Comparison of secondary organic aerosol formation from 1 toluene on initially wet and dry ammonium sulfate particles 2

The formation of secondary organic aerosol (SOA) has been widely studied in the presence of dry seed particles at low relative humidity (RH). At higher RH, seed particles can exist as dry or wet particles. Here, we investigated the formation of SOA from the photooxidation of toluene using an oxidation flow reactor under a range of OH exposures on initially wet or dry ammonium sulfate (AS) seed particles at an RH of 68 %. At an OH exposure of 4.66 × 10 10  molecules cm  -3  s, the ratio of the SOA yield on wet AS seeds to that on dry AS seeds was 1.31 ± 0.02. However, this ratio decreased to 1.01 ± 0.01 at an OH exposure of 5.28 × 10 11  molecules cm  -3  s. The decrease in the ratios of SOA yields as the increase of OH exposure may be due to the early deliquescence of initially dry AS seeds after coated by highly oxidized toluene-derived SOA. SOA formation lowered the deliquescence RH of AS and resulted in the uptake of water by both AS and SOA. Hence the initially dry AS seeds contained aerosol liquid water (ALW) soon after a large fraction of SOA formed and the SOA yield and ALW approached those of the initially wet AS seeds as OH exposure and ALW increased. However, a higher oxidation state of the SOA on initially wet AS seeds than that on dry AS seeds was observed at all levels of OH exposure. The difference in mass fractions of m/z  29, 43 and 44 of SOA mass spectra indicated that SOA formed on initially wet seeds may be enriched in earlier-generation products containing carbonyl functional groups at low OH exposures and later-generation products containing acidic functional groups at high exposures. Our results suggest that AS dry seeds soon turn to at least partially deliquesced particles during SOA formation and more studies on the interplay of SOA formation and ALW are warranted.

chamber used in this study had a volume of approximately 19 L (length 60 cm, diameter 126 20 cm). The total flow rate in the PAM chamber was set at 3 L min -1 using mass flow  156 The AS/SOA mixed particles were characterized for the chemical composition of non-

Characterization of non-refractory components
The collection efficiency (CE) of an AMS is dependent on the chemical 169 composition and acidity as well as the phase state of particles (Matthew et al., 2008;170 Middlebrook et al., 2012). Matthew et al. (2008) found that the CE for solid particles 171 thickly coated with liquid organics was 100%. In this study, experiments were 172 conducted at an RH of 68%, exceeding the RH threshold for the semisolid-to-liquid 173 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1008 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 8 November 2017 c Author(s) 2017. CC BY 4.0 License.
phase transition for toluene-derived SOA (Bateman et al., 2015;Song et al., 2016). A 174 CE of 1 was used for processing all AMS data since the concentration of sulfate 175 measured with the AMS varied by less than 5% of the average mass of sulfate after 176 coated by SOA for both wet and dry AS seeds conditions. For the quantification of 177 SOA, the contribution from background organic aerosols was subtracted from the total 178 organic aerosols. The ratio of SOA mass to background organic mass ranged from 7 to 179 59, indicating that the contribution from background organics was negligible. Aerosol 180 particles typically pass through a silica gel diffusion dryer to remove ALW before they 181 are measured by AMS. However, this may lead to some losses of semivolatile organics 182 through reversible partitioning (Wong et al., 2015;Faust et al., 2016). In this study, the 183 AS/SOA mixed particles stream passed through and bypassed a diffusion dryer 184 alternately before they were measured by AMS. Overall less than 8% of SOA were lost 185 for wet and dry AS seeds after passing the diffusion dryer ( Fig. S2 The ALW content of the initially dry AS was zero. However, as reactions proceed, SOA 199 themselves can uptake water and also lower the deliquescence RH of AS, leading to where VAS and VSOA represent the volume concentrations of dry AS and SOA particles, 206 κAS is the hygroscopicity parameter of AS particles obtained from Kreidenweis et al.

207
(2008), κSOA is the hygroscopicity parameter of toluene-derived SOA calculated using 208 the linear correlation between κSOA and the O:C ratios of SOA proposed by Lambe et al.

209
(2011b), the term f is the fraction of AS particles that dissolved, αw is the water activity 210 and ρw is the density of water (1.0 g cm -3 ). Here, αw was assumed to be equivalent to 211 RH/100 for simplicity. The volume concentrations of dry AS and SOA particles were 212 estimated from the measured mass concentration of AS and SOA assuming their 213 respective particle densities to be 1.77 g cm -3 and 1.4 g cm -3 (Ng et al., 2007).

214
For the initially wet AS seeds, all AS particles were completely aqueous and 215 therefore f = 1. For the initially dry AS seeds, before reactions, the AS particles were (DRH(ε)) of AS particles coated with toluene-derived SOA (Smith et al., 2013): The term ε is the volume fraction of SOA. The term εD, representing the volume fraction 222 of organics at which the mixture of SOA and AS particles deliquesced at an RH of 68%, 223 was estimated to be 0.75 based on the liquidus curve. loading and OH exposure but a lower RH with dry AS seeds (Ng et al., 2007;244 Hildebrandt et al., 2009). Note that the wall loss of particles was not corrected in this 245 study, so the SOA yields may be underestimated. As wet and dry AS seeds in this study 246 had similar particle number size distributions, the wall loss of particles would not affect 247 the comparison of SOA yield between wet and dry AS seeds.

248
As shown in Fig. 2a, a higher SOA yield was observed for wet AS seeds than for 249 dry AS seeds at the same OH exposure and the difference in SOA yield decreased as 250 the OH exposure increased. The ratio of SOA yields on wet AS seeds to those on dry 251 AS seeds was 1.31±0.02 at an OH exposure of 4.66×10 10 molecules cm -3 s but 252 decreased to 1.01±0.01 when the OH exposure was increased to 5.28×10 11 molecules 253 cm -3 s (Fig. 2b). These ratios are comparable to the 1.19±0.05 observed by Faust et al.

255
The formation of SOA on initially dry AS particles may alter the deliquescence 256 relative humidity (DRH) of AS particles. Smith et al. (2013) found that when coated 257 with toluene-derived SOA, the DRH of AS particles decreased from 80% to 58% as the 258 organic volume fraction increased from 0 to 0.8. Therefore, coating AS particles with  aging would cause f43 and f44 to converge toward the triangle apex (f43 = 0.02, f44 = 0.30).

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For both wet and dry AS seeds, f43 first increased and then decreased with the increase 304 of OH exposure, while f44 increased all the time. This reversing trend of f43 was the 305 result of the increase and subsequent decrease in C2H3O + (Fig. S4), an indicator of 306 products containing carbonyl functional groups. It was also observed for SOA formed 307 from other precursors such as alkanes and naphthalene (Lambe et al., 2011b). Before 308 the decrease in f43, SOA formed on wet AS seeds had higher f43 and similar f44 to SOA 309 formed on dry AS seeds at the same OH exposure. As OH exposure increased, SOA 310 formed on wet AS seeds had higher f44 and lower f43 than SOA formed on dry AS seeds.

311
The f43-f44 plot supports our earlier assertion that as OH exposure increased, the reaction 312 products changed from earlier-generation products containing carbonyl functional 313 groups to later-generation products containing acidic functional groups. In addition, as 314 OH exposure increased, SOA formed on wet AS seeds initially had more earlier-315 generation products but later had more acidic later-generation products than SOA 316 formed on dry AS seeds, likely due to the enhanced partitioning of these products on 317 initially wet AS seeds and/or enhanced uptake of water-soluble gases through aqueous 318 phase reactions.