, 2004] as a first approximation, the activity coefficients are also assumed to be unity in this work for the oxidized products in the aerosol droplets when the mixture contains similar compounds. [13] Once the saturation vapor pressure has been estimated, the calculation of K p,i for each condensable compound is an iterative procedure [Pankow, 1994b; Makar et al., 2003]. Mean molecular weight (MW om ), necessary for K p,i, of all compounds in the aerosol phase depends strongly on the fractions of individual compounds in that droplet. Iteratively, the fractions are then controlled by the partitioning coefficients Estimation of Saturation Vapor Pressure [14] Saturation vapor pressure (SVP) is one of the important physical properties that affect how organic compounds partition between gas and aerosol phases. It is a key parameter in the calculation of the partitioning coefficients in equation (1), and it depends strongly on temperature and the compound s molecular structure. [15] By making an assumption of the linear relationship for enthalpy at different temperatures, integrating the Clausius-Clapeyron equation from T b (normal boiling point) to T, and using the Trouton s rule, the SVP [Schwarzenbach et al., 2003] can be expressed as ln P 0 L 760 ¼ K Fð4:4 þ ln T b Þ 1:8 T b T 1 0:8ln T b T ; ð2þ where T b (K) denotes the normal boiling point, T (K) designates the temperature of interest, K F (no unit) is a Fishtine factor [Fishtine, 1963], and the unit of SVP is torr. For convenience, the above method to calculate the SVP is called the KF method. Note that the K F (with a subscript F) refers to Fishtine factor and KF (without subscript) is the name of the method to calculate the SVP in this work. [16] The factor K F is 1.0 for apolar and many monopolar compounds but sometimes can be as high as 1.3, depending on the molecular structure. Details about how to calculate the K F factor can be found in work by Fishtine [1963] or Sage and Sage [2000]. [17] The second parameter required in the estimation of the SVP in equation (2) is the normal boiling point (T b ). A group contribution method (Joback and Reid s [1987] fragmentation method, modified and extended by Stein and Brown [1994]) is used to estimate the normal boiling point (at pressure 1.0 atm). The boiling point estimation method is implemented in the MPBPVP model (MPBPVP model is available at pubs/episuitedl.htm), developed by the U.S. Environmental Protection Agency. [18] The predictions of SVP from the KF method and the universal functional activity coefficient (UNIFAC) p L 0 method from work by Asher et al. [2002] were also compared against measured data. The compounds for the intercomparison are taken from work by Asher et al. [2002] at a fixed temperature of 320 K. The results show that the two estimation methods are comparable with each other in terms of a standard error, which is defined as ¼ 1 Nc X Nc i¼1 log 10 PL;i o Predicted log 10 PL;i o exp ; ð3þ O O where P L,i Predicted and P L,i exp are the predicted and experimental SVPs (unit: atm) for compound i, and Nc is the total number of compounds of the study. [19] The of the UNIFAC-p 0 L is , and the of the KF method with estimated boiling point is However, the of the KF method is reduced to by using the experimental boiling points of the tested compounds. Although the evaluation is conducted with limited database here, the KF method shows the capability to predict the SVP for the organic condensable compounds, especially when the accurate data or estimation method [Lai et al., 1987] for the boiling points are available. [20] Moreover, the KF method has an advantage over the UNIFAC p 0 L method in that the current version of the UNIFAC p 0 L has a limitation to some functional groups in the organic aerosol phase compounds, such as oxygenated peroxide acids, nitrates, and peroxynitrate (PAN)-like compounds from the oxidation of -pinene. As a result, the KF method is chosen to estimate the SVP for modeling the SOA formation in this work. [21] Only the compounds with boiling points larger than 480 K have the tendency to form organic aerosol in the ambient air [Jenkin, 2004]. Before the calculation of SVP at the temperature of interest, the K F factor and the estimated boiling point for each compound are then determined by the corresponding estimation methods outlined above (if the experimental boiling points are available, the experimental boiling points are used instead). Finally, all these required parameters are utilized in the KF method to predict the SVP for each compound in the chemical system for the SOA formation. 3. Model Settings 3.1. Chemistry [22] In this work, only the subset of MCM describing -pinene oxidation, rather than the whole MCM, is chosen for the study of the SOA formation. The -pinene subset contains 928 chemical reactions and 310 chemical compounds. In order to run the model, 48 inorganic thermal chemical reactions and 21 inorganic compounds have been combined with that subset for -pinene. The inorganic reactions and the rate coefficients have been taken from a variety of evaluated chemical kinetics data [Atkinson, 1994; DeMore, 1994, 1997; Atkinson et al., 1997a, 1997b], and their full temperature and pressure dependencies, as well as 3of19

4 Table 1. Number of Compounds and Reactions in the Chemical System Inorganic Organic Total Number of compounds Number of reactions the variation in photolysis reactions with time, have been included. Of the organic compounds, 149 out of the total 331 have been identified first as potentially condensable on the basis of their estimated boiling points. The number of the compounds and reactions used in the chemical system is summarized in Table Model Evaluation of SOA Formation by Using Smog Chamber Data [23] The model in this work is evaluated against smog chamber data of -pinene oxidation. Again, this model consists of 331 species and 976 gas phase reactions, and the SOA are formed via Pankow s [1994a] gas/particle absorptive partitioning theory. In the model, aerosols are treated as bulk parameters, and the organic aerosols would evaporate back to the gas if the temperature is increased. During smog chamber studies, a time step of 2 min is used for the equilibrium absorptive partitioning process between gas and aerosol phases. [24] First of all, we evaluated our model against smog chamber experiments [Kamens et al., 1999], and we found that our model slightly overestimates decay rate of -pinene. Meanwhile, the same gas phase chemistry for -pinene oxidation was also very recently evaluated against environmental chamber data for a range of different initial HC/NO x conditions [Pinho et al., 2007]. Generally, model results are consistent with the observations of smog chamber experiments for -pinene oxidation. However, the model overestimates the formation rate of ozone and the decay rate of -pinene. This is similar to our model evaluation. [25] Next, a total of 22 recent smog chamber experiments [Kamens et al., 1999; Kamens and Jaoui, 2001; Takekawa et al., 2003; Presto et al., 2005; Presto and Donahue, 2006; Pathak et al., 2007] were used to evaluate our model performance for the SOA formation. These experiments were performed under different temperatures, different relative humidity, and different initial concentrations of -pinene, O 3, and NO x. Table 2 shows the initial conditions of these experiments, the measured SOA masses, the predicted SOA masses, and the ratios of the predicted to the measured SOA masses. Generally, a wide range of the ratios from to 1.57 for model performance is found here. [26] Interestingly, when the ratios of the predicted to the measured SOA masses are plotted, in Figure 1, against the temperature and the corresponding total SOA loading for the 22 smog chamber experiments, we find that the our Table 2. Evaluation of 22 Smog Chamber Experiments for the SOA Formation From -Pinene Oxidation Number Experiment -Pinene, ppbv O 3, ppbv Total NO x (NO/NO 2 ), a ppbv T, K Relative Humidity, % Measured SOA, g m 3 Predicted SOA, g m 3 Ratio of Predicted SOA to Measured SOA 1 b 05/08/ c 2140 c 9.77E-01 2 d 30/10/ / c 615 c 6.15E-01 3 b 11/03/ c 2319 c 1.25E+00 4 e PIN / f E-01 5 e PIN / f E-01 6 e PIN / f E-01 7 e PIN / f E-01 8 e PIN / f E-02 9 e PIN / f E g 30/03/ < E g 04/06/ < E g 15/09/ < E h 14/06/ < E h 28/06/ < E h 08/07/ < E h 13/07/ < E h 22/07/ < E i exp < E i exp < E i exp < E i exp < E i exp < E-04 a Total NO x concentration or the concentrations of NO and NO 2 respectively. b See Kamens et al. [1999]. c The maximum concentrations of the measured and predicted SOA mass for each smog chamber study. d See Kamens and Jaoui [2001]. e See Takekawa et al. [2003]. f Converted total concentration of SOA mass (SOA mass = M 0 /R s, where M 0 is the measured SOA concentration in the suspended aerosol phase and R s is the ratio of suspended to total concentration of the SOA) during the 4 h smog chamber experiment period. g See Presto et al. [2005]. h See Presto and Donahue [2006]. i See Pathak et al. [2007]. 4of19

5 Figure 1. Evaluation of model performance for 22 smog chamber experiments. The 22 -pinene oxidation experiments were performed under different temperatures with different total SOA mass loading. Each experiment is labeled (see also Table 2) with the ratio of the predicted to the measured SOA masses. model predictions for the total SOA are very close to the measured SOA mass either when the temperature is low, such as the data points at 283 K, or when the total SOA mass is larger than 200 g m 3. But the model underpredicts total SOA mass at high temperatures with low-soa mass loading. [27] Large discrepancy for the model performance at high temperature could be attributed to the following four factors: [28] 1. The SVP estimation method applied in this work has some uncertainty. [29] 2. Organic compounds interactions within the particles can lead to discrepancies in model performance (e.g., the activity coefficients for all organic compounds in the particles are assumed to be unity in this paper, and no interaction exists between the organic compounds and the water). [30] 3. The gas/particle absorptive partitioning theory has some limitations. (That is, perhaps some reactions, which might be very important at high temperatures with low-soa mass loading, are missing in current SOA model. Specifically, heterogeneous reactions [Jang et al., 2002] for the organic particles are not included. For example, the oligomerization [Kalberer et al., 2004; Liggio et al., 2005b] is a potential cause of the low bias in the SOA formation.) [31] 4. Aerosol microphysical processes are not included in current model. Only a bulk parameterization is used. [32] This new finding for the poor performance at high temperature is consistent with Jenkin s [2004] work, in which most of the evaluated smog chamber experiments were conducted above 290 K, and all partitioning coefficients were scaled up by a factor of 120 to fit the observations. Similarly, scaling factors from 5 to 500 were used by Johnson et al. [2004, 2005, 2006] for simulations of smog chamber experiments of aromatics and field measurements. [33] On the other hand, we also noticed that the model performance is quite close to the observations at high-soa mass loading. This could be because the dominant con- 5of19

6 Table 3. Initial Conditions and Emission Rates for -Pinene, O 3, NO, and NO 2 Compounds Initial Conditions, ppbv Emission Rates, Molecules cm 3 s 1 -pinene [0.1, 1, 4] [1, 2, 3] (Temp 303) e O 3 [50, 70, 90] 0 NO [1, 8, 15] [2, 4, 8] 10 6 NO 2 NO init /3 NO emission /3 densable chemical species at high-soa loading are different from those at low mass loading and the estimated SVP for these dominant species at high mass loading are more accurate than those under low mass loading. Under that condition, it is possible that the partitioning is a dominant process for the organic compounds over other chemical processes. Future investigation is needed for the model performance under different conditions Initial Conditions, Emission Rates, and Temperature Profile [34] The full chemical system is run in a zero-dimensional box model without deposition, and our primary objective is to examine the gas phase chemistry without perturbations from other loss mechanisms. This large chemical system is used as a basis for the examination of mechanism reduction techniques in a separate paper (A. G. Xia et al., submitted manuscript, 2008). [35] In order to study the chemical system for many different situations, a variety of initial concentrations and emission rates were generated, and these are listed in Table 3. The initial concentrations of -pinene, O 3, and NO x were chosen on the basis of typical values observed in the ambient atmosphere. In each scenario, the initial concentration and emission rate of NO 2 are set to be one third of NO [Bowman, 2005] on the basis of typical NO x emission profiles. The expression for the -pinene emission rate (E = E s e 0.09 x (Temp-303) ) follows the proposed algorithm of Guenther et al. [1993]. The three possible values for the basal emission rate, E s, ([1, 2, 3] 10 7 molecules cm 3 s 1 ) in Table 3, are comparable to the maximum measured in a forest during a field campaign of ECHO 2003, using the eddy covariance flux method [Spirig et al., 2005]. For example, the measured emission flux of 0.9 g m 2 s 1 corresponds to molecules cm 3 s 1 when a mixing height of 500 m is assumed. [36] In Table 3, there are five independent parameters: (1) emission rates of NO, (2) emission rates of -pinene (changing with temperature), (3) initial concentrations of ozone, (4) initial concentrations of -pinene, and (5) initial concentration of NO. Each parameter has three different values; therefore the total number of scenarios for the model runs is 243 (243 = 3 5 ) from three different values for each of the five parameters. [37] The model was run for 3 d starting from midnight for each scenario, and Figure 2 shows the profiles for the temperature and the relative humidity. The assumed anticorrelation between the temperature and relative humidity is typical from field measurements (PACIFIC 2001) in Vancouver [Shantz et al., 2004]. [38] The tested cases cover a broad range of conditions, and the ratios of average -pinene to average NO x, i.e., -pinene/no x (ppbvc/ppbv), are in the range of 0.18 to Specifically, in urban and suburban areas, -pinene is less than 1.0 ppbv, and NO x concentration is larger than 10.0 ppbv [National Research Council (NRC), 1991], then the maximum ratio is still less than 1.0 (ppbvc/ppbv). [39] However, in a forest area, -pinene concentration could reach as high as 1.5 ppbv in Canadian [Leaitch et al., 1999; Cheng et al., 2004] and German forests [Spirig et al., 2005]. Nevertheless, mean -pinene concentration is much smaller. For example, a maximum monthly mean of ppbv -pinene was observed in a northern European boreal forest [Hakola et al., 2000]. Meanwhile, NO x concentrations in the remote forest areas are in the range Figure 2. Profiles for the temperature and relative humidity for the 3 d model run. 6of19

7 Figure 3. Averaged mixing ratios of (a) -pinene and (b) NO x from the 108 selected scenarios. of 0.02 to 0.10 ppbv [NRC, 1991; Leaitch et al., 1999]. As a result, maximum -pinene/no x ratio (ppbvc/ppbv) in a remote forest could be larger than In general, our range of 0.18 to 8.43 for the HC/NO x ratio is still reasonable and representative of the ambient environment but is not allencompassing in that regard Numerical Method [40] The integration method used for the MCM gas phase chemistry was a Gear-type solver of ordinary differential equations (FACSIMILE 3.0), and the integration time step is 10 min, which is determined by the gas/particle partitioning process at every 10 min. A CPU time of 4 min per simulation day is needed on a Linux-based two-processor i686 workstation, and the gas/particle partitioning process takes 90% of the entire computational timing. [41] In the model, a time step of 10 min is used for the gas/ particle partitioning process during a 3-D simulation. This is different from the time step of 2 min used for smog chamber studies in section 3.2. Sensitivity tests of the time step also showed that the total SOA mass has an error less than 5% between the time steps of 10 min and those of 2 min. 4. Model Results and Discussions [42] The model for each scenario was run for 72 h (3 d) from midnight. The first day is used to spin up the chemistry. Following a similar method as in work by Whitehouse et al. [2004], the concentrations (or mixing ratios) of the compounds in the system are sampled on the last 48 hourly points and are used to analyze SOA formation. [43] For some scenarios, the O 3 mixing ratio is as high as 250 ppbv, and the NO x can reach 120 ppbv in other scenarios. The high concentrations for O 3 and NO x are partly due to the fact that nonchemical loss processes are not included in the model simulations. Model results from the 243 scenarios were therefore screened to remove those conditions unlike the ambient atmosphere, following four criteria: (1) maximum ozone mixing ratio should not exceed 200 ppbv, (2) maximum NO mixing ratio should not exceed 40 ppbv, (3) maximum NO 2 should not exceed 40 ppbv, and (4) maximum total organic aerosol should not exceed 100 g m 3. A total of 108 scenarios remained for further analysis after this screening step. All the criteria are based on ambient measurements in the urban, rural, and forest areas. [44] In order to describe the main or bulk features of the compounds from the 108 scenarios, the concentrations for all compounds have been averaged over the selected 48 h. The analysis which follows thus describes typical conditions during the 48 h period and eliminates short-term variability related to the diurnal forcing of the simulation. 7of19

8 Figure 4. (a) The formation of SOA depends strongly on the averaged HC/NO x ratios. (b) Averaged total SOA mass concentrations from the 108 selected scenarios. This selected 9 scenarios for the SOA formation. Figure 4b is a zoomed-in view of Figure 4a Model Results of the -Pinene, NO x, and Total SOA [45] Figures 3 and 4a show the summary of the averaged concentrations of gas phase compounds (-pinene and NO x ) and average SOA mass from the 108 selected scenarios, respectively, as a function of the ratio between average -pinene concentrations (HC in the figures of this paper refers to the concentration of the SOA precursor gas, -pinene) and average NO x. [46] A substantial difference in the simulated total SOA can be seen, depending on the initial conditions. In Figure 3, when the averaged -pinene concentration is as low as 0.19 ppbv and the NO x concentration is around 10.8 ppbv, the calculated HC/NO x ratio is (ppbvc/ppbv). For that particular case, the averaged total SOA formed is as low as 1.50 g m 3, shown in Figure 4a, because -pinene concentration is quite low. Alternatively, when -pinene concentration reaches 1.49 ppbv and the NO x concentration is 1.76 ppbv, the calculated HC/NO x ratio is 8.43 (ppbvc/ppbv). In this situation, the total SOA can reach as high as 70 g m 3. The averaged total SOA varies from 1.6 to 70 g m 3 for the 108 scenarios with different average HC/NO x ratios in Figure 4a. [47] Next, three scenarios in Figure 4a are selected for detailed analysis. Most of the conditions in the three scenarios, represented as squares in Figure 4b, are the same. They have the same initial conditions of O 3 (50 ppbv) and -pinene (4 ppbv), the same emission rates of -pinene ( e 0.09 (Temp 303) molecules s 1 ) and NO x ( molecules s 1 ), and the same profiles for the temperature and relative humidity, shown in Figure 2. The 8of19

9 Figure 5. Changes of nitrates, PANs, acids, and ROOHs with different initial concentrations of the NO x (a) in gas phase, (b) in aerosol phase, and (c) in both phases. only difference between the three scenarios is the different initial conditions of the NO x. [48] In Figure 4b, for scenario 1, the solid black square (with initial O 3 at 50 ppbv) has an initial NO x of 20.0 ppbv, and the total SOA formed from this situation is g m 3 with average HC/NO x ratio at 3.72 (ppbvc/ppbv). This may be contrasted with the high HC/NO x for scenario 3, and the same initial O 3 and an initial NO x of 1.3 ppbv forms much more total SOA, g m 3, with an average HC/ NO x ratio of 8.43 (ppbvc/ppbv). When the HC/NO x is very high, the chemical mechanism leads to the formation of more SOA mass. Similar results can be found for two other initial concentrations of O 3 (70 or 90 ppbv). [49] The effect of changes in SOA formation with changes in HC/NO x has been observed during several smog chamber studies in describing the oxidation of m-xylene [Odum et al., 1996; Song et al., 2005], oxidation of toluene [Hurley et al., 2001; Sato et al., 2004], and the ozonolysis of -pinene [Presto et al., 2005]. The effect results from the competing branching among NO x +RO 2,HO 2 +RO 2, and RO 2 +RO 2, with the latter two dominating in high HC/NO x ratios. A detailed analysis of this effect on SOA formation for the 50 ppbv ozone initial conditions follows. [50] At high NO x concentrations, NO x + RO 2 is the dominant path for the loss of RO 2, which leads to the generation of more nitrates and peroxynitrates. When the NO x concentration is low, the importance of the NO x + RO 2 path diminishes, and the production of nitrates and peroxynitrates decreases, while the chemical path for HO 2 + RO 2 becomes more important. This leads to the formation of more condensable organic peroxides (collectively, ROOHs) and organic acids from the gas phase oxidation of -pinene. The total mass (unit: g m 3 ) of the compounds in both gas and aerosol phases are shown by the summation of all ROOHs and acids in Figure 5c. [51] It is interesting to note that the relative proportion of ROOHs in the aerosol phase increases faster with decreasing NO x than the amount in the gas phase or the total across both phases. The additional organic aerosol at low NO x conditions is mostly due to increased ROOHs content in the aerosols, nitrates and peroxynitrates having a much smaller impact. The acids follow a similar trend as the ROOHs for SOA formation. The particle phase nitrates and peroxynitrates increase despite decreasing gas phase concentrations because of the acid and ROOHs increases; these latter species increase the total suspended particulate matter (TSP), in turn increasing the partitioning to the aerosol phase for the other compound groups. Similar findings have been noted by Bowman and Karamalegos [2002]: higher TSP leading to a positive feedback between individual SOA formation and the total SOA mass. Here the large increase in particle phase organic peroxides and acids at lower NO x conditions lead, through TSP increases, to increased particle phase partitioning from the nitrate and peroxynitrates groups. 9of19

10 Figure 6. Organic aerosol mass compositions for the four chemical groups at different HC/NO x ratios, showing (a) relative concentrations and (b) absolute concentrations. [52] To explain the NO x effect on the SOA formation, Presto et al. [2005] recently found that more volatile products were observed under high NO x conditions from smog chamber experiments. Here we suggest that the feedback between the total condensed phase and partitioning may also come into play: a reduction in the formation rate of some less volatile products (ROOHs and acids) changing the mass of substrate, hence changing partitioning of other species, beyond what would be expected from changes to the product volatility alone. Less ROOHs and acids are generated in both the gas and aerosol phases at higher NO x conditions, and this leads to decreases in nitrate and peroxynitrates partitioning to the aerosol phase, despite increases in the total mass of these species. [53] It is very important to note that the NO x effect on the SOA formation depends strongly on the structure of the precursors: High NO x does not necessarily lead to low SOA formation. For example, Zhang et al. [2006] recently found 10 of 19

11 Figure 7. The names, structures, formula, estimated boiling points (K), and the median aerosol mass fraction for the selected 28 condensable compounds. that more SOA are formed at high NO x conditions during ozonolysis of limonene because the gas phase oxidation of the exocyclic double bond dominates for that compound under high NO x conditions. [54] The initial concentration of O 3 affects the resulting HC/NO x in Figure 4b. Two different initial conditions of O 3 at 50 and 70 ppbv are compared here. When initial concentration of NO x is very high (20 ppbv), the additional 20 ppbv of initial O 3 increases the HC/NO x ratio by consuming more NO x via the reaction (NO 2 + O 3 ). In contrast, when the initial concentration of NO x is low (1.3 ppbv), the additional 20 ppbv of O 3 decreases the HC/NO x ratio by -pinene ozonolysis (O 3 + -pinene). Interestingly, no significant change is observed from changes in O 3 to the HC/NO x ratio when the initial NO x is 10.7 ppbv because of the balance between the above two reaction pathways. [55] In smog chamber experiments, sometimes the HC/ NO x ratio may be very small. For example, the initial HC/ NO x ratio is as low as 0.88 (ppbvc/ppbv) during the oxidation of toluene in work by Stroud et al. [2004]. It should be noted that the ratio of HC/NO x usually refers to the ratio at the starting time of smog chamber experiments in the literature. Here we suggest that average ratios may be a more appropriate metric for describing chamber conditions because of the variations in particle composition that may result from a similar set of initial conditions and because of the rapid change in HC/NO x that may occur during the course of a single chamber experiment. [56] From the above analysis, it can be seen that substantial increases in SOA formation may occur at low NO x concentrations, when the HC/NO x ratio ranges from to Extrapolation of aerosol yields from smog chamber simulations with a limited HC/NO x range [Odum et al., 1996, 1997; Griffin et al., 1999] may therefore result in underestimates or overestimates of aerosol mass at other HC/NO x conditions Investigation of Organic Aerosol Mass Composition by Species Groups [57] Next, we analyze the change of organic aerosol mass composition for the suite of 108 test cases. Figures 6a and 6b display the organic aerosol mass composition for the four groups (peroxynitrates, nitrates, organic peroxides, and acids) at different HC/NO x ratios. As in the analysis in section 4.1, of the four groups, the group of ROOHs is the dominant one when the HC/NO x ratio is larger than 1.0. [58] The importance of the ROOHs has been seen in smog chamber experiments and the model studies. Docherty et al. [2005] noted the dominance of the organic peroxides in the SOA formed from the ozonolysis of several monoterpenes and estimated that the organic peroxides could represent 47% of the total SOA during -pinene oxidation, and the fraction could be as high as 85% for -pinene oxidation. [59] In an earlier global model study, Bonn et al. [2004] used a simplified -pinene oxidation scheme to model SOA formation. The results showed that the calculated organic peroxides constitute 63% of the global SOA formation. [60] In our model studies, the fraction of the ROOHs in the SOA generated from -pinene oxidation is 7.5% at a low HC/NO x ratio of (ppbvc/ppbv). But the percentage of the organic peroxide reaches as high as 55% at a high 11 of 19

12 Figure 8. (a) Aerosol mass fractions of different compounds and (b) their cumulative aerosol mass fractions at one time point. The selected time point corresponds to 1200 LT (noontime) of the third-day model simulation for the scenario with HC/NO x at ppbvc/ppbv. HC/NO x ratio of 8.43 (ppbvc/ppbv). Organic acids follow a similar chemical path as the ROOHs, with the acids constituting about 4.5% of the total SOA at the low HC/NO x ratio and constituting about 20% at the high HC/NO x ratio. [61] In contrast, the mass fraction of the peroxynitrates decreases from 70 to 21% when the HC/NO x ratio increases from to 8.43 (ppbvc/ppbv). The nitrates follow the pattern of the peroxynitrates when the HC/NO x changes, with decreases from 18 to 2.5% when the HC/NO x ratio increases from to 8.43 (ppbvc/ppbv). In particular, Figure 6a shows that the decline of the relative concentrations of PANs and nitrates with increasing HC/NO x is due to the absolute concentrations of the organic peroxides and acids going up in Figure 6b. [62] Our simulations suggest that the nitrates and peroxynitrates dominate the SOA composition at lower HC/NO x ratios and organic peroxides and acids dominate the SOA mass at higher HC/NO x ratios. Further smog chamber experiments are needed to confirm these predictions, particularly for the thermally unstable peroxynitrates and nitrates. The effects of the HC/NO x on total SOA formation were identified previously by several smog chamber experiments [Odum et al., 1996; Hurley et al., 2001; Sato et al., 2004; Song et al., 2005; Presto et al., 2005]; here we have shown that these changes are accompanied by significant changes in the types of compounds within the organic aerosol Investigation of Aerosol Composition by Individual Product Species [63] Among the 149 condensable compounds examined here, some of the compounds always have a very high mass fraction in the aerosol phase, while many have a negligible aerosol mass fraction. In this section, we determine the dominant condensable compounds for the SOA formation during the oxidation of -pinene under a wide range of conditions. [64] First, we investigate the aerosol mass fraction for each of the 149 condensable compounds at every sampled time point of the 108 simulations in section 4. Figure 7 shows the names, molecular structures, and formula of selected condensable compounds. The 149 organic compounds are ranked, in descending order, according to their organic aerosol mass fraction at an example time point in Figure 8a (only the top 20 compounds are shown). The selected point corresponds to 1200 LT (noontime) of the third-day model simulation for the scenario with HC/NO x at ppbvc/ppbv. The corresponding cumulative aerosol mass fractions were calculated and are presented in Figure 8b. In this example, the first 14 compounds (up to C811CO3H) constitute 85% of the SOA mass. This mass cutoff of 85% is used as an arbitrary threshold to select the 12 of 19

13 Figure 9. Box plot of the organic aerosol mass composition for each of the 28 condensable compounds at the 5184 sampled time points. dominant SOA-forming compounds in this study. Therefore 14 compounds are selected as the important condensable compounds for this time point. The procedures to select important compounds are repeated at all 5184 sampled time points (5184 sampled time points = 48 h scenario scenarios). [65] The union of the selected dominant compounds from all sampled time points is analyzed on the basis of their frequencies (>96%) and the values of aerosol mass fractions (>0.002) during the 5184 time points. As a result, 28 organic compounds have been selected as important condensable compounds within the system of -pinene oxidation in this work. The selected 28 compounds, in fact, cover 87 94% of the total SOA mass across all simulations. [66] Before the discovery of the oligomers existing in the aerosol phase [Jang et al., 2002; Tolocka et al., 2004; Kalberer et al., 2004; Liggio et al., 2005a; Presto et al., 2005], Seinfeld and Pandis [1998] suggested that the compounds with more than six carbons have the potential to form SOA. Of the 28 condensable compounds in this 13 of 19 -pinene oxidation system, without considering the formation of oligomers, 27 compounds have carbon numbers larger than six, with the exception of H1C23C4PAN, a five-carbon compound. [67] Among the 28 compounds, 14 compounds are organic peroxides, and these ROOHs can cover almost half of the total SOA mass at high HC/NO x conditions, in accord with Docherty et al. [2005]. Seven ROOHs were also identified as significant compounds [Yu et al., 1999b; Jenkin, 2004] during the smog chamber studies in the ozonolysis of -pinene. The 14 organic peroxides identified here have different carbon numbers: one C7, two C8, four C9, and seven C10 peroxides. The abbreviated names, molecular structures, formula, and estimated boiling points of the 28 compounds are listed in Figure 7. [68] Figure 9 shows the relative importance of the individual 28 compounds via the aerosol mass fraction of the compounds at all 5184 time points. C920PAN and C811PAN (both peroxynitrates) are the top two dominant SOA compounds in terms of frequency of occurrence, with

14 Figure 10. Aerosol mass fraction of the 28 dominant condensable compounds at 5 different scenarios relative to (a) total SOA mass and (b) respective functional group. a mass fraction in the range of 5 to 35%. The estimated boiling points for the two compounds reach as high as 621 and 628 K, respectively. The frequencies of occurrence of organic peroxides are more distributed, with many species being equally likely to contribute to the organic aerosol mass. Finally, median aerosol mass fractions relative to total SOA mass with each compound for the 5184 points are also listed in the last column of Figure 7, in which nine compounds are identified to have median aerosol mass fraction larger than The nine compounds include five ROOHs (C812OOH, C813OOH, C921OOH, C922OOH, and C98OOH), one acid (PINONIC), and two PANs (C811PAN and C920PAN). [69] In order to study the effect of HC/NO x ratios on individual compounds, we have selected five scenarios, shown in Figures 10a and 10b, with the HC/NO x ratios of 0.19, 0.42, 1.00, 2.14, and In Figure 10a, most of the individual compounds follow the same pattern indicated in 14 of 19

15 Figure 10. (continued) Figure 6a for the compounds in that group: The aerosol mass fractions for the nitrates and peroxynitrates decrease with the HC/NO x ratio; the organic peroxide and acid mass fractions increase as the HC/NO x ratio increases, with the exception of the four compounds (C922OOH, C98OOH, NAPINAOOH, and NC102OOH). The four exceptions, especially for the NAPINAOOH and NC102OOH, reach their maximum aerosol mass fractions at an intermediate HC/NO x ratio because NO x affects the formation of the NAPINAOOH and NC102OOH in two ways: introducing the nitrogen-containing VOC compounds and controlling the two different paths between RO 2 +NO x and RO 2 +HO 2. [70] Figure 10b shows the fractions of the compounds relative to their chemical group, not to the total SOA mass. Now it is much evident to see the contributions of the compounds vary within each group. For example, within the 15 of 19

16 ROOHs group, the contributions of NAPINAOOH and NAPINBOOH decrease dramatically when the HC/NO x ratio increases from 0.19 to 8.43 (ppbvc/ppbv). Moreover, PINONIC is the most dominant compound within the acids group. [71] Interestingly, fractions relative to the total SOA mass with the seven compounds from the PANs group, shown in Figure 10a, decrease with the HC/NO x ratio. The fractions relative to the PANs group, shown in Figure 10b, also decrease with the HC/NO x ratio, except C811PAN. This is because C811PAN is less sensitive to the change of the HC/ NO x ratio than the rest six compounds from the PANs group Implications for the Ambient Atmosphere [72] In the real atmosphere, the chemistry is more complicated than that from the box model studied here because of the large number of other SOA precursor organic compounds that may exist. To date, only 10 15% of the organic mass has been resolved on the molecular level [Rogge et al., 1993; Saxena and Hildemann, 1996]. The chemical and physical processes [Boudries et al., 2004; Kanakidou et al., 2005] for the aerosol formation are complex and include chemical reactions within aerosol particles and the formation of the oligomers [Baltensperger et al., 2005; Liggio et al., 2005b], etc. [73] The concentrations for the compounds of interest in the gas and aerosol phases are of great concern. [Yu et al., 1999a; Leaitch et al., 1999]. For example, the -pinene concentration could reach as high as 1.5 ppbv in Canadian [Leaitch et al., 1999; Cheng et al., 2004] and German forests [Spirig et al., 2005]. NO x concentration has a large variation from 20 pptv in remote forest areas [NRC, 1991; Leaitch et al., 1999] up to 200 ppbv in urban area in Mexico City [Shirley et al., 2005]. The test range chosen in this work is relevant for ambient atmospheric chemistry. [74] Our findings suggest the composition of secondary organic aerosol from -pinene may change with distance from the urban sources and advection into surrounding rural areas. More nitrates and peroxynitrates will be formed in urban source regions where NO x sources are strong, and more organic peroxides and acids will be formed in the countryside with increasing distance from the NO x source and increasing influence of local monoterpene emissions. [75] We would like to point out that the actual chemical composition could be different from the model results presented here because the application of Pankow s [1994a] absorptive partitioning theory could be inadequate to describe ambient SOA formation with current SVP estimation methods. For example, the formation of the oligomers in ambient atmosphere [Baltensperger et al., 2005] could modify actual chemical compositions of the organic aerosols. 5. Summary and Conclusions [76] In this work, the subset of the explicit chemical mechanism of MCM for -pinene oxidation has been first evaluated against a series of smog chamber experiments. Interestingly, our model results are very close to observations either at low temperature or with high-soa mass loading. Next, the model has been studied extensively for the formation of SOA under a wide range of conditions. Generally, two dominant processes affect the SOA formation: the gas/particle partitioning process and chemical reactions of precursors. [77] The purely absorptive partitioning theory from work by Pankow [1994a] was adopted to describe the gas/particle partitioning for 149 condensable compounds in the system. In order to predict SVP for different functional condensable compounds, the KF method is used. [78] Chemically, the HC/NO x ratio is the critical factor controlling the chemical path of the condensable products. At higher HC/NO x, the chemical reaction of HO 2 +RO 2 is important for the formation of condensable organic peroxides and acids. In contrast, when the HC/NO x is lower, the chemical reaction of the NO x +RO 2 dominates, and the important condensable products are the peroxynitrates and the nitrates. [79] When the condensable compounds are lumped into four groups based on their chemical structures, the aerosol mass fraction from each group depends strongly on the HC/ NO x ratio. Specifically, a roughly linear relationship exists between the aerosol mass fraction and the logarithm of HC/ NO x ratios. [80] Although the effects of the HC/NO x on total SOA formation were identified previously by several smog chamber experiments, this is the first work to systematically model the HC/NO x effect on not only the total SOA mass but also on organic aerosol composition. Increasing NO x levels lead to a reduction in the formation of organic peroxides and acids, in turn reducing the total suspended particulate matter that may act as a substrate for the absorptive partitioning of other condensable species. A larger fraction of the total nitrate and peroxynitrate mass remains in the gas phase at high NO x concentrations as a result of this decrease of substrate mass. [81] Chemical compositions in ambient aerosols could be different from what we reported here for the following reasons. In the first place, although the MCM is near explicit, it ignores some minor reaction routes that might possibly yield low-volatility products that may make nonnegligible contributions to SOA, and it uses a greatly simplified representation of reactions of secondary products such as organic acids and peroxides. This might have a nonnegligible effect on the distribution of the low vapor pressure products predicted to be formed, particularly those with multiple functional groups that presumably come from secondary or tertiary reactions. In the second place, the test range chosen in the work, while relevant for ambient atmospheric chemistry, does not include all possible initial conditions. The range of HC/NO x conditions employed here is only one part of a potentially much larger range in the ambient atmosphere, and a further investigation of a larger set of starting conditions is recommended for future investigation. Our conclusions are limited to the range described. In the third place, the gas/particle absorptive partitioning theory and gas phase chemical reactions are not entirely responsible for SOA formation in ambient atmosphere. Other missing processes, such as the heterogeneous reactions for the organic aerosols, compounds interactions within the aerosols, and aerosol dynamics, are missing from current model. In the fourth place, the estimation method for the SVP of the condensable organic compounds could affect 16 of 19