Influence of Ozone and Radical Chemistry on Limonene Organic

Article pubs.acs.org/est

Influence of Ozone and Radical Chemistry on Limonene Organic Aerosol Production and Thermal Characteristics Ravi K. Pathak,† Kent Salo,†,§ Eva U. Emanuelsson,† Cilan Cai,† Anna Lutz,† Åsa M. Hallquist, and Mattias Hallquist*,†

†,‡,⊥

†

Atmospheric Science, Department of Chemistry and Molecular Biology, University of Gothenburg, S-412 96 Gothenburg, Sweden IVL Swedish Environmental Research Institute, P.O. Box 53021, S-400 14 Gothenburg, Sweden

‡

S Supporting Information *

ABSTRACT: Limonene has a strong tendency to form secondary organic aerosol (SOA) in the atmosphere and in indoor environments. Initial oxidation occurs mainly via ozone or OH radical chemistry. We studied the effect of O3 concentrations with or without a OH radical scavenger (2-butanol) on the SOA mass and thermal characteristics using the Gothenburg Flow Reactor for Oxidation Studies at Low Temperatures and a volatility tandem differential mobility analyzer. The SOA mass using 15 ppb limonene was strongly dependent on O3 concentrations and the presence of a scavenger. The SOA volatility in the presence of a scavenger decreased with increasing levels of O3, whereas without a scavenger, there was no significant change. A chemical kinetic model was developed to simulate the observations using vapor pressure estimates for compounds that potentially contributed to SOA. The model showed that the product distribution was affected by changes in both OH and ozone concentrations, which partly explained the observed changes in volatility, but was strongly dependent on accurate vapor pressure estimation methods. The model−experiment comparison indicated a need to consider organic peroxides as important SOA constituents. The experimental findings could be explained by secondary condensed-phase ozone chemistry, which competes with OH radicals for the oxidation of primary unsaturated products. and an exocyclic terminal double bond.13 These are responsible for limonene’s high tendency to form SOA because both double bonds can be oxidized with little or no carbon loss, generating polar, water-soluble products. Although some products have been identified from limonene ozonolysis, SOA formation via the reaction of limonene with ozone is complex and clearly needs further work. One important issue is the role of ozone and radical chemistry in the thermodynamic properties of SOA.14−24 Recently, we reported that gas-phase OH radical reactions can age SOA by converting semivolatile organic compounds (SVOCs) into low-volatile organic compounds (LVOCs).25 These findings are in agreement with those of Robinson et al.,26 who showed that a significant amount of SOA can be produced by gas-phase oxidation of emitted intermediate volatile compounds (IVOCs), producing either SVOCs or LVOCs. In the present study, the effects of ozone concentration and associated radical chemistry on the volatility of SOA from limonene oxidation were investigated in a laminar flow reactor under humid and dry conditions. The results were compared to chemical kinetics simulations using the mechanisms proposed by Li et al.19

1. INTRODUCTION Organic aerosol is the least understood component of the atmospheric aerosol, which adversely affects human health and influences the earth’s radiation balance through direct and indirect effects.1,2 A major fraction of the atmospheric organic aerosol is secondary organic aerosol (SOA), contributing up to 80% of the total organics.3,4 SOA is produced from the oxidative processing of volatile organic compounds (VOCs) in the atmosphere, but the mechanism requires further elucidation.2,5,6 Monoterpenes (C10H16) are one class of VOCs that have attracted large interest due to their large global emission and ability to form a high number of condensable products. 7 The total global monoterpene emissions have been estimated in several studies and range from 30 to 127 Tg C yr−1,8,9 with limonene comprising up to 20% of the total.10 Limonene is also the main component of the essential oil extracted from citrus rind and has extensive industrial uses. In particular, it has become a popular ingredient in household products, specifically citrus-scented air fresheners and cleaners. The use of limonene in these products makes it of special concern regarding indoor exposure because high concentrations of limonene and coexisting ozone are wellknown to produce SOA, which can cause pulmonary irritation.11,12 From a chemical point of view, limonene chemistry is interesting since it has two very different unsaturated bonds: an endocyclic trisubstituted double bond © 2012 American Chemical Society

Received: Revised: Accepted: Published: 11660

May 2, 2012 September 10, 2012 September 17, 2012 September 17, 2012 dx.doi.org/10.1021/es301750r | Environ. Sci. Technol. 2012, 46, 11660−11669

Environmental Science & Technology

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Table 1. Summary of Experimental Conditions with Measured VFR383K and SOA Massa initial conditions experiment 1 2 3 4 5b 6b 7b 8b 9d 10e 11b,e

[limonene] (μg m−3) 85.5 ± 4.5 85.5 ± 4.5 85.5 ± 4.5 85.5 ± 4.5 85.5 ± 4.5 85.5 ± 4.5 85.5 ± 4.5 85.5 ± 4.5 85.5 ± 4.5 85.5 ± 4.5 434 ± 21.7 85.5 ± 4.5 434 ± 21.7

experimental result

[O3] (μg m−3) 1200 3900 6900 9800 1200 3900 6900 9800 9800 3900

± ± ± ± ± ± ± ± ± ±

40 160 200 390 40 160 200 390 390 160

3900 ± 160

SOA mass (μg m−3) 3.4 ± 0.3 14.7 ± 0.3 18.2 ± 0.3 19.0 ± 0.4 16.0 ± 1.5 26.3 ± 0.4 28.6 ± 0.4 29.5 ± 0.4 9.0 ± 0.3 10.8 ± 1.0 123.9 ± 2.0 20.9 ± 1.0 131.0 ± 3.1

model result

VFR383K 0.410 0.460 0.475 0.510 0.467 0.464 0.459 0.463 0.557 0.486 0.523 0.498

± ± ± ± ± ± ± ± ± ± ± ±

0.003 0.003 0.004 0.004 0.003 0.003 0.004 0.004 0.004 0.004 0.004 0.005

Δ[limonene]c (μg m−3)

SOA mass (μg m−3)

Peroxides (μg m−3)

45.5 78.3 83.9 84.7 51.8 81.0 84.4 84.8 84.7

11.3 28.3 31.2 31.3 11.0 24.8 27.1 27.5 27.7

0.01 0.01 0.01 0.01 5.95 7.20 6.20 5.42 5.57

a

Also shown are the corresponding model simulation results on limonene consumed (Δ[limonene]) and SOA mass. All experiments were performed at 298 K, and the OH scavenger concentration was sufficiently high (∼3 × 105 μg m−3) to reduce OH reactions with the precursor compound by more than 95%. All experiments were done at humid conditions (40 ± 1% RH) with the exception of experiment 9 (5 ± 1% RH). b Experiments without OH scavenger; all other experiments used 2-butanol as the OH scavenger. cAmount of limonene reacted in the respective experiment estimated by the model. dDone at low humidity, 5 ± 1% RH. eDone in separate campaign with higher temperature, T ≈ 300 K, and lower atmospheric pressure.

2. EXPERIMENTAL SECTION Experiments were conducted in the Gothenburg Flow Reactor for Oxidation Studies at Low Temperatures (G-FROST). G-FROST has been used in several earlier studies on SOA formation processes, e.g., to study the effect of relative humidity (RH), temperature, and use of a OH scavenger on the ozonolysis of monoterpenes.22−24 Briefly, G-FROST is an experimental facility that consists of a vertical laminar flow reactor (191 cm long and 10 cm i.d.) housed in a temperaturecontrolled room with a working temperature range of 238−323 K. At the end of the flow reactor, there is a centered sampling funnel, which means that only the central part of the laminar flow is conveyed to the analytical instruments, while the excess flow is passed to a vent to reduce the effect of the slower flow close to the reactor walls. Mixing of the reactants was achieved using an injector equipped with a mixing plunger, ensuring rapid homogeneous mixing of the precursors and oxidant. Directly behind the mixing point, a stainless steel mesh was used to rapidly distribute the flow radially over the cross section of the cylinder, generating a laminar flow profile. The injector carrying the oxidant (ozone) consisted of a 6 mm Teflon tube supported by a stainless steel hose. The gaseous organic precursor limonene, with or without the OH scavenger (2-butanol), was carried within a humidified bulk flow and delivered to the top of the flow reactor. In this set of experiments, the aerosol was characterized with regard to number size distributions and thermal properties. Size distributions were measured using a scanning mobility particle sizer (SMPS) system, while thermal characterization was performed using a volatility tandem differential mobility analyzer (VTDMA). The VTDMA consisted of two differential mobility analyzers (DMA1 and DMA2) in tandem separated with a set of controlled heating ovens mounted in parallel, enabling a swift change of the desired oven.27,28 The sample flow was 0.3 SLPM (standard liters per minute; 298 K and 1 atm), giving a residence time in the heated part of the oven of 2.8 s, calculated assuming plug flow. A size-selected monodisperse aerosol (e.g., 55 nm) using DMA1 was fed to the heated ovens (298−493 K). The volume fraction remaining

(VFRT) of the aerosol was subsequently derived by measuring the modal diameter (Dp), using DMA2 and a condensation particle counter (CPC), at both the evaporative temperature (T) and reference temperature of 298 K (Dp298K), i.e., VFRT = (DpT/Dp298K)3. By making measurements at several evaporative temperatures, we were able to map the thermodynamic properties of SOA formed under selected conditions in G-FROST. It should be noted that the volatility measured using a VTDMA is based on evaporation where recondensation is avoided, i.e., nonequilibrium conditions.28 Formation of SOA was investigated under the experimental conditions summarized in Table 1. The average reaction time was kept constant (238 s), and the total flow in the system was 1.6 SLPM, 0.94 SLPM of which was conveyed via the sampling funnel. An SOA density of 1.4 g cm−3 was used in the calculation of SOA mass from number size distributions, as recommended in the review by Hallquist et al.2 The RH was 5% for dry conditions and 40% for humid conditions. Usually a stable aerosol was obtained about 45 min after new conditions were set. However, before detailed characterization was carried out, the system was allowed to equilibrate for 2−3 h for each new condition. This allowed slower processes that could be compound specific to equilibrate. A major general concern in all types of SOA experiments is wall effects. In the G-FROST experiments, wall effects were reduced by three measures. First, characterization was only performed in the central part of the flow. Second, the wall was coated with halocarbon wax to reduce uptake of radicals, oxidants, and water. Third, a long equilibrium time was used to ensure reduced net uptake of products. In this study, the focus was to investigate the effect of the ozone concentration in the absence or presence of a OH scavenger (2-butanol) under humid conditions. The amount of limonene consumed for each experimental condition was calculated on the basis of a chemical model presented by Li et al.19 and the initial concentrations used as shown in Table 1. The model calculations also provided extensive data on the amount of SOA produced and concentrations of intermediates or end products during limonene ozonolysis, i.e., from the time 11661

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of mixing until a time corresponding to the residence time in G-FROST. The model results were used to interpret the experimental results by comparing the aerosol mass and its thermal characteristics.

3. RESULTS Table 1 summarizes the experimental conditions, SOA mass, and thermal properties measured at 383 K, i.e., the VFR383K, together with the model results. For experiments 1−8, oxidation of limonene at an initial concentration of 85.5 μg m−3 (15 ppb) led to an SOA mass of 3.4−29.5 μg m−3, with the fraction of limonene consumed ranging from 53% to 99%. The consumed fraction of limonene is related to the residence time and oxidant concentration. In the presence of a OH scavenger, the lifetime of limonene is only related to the lifetime with respect to the ozone reaction (τlimonene+ozone = 1/([ozone]×kozone+limonene)). In the absence of a OH scavenger, the consumed fraction of limonene is also influenced by the OH concentration, which varies within the flow profile of the flow reactor. To account for additional OH chemistry, the model described by Li et al.19 was used to calculate the amount of consumed limonene given in Table 1. According to the model, 20% of limonene reacted with OH radicals. The model also provided time profiles of peroxy radicals (HO2 and RO2). In previous studies it has been demonstrated that HO2 and RO2 have a significant effect on SOA formation.24,29,30 The calculated fraction of HO2 of all peroxy radicals decreased with time in all experiments without scavenger, while the fraction increased in the scavenger experiments. This made it more complex to evaluate the SOA dependence on the HO2 to RO2 ratio. However, as described below, the formation of peroxides demonstrates the importance of the RO2 + HO2 reaction for SOA formation. In experiments 10 and 11, the ozone concentration was fixed, while the initial limonene concentration was steadily reduced from high (434 μg m−3, ∼78 ppb) to low (85.5 μg m−3, ∼15 ppb) over 24 h. Influence of Water, Ozone, and Radicals. In our previous studies in the G-FROST facility, we demonstrated that water had an effect on the produced mass and number of SOA.22−24 This effect was attributed to changes in the chemical mechanism. To further test if water also affects the thermal properties of the produced aerosol, one experiment conducted at low relative humidity was included in the present study (experiment 9). A clear change in VFR between dry and humid conditions was observed (Figure 1a). This may be due to adsorbed water on the SOA entering the VTDMA setup. However, it is known that this type of aerosol has a relatively low water-uptake efficiency,31 and most of the water effect, where the humid aerosol is more volatile than the dry, has been attributed to previously described changes in the chemical mechanism.22,32 This water effect on VFR is analogous to observations from AIDA (Aerosol Interactions and Dynamics in the Atmosphere) chamber experiments, which showed that fresh initial particles produced under humid conditions were more volatile than those in the corresponding dry experiment.27 The effect of water on volatility was also observed by Lee et al.33 using a thermodenuder system with a relatively long residence time of 16 s to allow more evaporation at low temperatures.34,35 In the first four experiments (experiments 1−4) listed in Table 1, the ozone concentration was increased from 1200 μg m−3 (∼600 ppb) to 9800 μg m−3 (∼5000 ppb) in the presence of 2-butanol as OH scavenger. The resulting VFRs

Figure 1. VFR at selected evaporation temperatures. The corresponding particle number size distributions are shown as insets. (a) VFR under humid (40% RH) and dry (5% RH) conditions in the presence of OH scavenger and 9800 μg m−3 ozone (experiments 4 and 9). (b) Experiments with 2-butanol as the OH scavenger, experiments 1−4. (c) Experiments without OH scavenger, experiments 5−8.

for all evaporative temperatures (298−493 K) and final size distributions are shown in Figure 1b. VFR at all temperatures was dependent on the ozone concentration used. The tabulated VFR383K values presented in Table 1 increase from 0.41 to 0.51 and are consistent with the modeled increased oxidation of the compounds contributing to the condensed phase. More limonene was also consumed at elevated ozone concentrations, and the resulting SOA mass increased from 3.4 to 19 μg m−3. It is notable that, at the two highest ozone concentrations, the small difference in consumption of limonene (0.8 μg m−3) correlates with a significant SOA production (0.8 μg m−3), which must involve conversion of first-generation products with a remaining double bond into second-generation products with lower vapor pressures. The number size distributions indicate that increased ozone concentrations also enhance nucleation, with a higher number peak shifted toward smaller diameters. In the high ozone concentration experiments, the initial rate of reaction is high, and thus, more nucleating low-volatile 11662



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products are produced per unit of time, reaching critical supersaturation and creating more critical clusters that can subsequently form particles. Figure 1c shows that in the absence of OH scavenger (experiments 5−8), VFR hardly changed with increasing ozone concentration for any of the evaporative temperatures. In contrast, the experiments with scavenger clearly showed an increase in VFR with increasing ozone concentration (Figure 1b). Increasing the ozone concentration affected the number size distributions both with and without scavenger: the peak shifted to smaller size, indicating faster or higher production of nucleating low-volatile products. Comparing the size distribution without scavenger (Figure 1c) with those in the presence of scavenger (Figure 1b), the shift in the peak toward smaller sizes is clearly more pronounced in the absence of scavenger. In the nonscavenger experiments, the oxidation of limonene may be enhanced due to reaction with OH radicals. Furthermore, OH radicals may react with some of the products, resulting in secondary oxidation, e.g., as described by Salo et al.25 The difference in the number of particles with and without scavenger can thus be attributed to OH chemistry, which enhances the rate of production of the nucleating compounds. In addition, the total mass of SOA produced without OH scavenger is higher, ranging from 16.0 to 29.5 μg m−3 for low and high ozone concentrations, respectively. This is partly due to more limonene being converted, but this minor change does not explain the large increase in SOA mass observed. Instead, the latter can be attributed to more efficient oxidation of the first-generation products, generating a larger amount of lowvolatile products. However, when comparing VFR383K for experiments 1−4 and experiments 5−8, values in the absence of scavenger are comparable to those with scavenger. This is rather surprising given that more oxidation takes place in the nonscavenger case. An explanation is that the increased oxidation does not primarily convert SVOCs to LVOCs but rather VOCs or IVOCs to SVOCs. To elucidate this further, the system was chemically modeled using the extensive kinetic mechanism proposed by Li et al.19 (see the modeling section) and two additional types of experiments were conducted. In experiments 10 and 11, the initial limonene concentration was steadily decreased from 434 μg m−3 (∼78 ppb) to 85.5 μg m−3 (∼15 ppb) with or without scavenger in the presence of 3900 μg m−3 (∼2000 ppb) ozone. This allowed continuous measurement of VFR383K over an extensive range of SOA mass (Figure 2). In the experiments without scavenger, the VFR383K was, within experimental scatter, not influenced by the decreasing initial concentration of limonene. In contrast, the experiment using OH scavenger showed a steady decrease in VFR383K with decreasing SOA mass. Parts b and c of Figure 1 compare the systems with and without OH scavenger for four ozone concentrations. For the SOA produced with OH scavenger, VFR increased as the ozone concentration increased, while no such effects was observed without OH scavenger. Compared to the data shown in Figure 2, the VFR383K with OH scavenger did increase with higher limonene concentration. Thus, rather than the VOC/ozone ratio, the most important factor seems to be the total amount of limonene converted. A plausible explanation for the increasing trend observed in VFR as a function of limonene and ozone concentrations in the scavenger experiments is the potential involvement of a second oxidation step between ozone and the primary products. Under conditions of high ozone and limonene concentrations, primary unsaturated products are generated rapidly, and these products

Figure 2. Scavenger effect on VFR383K at selected ozone/limonene ratios. Blue symbols indicate experiments with scavenger, and red symbols indicate experiments without scavenger. The plot shows the effect on VFR383K of gradually decreasing the initial limonene concentration at a constant ozone level of 3900 μg m−3 (experiments 10 and 11).

will be susceptible to ozonolysis during the contact time in the flow reactor. This may be related to multiphase ozonolysis involving the secondary ozonide as suggested by the study of Maksymiuk et al.13 In the absence of scavenger, the OH radical might inhibit this multiphase reaction because it is a more powerful oxidant than ozone, leading to OH addition instead of formation of the secondary ozonide, and thus reducing the effect of ozone on volatility. Chemical Simulations. To elucidate the chemical mechanism, limonene ozonolysis simulations were performed using the kinetic chemical mechanisms developed by the Kamens group.15,19,36 These mechanisms have been developed over several years using detailed gas kinetic data to simulate and evaluate processes taking place in large chamber experiments and link the results to processes under atmospheric conditions. Application of this model to simulate flow reactor experiments has, as far as we know, not been carried out before and may progress beyond the initial design stage. The FACSIMILE code, with some modification for the present simulation, was taken from Li et al.19 In the flow reactor simulations, the chemical mechanism for the limonene SOA formation involved the reaction of limonene with ozone in the dark followed by subsequent reactions of the first-generation products, e.g., C10and C9-oxygenated compounds, with O3 and OH radical. In the model describing the OH scavenger experiments, the concentration of 2-butanol was set to 3 × 105 μg m−3 (∼105 ppb), and the following chemistry was included in the model:

where kOH = 8.7 × 10−12 molecule cm3 s−1, kHO2 = (2.9 × 10−13)e1300/T[1 − e−0.245n] molecules cm3 s−1 (n is the number of carbons), kRO2 = 8.8 × 10−13 molecule cm3 s−1, and kRO = 11663



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Table 2. Species Contributing to the Aerosol Phase According to Model Simulations, Saturation Concentration, and Corresponding Classification of Volatilitya

a

Major aerosol contributors are in bold. Given are the oxidation state of the compounds and corresponding average oxidation state in modeled experiments. Notes: #, the oxidation state of each compound has been estimated as 2(O/C) − H/C according to Kroll et al.;49 *, the average OS (OS) is estimated as ∑OSif i, where OSi is the oxidation state of compound i and f i is the fraction of compound i in SOA for that experiment. 11664



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Figure 3. Volatility distribution profiles derived from predicted concentrations of the aerosol components according to model simulations of experiments 1, 4, 5, and 8. The products were classified into volatility bins according to Table 2. The green sections indicate the organic aerosol (OA) mass, while full bars are total organic mass (OM). The percentages are the contribution of the respective volatility bin to the total organic aerosol mass (i.e., sum of the green parts for all bins).

(2.0 × 1014)e−4714/T molecules cm3 s−1 (master chemical mechanism, MCM, http://mcm.leeds.ac.uk). The effect of introducing 2-butanol chemistry is a net consumption of OH radicals and production of HO2 and RO2 radicals. The gas−particle equilibrium was modeled explicitly using rate coefficients for desorption estimated from first principles, and the uptake rate coefficient was defined by the partitioning and desorption coefficients. The equilibrium partitioning coefficient (Kp) is the inverse of the saturation concentration for species i (Coi , μg m−3) and can be estimated from the respective vapor pressure.15 The vapor pressures used in the model were primarily estimated using the approach of Leungsakul et al.15 based on an equation from Mackay et al.37 This approach needs input data on boiling points and enthalpies of the compounds. These data were derived using three different methods. In method 1, boiling points were derived according to Nannoolal et al.37 and enthalpies as by Moller et al.38 In method 2, boiling points and enthalpies were derived on the basis of the method of Nannoolal et al.,39 whereas in method 3, boiling points were derived according to Stein and Brown40 and enthalpies as by Myrdal and Yalkowsky.41 For comparison, vapor pressures were also estimated by the E-AIM thermodynamic model (E-AIM = extended aerosol inorganics model; http://www.aim.env.uea.ac. uk/aim/aim.php) using the same set of methods for the input data as in the approach of Leungsakul et al. Estimation of the vapor pressure of atmospherically relevant compounds has recently been discussed by Booth et al.,42 who concluded that the decision about what methods to use is nontrivial as, e.g., it depends on the complexity and functional groups of the compounds. To determine the method to use in our study, vapor pressure estimates of pinonic acid (C10H17O3), a wellknown product of pinene oxidation, were compared to the experimentally determined value of Salo et al.28 and best

estimate from experimental observations of Bilde et al.43 The experimental value of Salo et al. was modified using a revised procedure to evaluate the flow profile, providing a slightly higher vapor pressure.42 All vapor pressure calculations are provided in the Supporting Information (Table S1). However, using all three methods, E-AIM predicted vapor pressures (μg m−3) for pinonic acid that were 1−4 orders of magnitude higher (method 1, 7.2 × 102 μg m−3; method 2, 4.6 × 103 μg m−3; method 3, 1.3 × 104 μg m−3) than the experimental values (Salo et al., 0.41 μg m−3; Bilde et al., 3.8−7.5 μg m−3), whereas the estimate obtained using the approach of Leungsakul et al. combined with method 2 was reasonably close (13 μg m−3) to the experimental values and was therefore the main method employed in subsequent model calculations. It was evident that the vapor pressure estimates are a source of major uncertainty in the SOA mass calculations as also pointed out recently by Compernolle et al.44 Values of the modeled SOA mass for all the experimental conditions using method 2 and the approach of Leungsakul et al. are shown in Table 1 (for experiments 1−9), while the results of all methods are presented in Table S1 in the Supporting Information (for experiments 1−8). The SOA masses predicted by the E-AIM model using the three different methods described above were very small compared to our experimental results. This is consistent with the overestimated value for the vapor pressure of pinonic acid obtained by E-AIM. The approach of Leungsakul et al.16 gave different results depending on the input (methods 1−3), and the range in predicted SOA mass from the three methods did cover the experimental results. However, a major concern was that the response to the ozone concentration and use of a scavenger within a method data set did not capture the experimental findings. Presented in Table 1 is the modeled sum of organic peroxides (C9−C10). In the present model these compounds 11665



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of a OH scavenger on the SOA mass imply that the current chemical scheme does not fully capture the features of SOA formation from limonene ozonolysis in the presence of OH scavenger. In the absence of OH scavenger, the data from experiments 5 and 8 in Figure 3 show a redistribution of the modeled organic aerosol among the volatility bins when ozone was increased from 1200 to 9800 μg m−3. For these experiments, the effect of ozone on the relative change in the SOA composition was quite similar to that of the non-OH scavenger experiments. A relatively large increase (from 8.4% to 29.3%) in the SOA mass fraction of the intermediate bins (log Coi = 0 and −1 μg m−3) was predicted with a simultaneous decrease of the SOA mass fraction of the high-volatility bin (log Coi = 1 μg m−3) from 68.7% to 53.2% and of the low-volatility bin (log Coi = −2 μg m−3) from 22.1% to 14.7%, indicating no major influence on volatility. This is consistent with the experimental observations on volatility, where no distinguishable change in the VFR values was seen in the absence of scavenger (see Figure 1c).

were defined to not contribute to aerosol mass. However, in a number of previous studies the importance and evidence for organic peroxides in SOA has been elucidated.24,45,46 The modeled organic peroxide concentration did primarily depend on the use of a scavenger and can explain the difference in SOA mass observed between experiments with and without OH scavenger. It should be stressed that our experimental results are in agreement with previous experimental studies on limonene24 and α-pinene24,47 ozonolysis, providing further support to include large organic peroxides as aerosolcontributing species.48 Chemical Simulation of SOA Products and Volatility. The model identified a number of compounds (15 compounds, excluding polymerization products and compounds contributing to the initial organic seeds) which made the largest contribution to the aerosol composition in the simulations. The saturation concentrations and relative distributions predicted by four simulations (corresponding to experiments 1, 4, 5, and 8) of all the products are listed in Table 2. More than 97% of the SOA mass was attributed to seven products, here referred to as limonic acid, ketolimononic acid, 7-hydroxyketolimonaldehyde, ketolimonalic acid, limononic acid, 7-hydroxy limononaldehyde, and limonalic acid, with corresponding chemical structures shown in Table 2. Furthermore, more than threefourths of the simulated SOA mass was attributed to five major aerosol products: limonic acid, limononic acid, 7-hydroxy limononaldehyde, limonalic acid, and ketolimononic acid. To simplify the analysis, we classified all compounds on the basis of their saturation concentration into low-volatility (log Coi ≤ −2 μg m−3), intermediate-volatility (−2 μg m−3 < log Coi ≤ 0 μg m−3), and high-volatility (log Coi > 0 μg m−3) bins. The most dominant fraction (about 53% or more) of the modeled SOA was due to the high-volatility compounds. Thus, any changes in the SOA concentrations of compounds in this bin would have a significant effect on the volatility characteristics of the SOA formed under various conditions. Furthermore, the model simulations enabled us to calculate the average oxidation state for the SOA produced (OS) in line with Kroll et al.49 According to the calculated OS (−0.9 to −0.81), the modeled SOA is comparable to a rather fresh atmospheric biogenic aerosol and to the initial stage of SOA in ozonolysis of monoterpene experiments.49 The response to the ozone concentration of the calculated OS was largest for the systems without OH scavenger. The influence of the ozone concentration on the modeled limonene ozonolysis products formed in the presence or absence of OH scavenger is shown in Figure 3 (experiments 1, 4, 5, and 8). The data for experiments 1 and 4 show that when the ozone concentration was increased from 1200 μg m−3 (experiment 1) to 9800 μg m−3 (experiment 4) in the presence of OH scavenger, the percentage of the total modeled organic mass (shown at the top of the bars for each bin) in the highvolatility (log Coi = 1 μg m−3) and low-volatility (log Coi = −2 μg m−3) bins decreased from 74.6% to 69.4% and from 21.2% to 14.9%, respectively, which, in turn, resulted in an increase in the intermediate volatility bins (log Coi = 0 and −1 μg m−3) from 3.5% to 14.0%. Thus, this redistribution, i.e., simultaneous decrease of the SOA fraction of both low- and high-volatility bins, would lead to no major differences in the volatility characteristics for low and high ozone concentrations. This is somewhat inconsistent with the experimental results, where SOA at high ozone concentration was found to be less volatile (Figure 1b). These discrepancies and the minor effect

4. DISCUSSION It has previously been argued that the volatility of limonene SOA is complex, in which both physical and chemical processes influence the thermal behavior.33 Previous studies have demonstrated that mass-transfer effects may be important in explaining the observed volatilities.33,35 In addition, Virtanen et al.50 showed that SOA particles may exhibit phase changes under certain conditions, which may also have implications for the kinetics of evaporation and partitioning.51,52 In the present work on limonene SOA, the volatility behavior in the absence of OH scavenger was a surprise because no effect of the ozone concentration was observed. Maksymiuk et al.13 have put forward evidence for multiphase ozonolysis involving secondary ozonide formation. This is also in line with our results with a scavenger, which showed that VFR and the SOA mass increased with increasing ozone concentrations. The observed increase in VFR with limonene concentrations provides further evidence for a second oxidation step, involving reaction of ozone with the primary products. For both high ozone and high limonene concentrations, the primary unsaturated products and SOA would be formed rapidly, enabling multiphase ozonolysis during the contact time in the flow reactor.53−56 However, the change in VFR with precursor concentrations was not observed in the absence of scavenger. We suggest that the OH radical can inhibit ozone multiphase reactions by being a much more reactive oxidant, leading to OH addition instead of forming the secondary ozonide, and thus reducing the effect of ozone on volatility that was so clearly observed in the nonscavenger experiments. The modeled OH radical concentrations did support a reaction with unsaturated compounds providing lifetimes of minutes or less, but if the OH reaction takes place in the gas phase57 or heterogeneously58 could not be resolved. Furthermore, several studies have highlighted the complexity of scavenger chemistry because endocyclic and exocyclic compounds appear to react differently.14,24,29,45 The model− experiment comparison with and without OH scavenger indicates organic peroxides to be important SOA constituents. The absolute modeled SOA mass depends strongly on vapor pressure estimations, and in addition to the six approach− method combinations used in the present work new ways of estimations are developed.59,60 However, considering this as a first attempt to directly transfer a photochemical model to 11666



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simulate flow reactor experiments, it revealed some interesting features and could, together with improved characterization of the SOA from G-FROST, prove a useful tool to elucidate the mechanism of SOA formation. This would aid in developing other more complex atmospheric SOA models for health or climate assessments,61−63 as well as enabling elucidation of short-time-scale processes of importance for indoor air quality.20,64

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aerosol concentrations during SCAQS. Atmos. Environ. 1995, 29 (23), 3527−3544. (5) Kroll, J. H.; Seinfeld, J. H. Chemistry of secondary organic aerosol: formation and evolution of low-volatility organics in the atmosphere. Atmos. Environ. 2008, 42 (16), 3593−3624. (6) Goldstein, A. H.; Galbally, I. E. Known and unexplored organic constituents in the earth’s atmosphere. Environ. Sci. Technol. 2007, 41 (5), 1514−1521. (7) Lee, A.; Goldstein, A. H.; Kroll, J. H.; Ng, N. L.; Varutbangkul, V.; Flagan, R. C.; Seinfeld, J. H., Gas-phase products and secondary aerosol yields from the photooxidation of 16 different terpenes. J. Geophys. Res., [Atmos.] 2006, 111, (D17). (8) Schurgers, G.; Arneth, A.; Holzinger, R.; Goldstein, A. H. Process-based modelling of biogenic monoterpene emissions combining production and release from storage. Atmos. Chem. Phys. 2009, 9 (10), 3409−3423. (9) Guenther, A.; Hewitt, C. N.; Erickson, D.; Fall, R.; Geron, C.; Graedel, T.; Harley, P.; Klinger, L.; Lerdau, M.; Mckay, W. A.; Pierce, T.; Scholes, B.; Steinbrecher, R.; Tallamraju, R.; Taylor, J.; Zimmerman, P. A global-model of natural volatile organic-compound emissions. J. Geophys. Res., [Atmos.] 1995, 100 (D5), 8873−8892. (10) Geron, C.; Rasmussen, R.; Arnts, R. R.; Guenther, A. A review and synthesis of monoterpene speciation from forests in the United States. Atmos. Environ. 2000, 34 (11), 1761−1781. (11) Walser, M. L.; Desyaterik, Y.; Laskin, J.; Laskin, A.; Nizkorodov, S. A. High-resolution mass spectrometric analysis of secondary organic aerosol produced by ozonation of limonene. Phys. Chem. Chem. Phys. 2008, 10 (7), 1009−1022. (12) Sunil, V. R.; Laumbach, R. J.; Patel, K. J.; Turpin, B. J.; Lim, H. J.; Kipen, H. M.; Laskin, J. D.; Laskin, D. L. Pulmonary effects of inhaled limonene ozone reaction products in elderly rats. Toxicol. Appl. Pharmacol. 2007, 222 (2), 211−220. (13) Maksymiuk, C. S.; Gayahtri, C.; Gil, R. R.; Donahue, N. M. Secondary organic aerosol formation from multiphase oxidation of limonene by ozone: mechanistic constraints via two-dimensional heteronuclear NMR spectroscopy. Phys. Chem. Chem. Phys. 2009, 11 (36), 7810−7818. (14) Iinuma, Y.; Boge, O.; Miao, Y.; Sierau, B.; Gnauk, T.; Herrmann, H. Laboratory studies on secondary organic aerosol formation from terpenes. Faraday Discuss. 2005, 130, 279−294. (15) Leungsakul, S.; Jaoui, M.; Kamens, R. M. Kinetic mechanism for predicting secondary organic aerosol formation from the reaction of dlimonene with ozone. Environ. Sci. Technol. 2005, 39 (24), 9583−9594. (16) Leungsakul, S.; Jeffries, H. E.; Kamens, R. M. A kinetic mechanism for predicting secondary aerosol formation from the reactions of d-limonene in the presence of oxides of nitrogen and natural sunlight. Atmos. Environ. 2005, 39 (37), 7063−7082. (17) Donahue, N. M.; Tischuk, J. E.; Marquis, B. J.; Hartz, K. E. H. Secondary organic aerosol from limona ketone: insights into terpene ozonolysis via synthesis of key intermediates. Phys. Chem. Chem. Phys. 2007, 9 (23), 2991−2998. (18) Iinuma, Y.; Muller, C.; Boge, O.; Gnauk, T.; Herrmann, H. The formation of organic sulfate esters in the limonene ozonolysis secondary organic aerosol (SOA) under acidic conditions. Atmos. Environ. 2007, 41 (27), 5571−5583. (19) Li, Q. F.; Hu, D.; Leungsakul, S.; Kamens, R. M. Large outdoor chamber experiments and computer simulations: (I) secondary organic aerosol formation from the oxidation of a mixture of dlimonene and α-pinene. Atmos. Environ. 2007, 41 (40), 9341−9352. (20) Chen, X.; Hopke, P. K. A chamber study of secondary organic aerosol formation by limonene ozonolysis. Indoor Air 2010, 20 (4), 320−328. (21) Johnson, D.; Marston, G. The gas-phase ozonolysis of unsaturated volatile organic compounds in the troposphere. Chem. Soc. Rev. 2008, 37 (4), 699−716. (22) Jonsson, Å. M.; Hallquist, M.; Ljungstrom, E. Impact of humidity on the ozone initiated oxidation of limonene, Δ3-carene, and α-pinene. Environ. Sci. Technol. 2006, 40 (1), 188−194.

ASSOCIATED CONTENT

* Supporting Information S

Table listing the estimated vapor pressures of individual products and the total model SOA mass for experiments 1−8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Earth and Space Sciences, Chalmers University of Technology, S-412 96 Gothenburg, Sweden. Notes

The authors declare no competing financial interest. ⊥ Maiden name: Jonsson.

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ACKNOWLEDGMENTS The research presented is a contribution to the Swedish strategic research area Modelling the Regional and Global Earth system, MERGE. This work was supported by Formas (Grant 214-2010-1756) and the Swedish Research Council (Grant 80475101). E.U.E., K.S., and Å.M.H. acknowledge support from the platform initiatives at the Faculty of Science, University of Gothenburg. The Kamens group is acknowledged for making their model openly available to the community.

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REFERENCES

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Influence of Ozone and Radical Chemistry on Limonene Organic

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