Temperature and acidity dependence of secondary organic aerosol formation from α-pinene ozonolysis with a compact chamber system

Secondary organic aerosol (SOA) is of great importance, affecting human health and the prediction of climate change; however, the factors (e.g., temperature, acidity of pre-existing particles, and oxidants) influencing its formation are not sufficiently resolved. In this study, a compact Teflon atmospheric simulation chamber is developed, in which reactions in 10 atmospheric pressure conditions can be performed with controlled temperature, humidity, oxidation agents, and seed particle acidity. Using the chamber, α-pinene ozonolysis SOA formation was simulated under temperatures of 278 K, 288 K, and 298 K with neutral/acidic seed aerosols. The SOA components of m/z less than 400 were analyzed using negative electrospray ionization liquid-chromatography time-of-flight mass spectrometry. The temperature and acidity dependence of SOA yields and chemical components were investigated. From the slightly negative temperature dependence of the SOA yields, the 15 enthalpy of vaporization in neutral and acidic seed conditions was estimated to be 25 and 44 kJ mol−1, respectively. With these values, the volatility distributions of the identified SOA compounds were consistently explained. Acidity dependence analysis of the chemical formula, molecular mass, and O:C ratio of the detected compounds indicated the enhanced formation of many oligomers in the wide molecular mass range with a wide range of O:C ratios under acidic seed conditions. The acidity dependence of certain major compounds could be explained by acid-catalyzed heterogeneous reactions (e.g., m/z 171, 185, 20 343, and 357) or acid-catalyzed decomposition of hydroperoxides (e.g., m/z 215 and 197). In addition, the formation of organosulfates (OS) was observed under acidic seed conditions. We proposed that six of the eleven detected OS were possibly formed through the aldehyde + HSO4 pathway. Further studies on the OS formation during α-pinene ozonolysis are warranted.


Introduction
Secondary organic aerosol (SOA) in the atmosphere is a complex of organic compounds which are formed through 25 photooxidation of precursor volatile organic compounds (VOCs) of either biogenic (e.g., monoterpene and isoprene) or anthropogenic (e.g., alkanes and aromatics) origins, or both (Hallquist et al., 2009). SOA plays important roles in the aerosol effect on climate (Tilmes et al., 2019), air quality (Parrish et al., 2011), as well as human health (Shiraiwa et al., 2017).
Nevertheless, it is noted in the IPCC Fifth Assessment Report (AR5) that the formation of SOA has not been included in the estimation of the radiative forcing from aerosols because the formation is influenced by a variety of factors not yet sufficiently 30 quantified (Stocker et al., 2013). However, in association with the advance of research technologies, processes that influence the growth of SOA particles to sizes relevant for clouds and radiative forcing have been intensively investigated (Shrivastava et al., 2017).
The importance of the formation of low volatility organic compounds (LVOCs) with saturate concentrations of less than 10 −0.5 μg m −3 has been highlighted in the SOA formation mechanisms of recent studies (Ehn et al., 2014;Shrivastava et al., 2017). 35 Semi-volatile organic compounds (SVOCs) are generated from oxidation reactions of VOCs in the gas phase.
Heterogenous/multiphase reactions of SVOCs on particles are thought to contribute to the formation of LVOCs in SOA. Earlier acid-catalyzed heterogeneous reaction studies (Jang et al., 2002;Hallquist et al., 2009) proposed the formation of hemiacetals, https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License. decreases of SOA yields under acidic seed conditions. The variety of the SOA yield differences between acidic and neutral 80 seed conditions could probably be related to different experimental settings among the studies.
With respect to chemical compositions, Kristensen et al. (2017) compared the chemical compositions of α-pinene ozonolysis SOA formed at temperatures of 293 and 258 K and found that the mass fraction of carboxylic acids increased at 258 K compared to 293 K, while the formation of dimer esters was suppressed at the sub-zero reaction temperature. Compared with neutral seed conditions, enhanced formation of large molecules under acidic seed conditions has been reported by Gao et al. 85 (2004) and Iinuma et al. (2004). Gao et al. (2004) additionally reported less abundant small oligomers (e.g., a compound with molecular mass of 358) under acidic seed conditions. Their research indicates that systematic studies of α-pinene ozonolysis SOA formation under specific experimental settings are warranted to clarify temperature and acidity dependence.
Furthermore, enhanced formation of organosulfates (OS) from α-pinene oxidation under acidic seed conditions has been suggested by previous studies (Surratt et al., 2007(Surratt et al., , 2008Iinuma et al., 2009;Duporté et al., 2020). OS has been regarded as 90 an important aerosol component, accounting for up to 30 % of organic mass in PM10, and also as an anthropogenic pollution marker in the past two decades (Iinuma et al., 2007Surratt et al., 2007Surratt et al., , 2008Riva et al., 2015Riva et al., , 2016Duporté et al., 2016Duporté et al., , 2020. The formation mechanisms of OS from α-pinene oxidations have been studied under different experimental settings. For example, Surratt et al. (2007) proposed OS formation through esterification of hydroxyl or carbonyl groups in photooxidation experiments. Surratt et al. (2008) proposed nitrooxy organosulfates formation through esterification of 95 hydroxyl groups in nighttime oxidation (i.e., NO3-initiated oxidation under dark conditions). Iinuma et al. (2009) proposed the acid-catalyzed ring-opening of epoxides mechanism through the α-pinene oxide/acidic sulfate particle experiment. Nozière et al. (2010) proposed a sulfate radical initiated process of α-pinene in irradiated sulfate solutions. For α-pinene ozonolysis experiments, the formation of OS through reactions between SO2 and stabilized Criegee intermediates under dry conditions or organic peroxides in aqueous phase has been recently suggested (Ye et al., 2018;Stangl et al., 2019;Wang et al., 2019). 100 However, based on our knowledge, no study concerning OS formation from α-pinene ozonolysis in the presence of sulfate particles exists.
In this study, α-pinene ozonolysis experiments have been conducted under dark conditions in the presence of seed particles and an OH scavenger. The study aimed to clarify the temperature and acidity dependence of the yield and chemical composition of α-pinene ozonolysis SOA. Moreover, OS compounds and its possible formation mechanisms have been targeted during the 105 analysis.

Chamber description and operation
A temperature-controllable chamber system has been developed for the simulation of SOA formation (Fig. S1). The chamber is a cuboid shape Teflon bag (FEP; 0.7 m 3 volume, 900 mm × 600 mm × 1300 mm; 50 µm thickness; Takesue, Japan) contained 110 in a constant temperature cabinet (HCLP-1240; W1200 mm × D703 mm × H1466 mm; NK System, Japan). The chamber is collapsible and is operated in atmosphere pressure. For a typical experimental run, a total volume of more than 0.6 standard cubic meters (sm 3 ) G3 pure air (CO2 < 1 ppmv, CO < 1 ppmv, THC < 1 ppmv, and dew temperature < −70 °C) with variable relative humidity (RH) was introduced into the Teflon chamber. The RH of the air was adjusted by passing the G3 pure air through MiliQ water (resistivity of 18.2 MΩ·cm) before entering the Teflon bag. α-Pinene liquid (Wako Chemicals, Japan) 115 was injected into the G3 pure air line through a septum equipped in a Swagelock PFA Tee connector using a micro-syringe (ITO corporation, Japan) in the middle of the injection of pure air. Diethyl ether in nitrogen gas (mass fraction of 0.4 %; Takachiho, Japan) used as an OH radical scavenger was introduced to the chamber in excess amounts (approximately 53 ppmv; 164-1963 times the initial concentration of α-pinene) after the introduction of pure air. Seed aerosol particles generated by a https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License. commercial atomizer (ATM220S, TOPAS GmbH, Germany) were subsequently introduced into the Teflon chamber. Neutral 120 seed aerosols were generated from a 0.6/0.3 mol L −1 (NH4)2SO4 solution and acidic seed aerosols were generated using a solution mixture of 0.25 mol L −1 (NH4)2SO4 and 0.25 mol L −1 H2SO4. After 30 min of stabilization, the initial concentrations of α-pinene and seed aerosols were measured with a quadrupole-type proton transfer reaction mass spectrometry instrument (PTR-QMS500, Ionicon Analytik Gesellschaft m.b.H., Innsbruck, Austria) and a scanning mobility particle sizer (SMPS, TSI classifier model 3082; differential mobility analyser model 3081; condensation particle counter model 3772; USA), 125 respectively. The SMPS was contained in a smaller constant temperature cabinet (LP-280-E, NK System, Japan), whose temperature was adjusted to be the same as the cabinet containing the Teflon chamber. The sheath and sample flow rates of the SMPS were 3.0 and 0.3 L min −1 , respectively. The measured diameter range of the SMPS was 13.8-685.4 nm, and the data were collected every 5 min. After obtaining the initial concentration of α-pinene and seed aerosols, excess ozone produced by irradiation of pure O2 with vacuum ultraviolet light from a low-pressure mercury lamp ozone generator (Model 600, Jelight 130 Compony Inc., USA) was introduced into the chamber to start the ozonolysis reactions. The ozone concentration in the chamber was measured with an ozone monitor (Model 1200, Dylec, Japan) immediately after its introduction. For some experiments, the order of the introduction of α-pinene and O3 was inverted (see Tables S1 and S2). The concentrations of both the α-pinene and aerosols were continually measured until the end of the experiment, which is defined in this study as 90 min after the start of the α-pinene ozonolysis reaction. The concentration of ozone after 90 min of ozonolysis reactions was also measured. In 135 total, 40 experimental runs were executed under neutral or acidic seed aerosol conditions at temperatures of 278, 288, or 298 K. For experiments with the same temperature setting, the RH settings were also similar (Tables S1 and S2). Notably, when the chamber temperature was set to 278 K, the temperature in the small cabinet was set to 280 K, which is the lowest work temperature of the SMPS. The Teflon bag was flushed with G3 pure air at least three times between two successive experimental runs, which took approximately 40 min. 140 One Teflon filter (PF020, 47 mm diameter, Advantec MFS) aerosol sample was collected for each different combination of seed and temperature condition. The sample volume was 0.5 m 3 for each sample. In total, six aerosol samples were collected, and they were subjected to negative electrospray ionization liquid-chromatography time-of-flight mass spectrometry analyses (Sect. 2.2). The acidity of the seed particles was measured in a separate experiment where the seed particles were sampled on a Teflon filter which was then extracted into 10 mL MiliQ water. The pH of the water solution was measured with a pH meter 145 (FPH70, AS ONE, Japan). The H + concentration was ~220 nmol m −3 under the acidic seed conditions.

LC-TOF-MS analysis
Chemical composition analysis of the Teflon filter samples was conducted using electrospray ionization liquidchromatography time-of-flight mass spectrometry (LC-TOF-MS) (Agilent Technologies, UK) similarly as in previous studies (Sato et al., 2018(Sato et al., , 2019 except that negative-mode was used in this study whereas positive-mode was used in those previous 150 studies. The mass calibration and lock-mass correction were conducted using G1969-85000 and G1969-85001 tuning mixtures (Agilent Technologies, UK), respectively. The mass resolution of the mass spectrometer (full width at half maximum) was > 20000. For the analysis, the Teflon filter sample was sonicated in 5 mL methanol for 30 min. The filter extract was concentrated to near dryness under a stream of nitrogen (∼1 L min −1 ). A 1 mL formic-acid-methanol-water solution (v/v/v = 0.05/100/99.95) was added to the concentrated extract to obtain the analytical sample. A 10 µL aliquot of the analytical sample 155 was injected into the LC-TOF-MS instrument and separated with an octadecyl silica gel column (Inertsil ODS-3; GL Science, Japan; 0.5 µm × 3.0 mm × 150 mm). A formic-acid-water solution (0.05 % v∕v) and methanol were used as mobile phases. The total flow of the mobile phases was 0.4 mLmin −1 . The methanol fraction during each analysis was set at 10 % (0 min), 90 % (30 min), 90 % (40 min), 10 % (45 min), and 10 % (60 min). As reported previously (Sato et al., 2007), the recovery of malic acid, whose saturation concentration was estimated to be 157 µg m −3 , was determined to be >90 %, suggesting that evaporation 160 https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License. loss during pre-treatment is negligible for molecules with saturated concentrations of ~10 2 µg m −3 or less. We determined the chemical formulae and signal intensities for 362 products (including eleven organosulfates) of different m/z (Table S3).

Wall-loss experiments
The wall-loss rate of particles in the chamber was evaluated by measuring the time evolution of the volume-size distributions of seed-only particles using the SMPS. The measurements were carried out whenever a new Teflon bag was used or the 165 experimental conditions (i.e., temperature or seed particle acidity) were changed under humid air conditions. The latest measured bulk wall-loss rate (Sect. 3.1) was applied for each SOA formation experiment.
In Fig. S2, size-resolved aerosol wall-loss rates, which were determined assuming first-order wall-loss constants (Wang et al., 2018a), were shown for seed particles of different size-distributions. Large wall-loss was observed for particles with mobility diameters of less than 100 nm, caused mainly by coagulation (Wang et al., 2018a). Large wall-loss for particles with mobility 170 diameters of larger than 200 nm was also observed. This is a shortcoming of this chamber and could be explained by the shorter sedimentation time in the compact space compared with large chambers. Figure S2 also indicates that the wall-loss rates of super-200 nm particles were relatively high when the mean diameter of the seed particles was relatively small. Therefore, we waited for 30 min after the introduction of seed particles to start the ozonolysis reaction so that the size distribution of the seed particles could shift to the larger size end due to coagulation and loss of small particles. In addition, we used high concentration 175 solutions for the generation of seed particles to produce large particles in this study (Sect. 2.1). where mSOA was calculated assuming SOA density of 1.34 g cm −3 (Sato et al., 2018) and equal to the arithmetic mean of the last three data of each experimental run. It was corrected for wall loss using the bulk-volume wall-loss rate, assuming a firstorder wall-loss constant which is independent of particle size and reaction time (Pathak et al., 2007b). The size-resolved wallloss rates were not applied because the bulk wall-loss rates were very close to the size-resolved rates at approximately 300 nm 185 ( Fig. S2). The mode diameters of the volume-size distributions of the seed particles ( Fig. S3) and volume-size distributions of aerosols at the end of the ozonolysis reactions were also approximately 300 nm (Fig. S3).
In addition, a four-product volatility basis-set (VBS) gas/particle partitioning absorption model (Eq. (2), Donahue et al., 2006;Lane et al., 2008) was applied to assist the interpretation of the observed responses of Y to the chamber temperature and the acidity of the seed aerosol. 190 where αi is the mass-based stoichiometric yield for product i, and * is the effective saturation concentration of i in µg m −3 . In this study, αi is assumed to be temperature-independent whereas the temperature dependence of * is accounted using the Clausius-Clapeyron equation as follows: * = , * exp ∆ , − (3) 195 https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License.
where T0 is the reference temperature, which is 298 K in this study; , * is the effective saturation concentration of i at T0; R is the ideal gas constant; and ΔHvap,i is the effective enthalpy of vaporization. The temperature dependence of Y is then represented by the substitution of Eq.
(2). We further assume a constant effective ΔHvap for all condensable organic compounds. Thus, the four-product basis set has five free parameters: α1, α2, α3, α4, and ΔHvap. Here, * = {1, 10, 100, 1000} µg m −3 , which was set based on the measured range of mSOA (Sect. 4.1) in this study. Microsoft Excel Solver GRG Nonlinear 200 engine was used for the derivation of the five parameters under neutral or acidic seed aerosol conditions.

Volatility distribution analysis
SOA compounds identified from the six filter samples through LC-TOF-MS analysis were subjected to volatility distribution analysis. The saturation concentration (C * ) of each chemical compound was calculated then ascribed to the volatility basis-set (Donahue et al., 2006). Again, 298 K was used as the reference temperature (T0). For compounds whose chemical structures 205 have been suggested by previous researchers, the SPARC online calculator (Hilal et al., 2003;Sato et al., 2018) was used for the derivation of their * . For other compounds, including organosulfates, the following equation from Li et al. (2016) was applied: where is the reference carbon number; nC, nO, and nS are the numbers of carbon, oxygen, and sulfur atoms in the molecule, 210 respectively; bC, bO, and bS are the respective contribution of each atom to log * ; and bCO is the carbon-oxygen nonideality.  Table S3. 215 4 Results and discussion 4.1 Temperature and acidity dependence of SOA yield An example experimental run of SOA formation from α-pinene ozonolysis is presented in Fig. 1 (Exp. No. 27). The initial concentrations of α-pinene and ozone were 145 and >824 ppbv, respectively. The number concentration of seed aerosols was 9.6 × 10 3 cm −3 and they were concentrated in the diameter range of 80-200 nm. SOA was formed while α-pinene was consumed 220 immediately at the introduction of excess O3. The mass concentration of SOA reached its maximum while α-pinene was almost totally consumed approximately 50 min after the introduction of ozone. The time variation of α-pinene conforms to an αpinene limited first-order chemical reaction (5 = 47 min). With the wall-loss correction, the SOA loading at the end of this experiment was calculated to be 200 μg m −3 , which resulted in a final SOA yield of 26.3 % (Table S2). As the SOA concentration was kept constant after 50 min, we consider the wall-loss correction applied here was reasonable (Ng et al., 225 2006). Without wall-loss correction, the concentration of SOA at 90 min could be underestimated by approximately 40 %. The observed aerosol number-size distribution shifted to a much greater but narrower size range of 200-300 nm at the end of the experiment.
https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License.  (Table S2). The measured SOA mass yields at 298 K under neutral seed conditions were compared with that of Pathak et al. (2007a) (Fig.   2). In this study, seven experiments with varied initial α-pinene concentrations (54-323 ppbv) at 298 K under neutral seed conditions were conducted under the RH conditions of approximately 26-27 % (Table S1). In Pathak et al. (2007a), the SOA mass yields from pre-existing studies under low NOx dark ozonolysis conditions were summarized and categorized into two 245 groups according to experimental RH conditions: RH<10 % and RH=50-73 %. They further fitted the data in each group using the multiple product basis-set approach (Pathak et al., 2007a). The four-product basis-set fitting was adopted to compare with the experiment results of this study (Fig. 2). The SOA yields in this study were 25-60 % lower than that of Pathak et al. (2007a). Possible reasons may include no consideration of the wall-loss of oxidized organic vapors because the surface to volume ratio of the chamber used in this study could be much greater than those of previous studies (the volume of the chamber 250 used in this study is 0.7 m 3 , whereas those of previous studies are 10-200 m 3 (Pathak et al., 2007a and references therein)).
According to Zhang et al. (2014), the vapor wall-loss bias factor, Rwall (defined as the ratio of the SOA mass when the vapor wall loss was assumed to be zero, to the SOA mass when the optimal vapor wall loss rate was applied), was reported to be ~4 at the initial seed surface area of ~2 × 10 3 μm 2 cm −3 (seed-to-chamber surface area ratio ~1 × 10 −3 ) and ~2 at the initial seed surface area of >6 × 10 3 μm 2 cm −3 (seed-to-chamber surface area ratio >3 × 10 −3 ) during the photooxidation of toluene. As the 255 initial seed surface area in the present study was in the range of (1−3) × 10 3 μm 2 cm −3 (seed-to-chamber surface area ratio (1−4) × 10 −4 ), Rwall in the Teflon bag might be at least twice that of the large chambers. This leads to the underestimation of the SOA yield of 50 % compared with the values obtained from the large chambers. We note that this is a shortcoming of the compact chamber with 0.7 m 3 volume used in this study. In the following paragraph, the temperature dependence of SOA yields will be discussed assuming that the underestimation of the SOA yield due to the wall-loss of oxidized organic vapors 260 does not affect the temperature dependence.
The yields of SOA from α-pinene ozonolysis under different experimental conditions in this study are summarized in Fig. 3.
Results under neutral and acidic seed conditions are presented separately in panels (a) and (b). In each panel, the measured SOA yields as a function of SOA mass loadings are indicated by markers and the four-product VBS model fitting results are shown by curves. The fitted parameters with the four-product VBS model under neutral and acidic seed conditions are 265 summarized in Table 1. Under neutral seed conditions, the fitted temperature independent stoichiometric yields α = {0.00, 0.09, 0.09, 0.52} and ΔHvap is 25 kJ mol −1 . Under acidic seed conditions, α = {0.00, 0.14, 0.05, 0.43} and ΔHvap is 44 kJ mol −1 .
The fitting results pointed out that most of the detected SOA compounds are of relatively high saturation concentration (i.e., c * at 298 K of 1000 µg m −3 ) under both neutral and acidic seed conditions. Weak increases of SOA yields with decreases of the chamber temperature can be observed under both seed conditions. This is consistent with the result in Pahtak et al. (2007b), 270 which found a weak dependence of SOA yields on temperature in the range of 288-303 K. As temperature decreases, SVOCs tend to partition into the particle phase because of the lowering of their volatilities.
The effective ΔHvap of α-pinene ozonolysis SOA derived from the four-product VBS fitting in this study was compared with previous studies ( Table 2). The ΔHvap values derived in this study were comparable to those in Saathoff et al. (2009) andPathak et al. (2007a) where the experiments were executed in large chambers of 10-200 m 3 . It may support our assumption that the 275 temperature dependence of SOA yields was not influenced by vapor wall-loss. It is also in agreement with the ΔHvap of 40 kJ mol −1 applied in the CMAQv4.7 model (Carlton et al., 2010). However, they are lower than that of Saha et al. (2016) and much lower than that of Epstein et al. (2010). While the ΔHvap in Saha et al. (2016) was derived based on thermodeneuder measurements which attributed 20-40 % of SOA mass to low-volatility material (C * < 0.3 µg m −3 ), most of the measured SOA in this study are of relatively high volatility as aforementioned. The differences in the volatility of SOA and the derived ΔHvap 280 between Saha et al. (2016) and this study are consistent with the general phenomenon that ΔHvap is conversely related with volatility (Epstein et al., 2010). The ΔHvap in Epstein et al. (2010) was derived from published experimental vapor pressure https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License. data of organic compounds. The reason for the difference in ΔHvap between this study and Epstein et al. (2010) cannot be currently explained.
The dependence of SOA yields on the acidity of seed particles seems to be small in this study. According to previous α-pinene 285 ozonolysis studies, SOA yields can be enhanced under acidic seed particle conditions compared to neutral seed conditions (Czoschke et al., 2003;Gao et al., 2004;Iinuma et al., 2004Iinuma et al., , 2005Czoschke and Jang, 2006;Jang et al., 2006;Northcross and Jang, 2007). But the degree of the enhancement seemed to be affected by the experimental settings. It is noted that all those previous studies except Gao et al. (2004) and Iinuma et al. (2005) did not use an OH radical scavenger, which plays an important role in influencing SOA yields (Iinuma et al., 2005;Na et al., 2007). In addition, the enhancement of SOA formation 290 under acidic seed conditions can be influenced by the SOA mass loadings. This is probably because the SOA components can be of high viscosity under conditions when RH is smaller than around 50 %, and if high SOA mass loadings coated the seed particles, the acid-catalyzed heterogeneous SOA formation reactions could be impeded (Shiraiwa et al., 2013b;Zhou et al., 2013). In Gao et al. (2004), obvious initial α-pinene concentration dependence of the enhancement of SOA yields under acidic compared with neutral seed conditions was observed. For initial α-pinene concentrations of 12, 25, 48, 52, 96, 120, and 135 295 ppbv, the relative enhancements of SOA yields were 37, 34, 26, 24, 15, 10, and 8 %, respectively. This is consistent with the statement that viscous material hinders acid-catalyzed reactions. In this study, the initial concentrations of α-pinene were 54-323 ppbv at 298 K, suggesting that the enhancement could be less than 24 %. Although with a high uncertainty, the fourproduct VBS model fitting results from this study indicate that the SOA yields could have been enhanced under acidic seed conditions when the SOA mass loadings were low (Fig. S4). When SOA loadings were high, the enhancement disappeared 300 (Fig. S4). When the SOA volume loading was 50 µm 3 cm −3 , the fitted SOA yields under acidic conditions were enhanced by 11, 17, and 25 % compared to neutral seed conditions under 298, 288, and 278 K, respectively. This is consistent with Gao et al. (2004) and is also comparable to the results of Iinuma et al. (2005). In Iinuma et al. (2005), the experiment with 2-Butanol as an OH radical scavenger under room temperature (294-298 K) found an enhancement of 19 % with the final SOA volume concentration of approximately 50 µm 3 cm −3 . Furthermore, the degree of acidity of the seed aerosols could also have influenced 305 the enhancement (Gao et al., 2004;Czoschke and Jang, 2006). Further comprehensive studies are warranted (including the consideration of the aerosol viscosity and phase separation) on the influence of seed aerosol acidity on α-pinene ozonolysis SOA formation.   Among the 362 compounds identified through LC-ToF-MS analysis in this study (Table S3), 331 compounds were ascribed to VBS bin ranges of −8-3. The other 31 compounds were ascribed to higher VBS bin ranges of 4-6, which is unrealistic for the compounds in the aerosol phase. Additionally, those latter compounds only accounted for an average of 12 % of the total mass of identified compounds. Thus, only the former 331 compounds are targeted in the following discussions. were normalized by their total intensity for each experiment and are presented in Fig. 4a. Compounds that were attributed to VBS bins between 0 and 3 are known as SVOC (Li et al., 2016). Those in bins 2 and 3 generally presented a decreasing tendency with the increase of experimental temperatures, and there was no obvious temperature dependence for those in bins 0 and 1 (Fig. 4a). The corresponding gas phase concentrations of each compound were derived assuming gas-particle partitioning equilibrium (Odum et al., 1996). The ΔHvap values derived in this study (Table 1) were used to calculate the 330 saturation concentration under 278 and 288 K following Eq. (3). The intensity of both the aerosol and gas phases were then normalized by their total amount and are presented in Fig. 4b. With the inclusion of the corresponding gaseous phase compounds, the temperature dependence of compounds in VBS bin 3 changed to positive (i.e., increased with temperature) whereas the tendency in bin 2 remained negative (Fig. 4b). As the α-pinene ozonolysis rate constant does not vary much under the temperature range of 278-298 K (Akimoto, 2016) and α-pinene was completely consumed at the reaction time of 90 min, 335 the total amount of formed SVOCs should be similar at the three temperatures. Hence, the total amounts of gas and aerosol phase compounds in each VBS bin should be independent of experimental temperatures. If a larger ΔHvap were applied for the derivation of the intensity of gas phase compounds (e.g., the semiempirical equation in Epstein et al. (2010)), the total amount in VBS bin 3 at 298 K would be much higher than at other temperatures, which is unreasonable. This suggested the appropriateness of the ΔHvap values derived from the four-product VBS fitting of the SOA yields in Sect. 4.1. In addition, we 340 note that the volatility distribution pattern derived in this study is similar to that of the experimental runs 1 and 6 (derived using the same method) of Sato et al. (2018), although positive electrospray ionization analysis was used and the α-pinene ozonolysis experiments were carried out under dry conditions in the latter. In Fig. 4b, the volatility distributions determined from SOA yield curves were also presented. The mass fractions determined from the LC-TOF-MS data at VBS bins 1 and 3, and 0 and 2, were smaller and larger, respectively, than those determined from the SOA yield curves. These differences are probably 345 https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License. caused by uncertainties in saturation concentration and sensitivity parameterization as well as the existence of undetected molecules in the LC-TOF-MS analysis (Sato et al., 2018). intensity of the aerosol phase and its equilibrium gas phase in each experiment. Curves and markers imposed in panel (b) are the volatility distributions under neutral (dark color) and acidic (light color) seed conditions determined from SOA yield curves (Sato et al., 2018). conditions were more than 1.1 (red symbols) or less than 0.9 (black symbols) times those of neutral seed conditions. Whiskers represent the standard deviations of the three experiments at different temperatures.
https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License. Figure 6: Atomic H to C ratio (H:C) versus O to C ratio (O:C). Star and Circle markers respectively indicate compounds whose 360 aerosol phase mass fractions under acidic seed conditions were more than 1.1 and less than 0.9 times those of neutral seed conditions. Color scale indicates the mean mass fraction of identified compounds across all six experiments. Notably, organosulfates were not presented.
To gain more insights into the acidity dependence of the α-pinene ozonolysis SOA, the relative intensities of the compounds identified by LC-TOF-MS under acidic and neutral seed conditions were compared. For compounds whose intensity under 365 acidic seed conditions was more than 1.1 times that of neutral seed conditions, monomers with a chemical formula of C10H9-18O4-14 accounted for 32 % and oligomers with a chemical formula of C11-22H7-35O2-12 accounted for 68 % of the total intensity. Conversely, for compounds whose intensity under acidic conditions was less than 0.9 times that of neutral seed conditions, monomers with a chemical formula of C6-10H7-15O4-6 accounted for 87 % and oligomers with a chemical formula of C11-21H9-27O5-10 accounted for 13 % of the total intensity. 370 Figure 5 presents the aerosol phase mass distributions of compounds whose mass fractions under acidic seed conditions were more than 1.1 (or less than 0.9) times those of neutral seed conditions. The molecular mass of those compounds with greater mass fractions under acidic seed conditions was generally distributed in the higher mass end compared with those with greater mass fractions under neutral seed conditions. The mass fraction weighted mean molecular mass of those compounds presented greater intensities under acidic seed conditions was 284 ± 14 (mean ± standard deviation) g mol −1 , whereas of those presented 375 greater intensities under neutral seed conditions was 204 ± 4 g mol −1 . Figure 6 presents the identified compounds whose intensity under acidic seed conditions was more than 1.1 (or less than 0.9) times that of neutral seed conditions in the atomic H to C ratio (H:C) versus O to C ratio (O:C) space. The color scale indicates the mean mass fraction of identified compounds across all six experiments. It is suggested that compounds with a lower intensity under acidic seed conditions were concentratedly distributed in the O:C ratio range of 0.4-0.75 and H:C ratio range 380 of 1.1-1.7, whereas those with higher intensity under acidic seed conditions were more broadly distributed in the H:C versus O:C space. Compounds with O:C ratios less than 0.4 were oligomers with a chemical formula of C11-22H19-35O2-7 and were distributed in the VBS bins of −3 to 3. Compounds with O:C ratios greater than 0.75 were likely to be highly oxidized molecules with a chemical formula of C10-14H9-21O9-14, and were attributed to VBS bins −8 to −2. Furthermore, oligomers with O:C ratios of less than 0.4 accounted for 61 % of those oligomers with high relative intensity under acidic conditions, whereas 385 those with O:C ratios of greater than 0.75 accounted for only 1 %. This, together with the aforementioned chemical formula and molecular mass distributions, indicated that the formation of many oligomers, especially with small O:C ratios, was enhanced under acidic seed conditions. Figure 7 presents the compounds whose normalized intensity under acidic seed conditions was less than 0.9 (panel a) or more than 1.1 (panel b) times that of neutral seed conditions in the VBS space. The acidity dependence of the major compounds in 390 https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License. VBS bins was tentatively explained from the viewpoints of acid-catalyzed decomposition reactions or acid-catalyzed heterogenous reactions. Compounds that presented lower intensities under acidic than neutral seed conditions were mainly distributed in VBS bins 3, 2, −1, −4, and −6 (Fig. 7a). The respective compounds that presented the highest intensity in VBS  Seubold and Vaughan, 1953). Next, previous studies indicate that m/z 171 and 185 were monomer precursors of dimers m/z 343 and 357, and all four products exist in both laboratory α-pinene ozonolysis SOA samples and field samples 400 (Gao et al., 2004;Yasmeen et al., 2010;Kristensen et al., 2014;Zhang et al., 2015). The chemical structure of m/z 185 has been proposed as pinic acid (Gao et al., 2004;Yasmeen et al., 2010;Kristensen et al., 2014), and m/z 171 as terpenylic acid (Yasmeen et al., 2010;Kristensen et al., 2014) and norpinic acid (Gao et al., 2004). In this study, excess OH scavengers could have minimized the formation of an OH functional group in the α-pinene ozonolysis products whereas the aldehydes should dominate the products (Gaona-Colmán et al., 2017). Thus, it is possible that the acid-catalyzed heterogenous esterification 405 between acid products such as m/z 171 and 185, and aldehyde products such as pinonaldehyde (Hallquist et al., 2009), led to the decreased intensity of m/z 171 and 185 under acidic seed conditions, which further led to the decreased intensity of the related dimers of m/z 343 and 357. Scheme 2 presents a possible esterification reaction between terpenylic acid (a possible isomer of m/z 171) and pinonaldehyde. The product, m/z 339.180 (C18H27O6), ascribed to VBS bin −1, presented higher relative intensity under acidic seed conditions. 410 The compounds with higher contributions under acidic than neutral seed conditions were mainly distributed in VBS bins −3, −2, 0 and 2 (Fig. 7b). The most abundant compounds of those whose intensity under acidic seed conditions was more than 1.1 times that of neutral conditions in bins 2, −2, and −3 were m/z 197 (C10H13O4), 355.175 (C18H27O7), and 383.170 (C19H27O8), respectively. Their contributions to the total signal of their respective bins were 14, 7, and 5 %, and the ratios of their intensities under acidic seed conditions to those under neutral seed conditions were 5.4, 1.8, and 1.3, respectively. The higher intensity 415 of m/z 197 under acidic than neutral conditions might be explained partly by the acid-catalyzed decomposition of m/z 215 (Scheme 1a). Zhang et al. (2015) proposed m/z 355 as a dimer ester with a hydroperoxide function. Another possible assignment of m/z 355 is an ester from the reaction of a Criegee intermediate (C10H16O3) with terpenylic acid and/or norpinic acid (C8H12O4) (Kristensen et al., 2017). The latter assignment of the product also has a hydroperoxide group. The formation of hydroperoxides involving Criegee intermediates has been observed in the gas phase (Sekimoto et al., 2020) and on the 420 surface of aqueous droplets . The relatively high intensity of m/z 355 under acidic seed conditions might be explained by the fact that the hydrolysis of ester hydroperoxides becomes slower under acidic conditions (Zhao et al., 2018). Kristensen et al. (2017) proposed m/z 383 as a dimer ester, whereas the molecular structure is to be identified. In bin 0, the intensities of m/z 311.149 (C16H23O6) and m/z 313.165 (C16H25O6) were the highest (contributing on average 10 and 9 % of the total intensity of the bin, respectively), and under acidic seed conditions were 4.8 and 1.5 times those of neutral seed 425 conditions, respectively. Although the structure of m/z 311 is yet to be identified, m/z 313 was proposed as a dimer ester by Zhang et al. (2015). These dimer esters at m/z 383 and 313 might be formed through acid-catalyzed heterogeneous reactions.
In addition, the eleven identified organosulfate compounds were ascribed to the VBS bins of −3 to 0 ( Table 3). All of them presented greater intensity on average under acidic conditions than under neutral seed conditions (Fig. S5). Four of the OS were in VBS bin 0, which together contributed 9.2 % of the intensity of that bin. The contributions of the two, three, and two 430 OS compounds (which respectively belongs to VBS bins of −1, −2, and −3) to their respective bins were 0.4, 4.7, and 2.6 %.
The formation mechanisms of those OS compounds will be discussed in the next section. It should be noted that the estimated https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License.
intensities of OS compounds in this study might be of a high uncertainty because methanol, which could react with carbonyl and carboxylic compounds in the SOA (Bateman et al., 2008), was used as the extraction solution. Acetonitrile should be used as the extraction solution in future studies to confirm the influence (Bateman et al., 2008). 435 Figure 7: Volatility distribution of SOA compounds whose aerosol phase mass fractions under acidic seed conditions were (a) less than 0.9 and (b) more than 1.1 times those of neutral seed conditions. Note the mass fractions presented here are same as those in Fig. 4a

OS formation under acidic conditions
In this study, significant signals were identified for eleven organosulfates under acidic seed conditions (Fig. S5). Figure 8 presents the molecular yields of those OS compounds estimated from the LC-TOF-MS analysis. M/z 223 was the most abundant OS identified at 298 k, followed by m/z 279, 281, 269, 283, 265, 253, 267, 251, 249, and 247.
The possible formation mechanisms of the OS compounds were proposed based on previous literatures combined with the 450 experimental settings of this study. To our knowledge, while the formation of OS through α-pinene ozonolysis in the https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License.
presence of an OH radical scavenger has been reported very recently (Ye et al., 2018;Stangl et al., 2019;Wang et al., 2019), these studies focused on the reactions between α-pinene ozonolysis products with SO2 rather the identification of OS compounds. Earlier studies on the identification and formation mechanisms of OS with α-pinene as the precursor VOC related to photooxidation (i.e., OH-initiated oxidation) or nighttime oxidation (i.e., NO3-initiated oxidation under dark 455 conditions) (Surratt et al., 2007(Surratt et al., , 2008. In this study, the formation of OS from α-pinene ozonolysis in the presence of an OH radical scavenger and acidic seed aerosols was investigated. Table 3 summarizes the possible chemical structures of the eleven detected OS compounds and their precursor α-pinene oxidation products. Among the eleven OS compounds detected in this study, eight of them have been detected in previous laboratory and/or field studies of α-pinene oxidations (Table 3), and the other three (m/z 251, 253, and 269) were detected 460 for the first time in this study. While all eleven OS compounds detected in this study were very likely from α-pinene ozonolysis reaction because the presence of excess OH scavenger in this study, five of the previously identified OS compounds (m/z 223, 247, 249, 265, and 279) were detected in α-pinene photooxidation and/or nighttime oxidation experiments (Surratt et al., 2008) and the other three (m/z 267, 281, and 283) were only identified in ambient aerosol.
The chemical structures of five of the OS compounds (i.e., m/z 223, 247, 249, 265, and 279) have been proposed in previous 465 studies. Four of them (m/z 223, 249, and 279) were likely to be sulfate esters formed from alcohols and sulfate (Surratt et al., 2007(Surratt et al., , 2008Zhang et al., 2015;Hettiyadura et al., 2019), and one (m/z 265) was from the sulfacation of the aldehyde compound, specifically pinonaldehyde (Liggio and Li, 2006;Surratt et al., 2007). The former is referred hereafter as the alcohol pathway, and the latter the aldehyde pathway (aldehyde + HSO4 − ; Surratt et al., 2007). Because aldehydes should dominate the products from α-pinene ozonolysis in the presence of an OH scavenger (Gaona-Colmán et al., 2017) and 470 pinonaldehyde has been detected in previous α-pinene ozonolysis studies with an OH scavenger (Jackson et al., 2017), we propose that the aldehyde pathway could be one of the dominant OS formation pathways in this study. In addition to m/z 265, we propose possible aldehyde precursors formed in α-pinene ozonolysis experiments for five other OS compounds identified in this study (i.e., m/z 251, 253, 269, 279, and 283). Except for the study where the aldehyde precursor of m/z 269 was found (Reinnig et al., 2009), an OH scavenger was used in all other studies where the aldehyde precursors were found 475 (Yu et al., 1999;Ma et al., 2008;Jackson et al., 2017). Presently, we suppose that the other five OS compounds were possibly formed through the alcohol pathway although no relevant information has been found for m/z 247 and 249.
Notably, another possible precursor with a hydroxyl function, which is formed in α-pinene ozonolysis experiments with an OH scavenger, has also been found for the OS at m/z 269 (Kristensen et al., 2014). Additionally, the proposed precursor for m/z 283 (Table 3) also contains a hydroxyl function, which can form another OS of m/z 265.038 (C9H13O7S) through the 480 alcohol pathway. A weak signal of this OS was observed (not presented in Fig. S5).
As shown in Fig. 8, the yields of m/z 247, 249, and 265 decreased with the increase of temperature. Conversely, the yields of other OS compounds increased with the temperature, or at least the yields at 278 K were the lowest among the three temperature conditions. The temperature dependence of OS yields seems not to be directly related to the formation mechanisms of either the alcohol pathway or the aldehyde pathway. It should be noted that the OS at m/z 247 and 249 could only be observed at 485 relatively low temperatures. As eleven OS (including three unreported) were observed in the α-pinene ozonolysis reaction with an OH scavenger and acidic seed particles, the identification of the structure of these OS using high-resolution ion mobility mass spectrometry is planned for the next step. When the structure of the OS can be determined and the precursor compound can be expected, the formation mechanism of the OS in α-pinene ozonolysis with acidic seeds will be confirmed.     Surratt et al., 2008;d Yassine et al., 2012;e Zhang et al., 2015;f Tao et al., 2014;g Ma et al., 2014;h Wang et al., 2018b;i Yu et al., 1999;j Surratt et al., 2007;k Jackson et al., 2017;l Liggio and Li, 2006;m Kahnt et al., 2014a;n Kahnt et al., 2014b;o Meade et al., 2016;p Reinnig et al., 2009;q Kristensen et al., 2014;r Aschmann et al., 1998r Aschmann et al., , 2002 500 s Brüggemann et al., 2019; t Wang et al., 2016;u Brüggemann et al., 2017;v Ma et al., 2008. https://doi.org/10.5194/acp-2020-798 Preprint. Discussion started: 17 September 2020 c Author(s) 2020. CC BY 4.0 License.

Summary and conclusions 505
A temperature controllable chamber system has been developed for studying SOA formation. In this study, SOA formation from α-pinene ozonolysis was studied with diethyl ether as an OH radical scavenger at temperatures of 278, 288, and 298 K under acidic/neutral seed conditions. The SOA yields and compounds with a molecular mass of less than 400 Da determined by a LC-TOF-MS were analyzed from the perspectives of temperature and seed particle acidity dependence.
The SOA yield slightly increased with the decrease of chamber temperature. The enthalpy of vaporization in neutral and acidic 510 seed conditions was estimated to be 25 and 44 kJ mol −1 , respectively. The acidity dependence of the SOA yield at low SOA loadings was comparable to those reported in Gao et al. (2004) and Iinuma et al. (2005).
Among the 362 identified compounds, the volatility of 331 was distributed in the VBS bins between −8 and 3. The temperature dependence of the volatility distribution of those identified compounds (particle phase + gas phase) could be consistently explained by the enthalpy of vaporization derived in this study. 515 The compounds whose intensities under acidic seed conditions were less than 0.9 times those of neutral seed conditions were dominated by monomers, whereas the compounds whose intensities under acidic seed conditions were more than 1.1 times those of neutral seed conditions were dominated by oligomers. The O:C ratios of the former were concentrated in the range of 0.4-0.75. The O:C ratios of the latter were broadly distributed. The compounds with O:C ratios less than 0.4 were all oligomers that accounted for 61 % of the oligomers with high relative intensity under acidic conditions, whereas those with O:C ratios 520 of greater than 0.75 were highly oxidized molecules and only contributed to 1 % of those oligomers. In addition, the mean molecular mass of the former compounds (204 ± 4 g mol −1 ) were evidently lower than those of the latter (284 ± 14 g mol −1 ).
These differences indicated that the formation of many oligomers, especially with small O:C ratios, was enhanced under acidic seed conditions. The acidity-dependence of certain major compounds might be explained by acid-catalyzed heterogenous reactions (e.g., m/z 171, 185, 343, and 357) or acid-catalyzed decomposition reactions (e.g., m/z 215 and 197). 525 For the first time, organosulfate compounds were studied for α-pinene ozonolysis reaction at the presence of an OH scavenger and acidic seed aerosols. Eleven OS compounds were determined from LC-TOF-MS analysis. All of them on average presented a higher yield under acidic than neutral seed conditions. Six of those OS compounds were possibly formed through the aldehyde + HSO4 − pathway, which should be confirmed in future studies through high resolution mass spectrometry analysis. 530 Data availability.
All the final data supporting the findings of this study are available in the manuscript or in the supplement. Raw data used to derive the final data are available on request to the corresponding author.

Author contributions
SI, YD, SE, and HT did chamber experiments; KS and SR did LC-TOF-MS analyses; SI, YD, SE, and MY did data analyses; 535 YD, SI, and HT made the manuscript; and all authors contributed to the revisions of the manuscript.