Evolution of source attributed organic aerosols and gases in a 1 megacity of central China 2

. The secondary production of the oxygenated organic aerosol (OOA) impacts air quality, climate and human 18 health. The importance of various sources in contributing to the OOA loading and associated different aging mechanisms 19 remains to be elucidated. Here we present concurrent observation and factorization analysis on the mass spectra of organic 20 aerosol (OA) by a high-resolution aerosol mass spectrometer and volatile organic compounds (VOCs) by a proton-transfer- 21 reaction mass spectrometer in Wuhan, a megacity in central China during autumntime. The full mass spectra of organics 22 with two principle anthropogenic sources were identified as the traffic and cooking sources, for their primary emission 23 profiles in aerosol and gas phases, the evolutions, and their respective roles in producing OOA and secondary VOCs.


Introduction 32
The transformation between gas and aerosol phase of organic species produces secondary organic aerosols (SOA) 33  Kroll and Seinfeld, 2008). Among these sources, SOA has been 50 found to be the main contributor of OA mass loading (41~69%) in urban environment of East Asia Hu et 51 al., 2017), especially in warm season when primary emissions were low, along with high ambient temperature and more 52 intensive chemical reactions (Hu et al., 2016). The formation of SOA from VOCs may experience a few reaction generations 53 (Knote et al., 2014) and could interact with other sources of species during the process (Shrivastava et al., 2017), hereby 54 complicating the goal of identifying the key precursors in contributing to the consequent SOA. In addition, some primary 55 gases already have somehow low volatility and may not require a long reaction chain to become condensable, such as some 56 primarily emitted intermediate volatility organic compounds (IVOCs) may substantially contribute to the SOA (Robinson et 57 al., 2007;Huang et al., 2021). An understanding of source profiles of primary emissions in both gas and aerosol phases is 58 therefore important to rule out the source-dependant production of SOA. The above necessitates the concurrent investigation 59 on the compositions of gases and aerosols at a receptor site, along with their evolution and interaction, in order to elucidate 60 the role of each source in contributing to SOA. 61 In this study, we performed online continuous measurements on the detailed mass spectra of organics concurrently on 62 aerosol and gas phases, in a typical anthropogenically polluted region in central China, where such data had been rarely 63 https://doi.org/10.5194/acp-2022-141 Preprint. Discussion started: 2 March 2022 c Author(s) 2022. CC BY 4.0 License.   Table S1). The diurnal pattern of this factor showed peaks in the morning and afternoon rush-hour (Fig. 2i), 165 with a major increase from 5:30, reaching a peak value of 1.5 µg m -3 at 8:30. The concentration gradually decreased around 166 noontime due to boundary layer dilution until 15:00 and reached a minimum of 0.6 µg m -3 . This spectrum also contained 167 fragment marker for possible coal combustion OA (CCOA), i.e. C9H7 + (m/z 115, r = 0.73) (Hu et al., 2013). This factor was 168 not distinctly resolved in this dataset maybe due to the urban nature of the site, where the traffic source may have 169 overwhelmed, due to the less significant coal combustion pollutants during the sampling period.

7
The second factor was characterized by m/z 55 (C4H7 + , C3H3O + ) and 57 (C4H9 + , C3H5O + ), accounting for 10% and 3.5% 171 of the total spectrum, respectively (Fig. 2b), with the lowest O/C ratio among factors (0.16). This factor has a 172 C3H3O + /C3H5O + of 3 and C4H7 + /C4H9 + of 2 (1 is usually for HOA), indicating it as cooking source rather than HOA (Mohr et 173 al., 2012). The correlation coefficient of the COA factor and marker ion C6H10O + was 0.91, which is also similar to a 174 previous study (Sun et al., 2011). The diurnal pattern of this factor showed a major peaking during 18:00 -20:00, reaching 175 up to 4.0 µg m -3 on average, in addition to a smaller peak at lunch time, which corroborated the diurnal cooking activities.    Table S1), with a major increase during the early morning, reaching a peak value of 12.1 ± 1.3 ppb at 08:30, further 208 corroborating the traffic source of this factor. The concentration decreased around noontime until 15:00 because of the 209 dilution by well-developed boundary layer and its consumption through photochemical reaction due to intense solar 210 radiation. The diurnal pattern was also high at night, although the peak was lower than early morning, the average 211 concentration was higher by a factor of 2 than daytime. This suggests that traffic VOCs prefer to participate in 212 photochemical reaction, and other primary emissions may be precursors during nocturnal chemistry. 213  and C9H12 (C9-aromatics) at m/z 107.086 and 121.101 in VOC mass spectra (Fig. 3b), which are footprint VOCs identified 216 from primary cooking emission during the charbroiling and frying (Klein et al., 2016). This factor had similar time series 217 ( Fig. 3g) with COA and had high correlation (r=0.67, Table S1). The concentration of this factor decreased during the 218 daytime, and yet surged after 18:00 with a peak value at 19:00 (17.2 ± 3.0 ppb). As shown in Fig. 3l, the diurnal pattern 219 decreased strongly after emission throughout the night, suggesting that cooking VOCs may be major precursors and were 220 consumed during night. SecVOC1 factor featured with some less-oxygenated VOCs e.g., C2H2O2, C6H10O2 and C10H14O (Fig. 3d). This factor 234 had a peak at 19:00 which was consistent with the primary cooking VOCs factor, but also increased throughout the night, 235 peaking at midnight.This factor had a similar temporal trend with OOA1 (r = 0.76, Fig. 3i), which was less oxygenated than 236 photochemistry dominated OOA2. Combining the features above, SecVOC1 tended to be contributed by some immediately 237 reacted species from emissions in the late afternoon and early night. A particular factor (Fig. 3e) is significantly composed of 238 large molecular weight (large-MW) oxidized VOCs, i.e. the average on relative contribution of ionic compounds with 239 m/z>120 was above 50% in this factor (Fig. S5e), which was much higher than that in SecVOC2 (Fig. S5c). Fig. 3e shows 240 its signature compounds of C8H14O, C6H12O3 and C8H4O3, and some are nitrogen-containing VOCs, such as C6H5NO3 and 241 C8H9NO3. These VOCs with m/z>120 tend to be intermediate-volatility organic compounds (IVOCs) as the estimated vapor 242 saturation concentration is less than 6.5 μg m -3 (Fig. S2). This factor is hereby termed as large-MW VOCs to indicate the 243 fraction of IVOCs, which only require few oxidation steps to become semi-volatile (Robinson et al., 2007). Fig. 3o shows an 244 increase of this factor at mid-night, later than the peak of SecVOC1, which may imply the ageing process in producing these 245 VOCs.

Oxidation process of organics in the day and night 253
After source attribution of organics in aerosol and gas phase, we are able to identify the emission structure of primary 254 sources and the consequent evolution. Given the identified primary traffic and cooking sources were emitted in the day and 255 late afternoon respectively, this provides the potential opportunity to study the evolution of different primary sources in the 256 potentially contrasting ageing mechanisms in the day and night. 257 the day is higher than that at night by 0.2 (Fig. 4a), corresponding with the increase of highly oxygenated fragments of AMS 259 f44 (CO2 + ) in the day (Fig. 4b), but increase of moderately oxygenated fragment f43 (C2H3O + ) in the night. The diurnal 260 variation of atomic ratio O/C of OA (Fig. 4e)    Considering the diurnal pattern of anthropogenic activities, the traffic emission at rush-hour is deemed to be the major 266 source in contributing to the daytime production of OOA. The ratio OOA2/HOA is thus used to indicate the daytime 267 oxidation of OOA. As Fig. 4d and 4f shown, OOA2/HOA had a clear peak during daytime, and increased after 8:00 and 268 peaked at 14:00. After 15:00, the ratio gradually decreased to minimum at 20:00 and maintained to be low throughout the 269 night. This clearly demonstrated the photooxidation in producing OOA2 from oxidizing HOA. Fig. 4c and 4g give a few  270 examples of photooxidation in gas phase: the toluene showed a production of C3H6O3 (hydroxypropionic acid) and C4H6O3 271 (acetic anhydride) by a factor of 2 in 3 hours (Fig. S7c); the C3H4O (acrolein) produced the oxidized product C2H2O2 272 (glyoxal) by a factor of 1.3 (Fig. 4h) The nocturnal oxidation is mainly contributed by the sources emitting from late afternoon throughout the midnight. 281 Both traffic and cooking sources contributed to the emission since late afternoon, with cooking source as the predominant 282 contributor in both aerosol (Fig. 2j) and gas (Fig. 3l) phases. The ratio between the nighttime OOA1 and cooking aerosol 283 (OOA1/COA) is therefore used to indicate the nocturnal oxidation of SOA ( Fig. 4d and j). There was a sudden increase of 284 OOA1/COA during the daytime because COA was consumed rapidly in the afternoon after a small amount of emission at 285 noon (Fig. 2j). The lowest OOA1/COA at 0.5 corresponded with the fresh cooking emission at 18:00, and kept increasing 286 until peaking in the early morning at 6:00 up to 3, which was an increase by a factor of 6 compared to the minima (Fig. 4j). 287 In addition to evidence for daytime reaction, Fig. 4h  odd oxygen Ox (O3+NO2) has been widely used to generally indicate the activity of daytime photochemistry (Hu et al., 313 2017), and the enhanced moisture is main driving factor for nighttime chemistry. The Ox concentration and RH are therefore 314 used as references with which the variations of species were correlated in the day and night, respectively. As Fig. 5a shown, 315 the traffic primary emissions in both gas (traffic VOCs) and aerosol phase (HOA) declined with increased Ox by 60 % and 316 40% respectively, suggesting their roles as precursors in the daytime reaction. The produced species are oxygenated 317 SecVOC2 and OOA2, showing enhancement with Ox, peaking at midday-afternoon. This process was rapid as the SOA 318 production by a factor of 2.5 and the oxygenated VOCs production by a factor of 1.7 within 6 hours. The traffic VOCs are 319 widely observed to contribute to SOA production, with aromatic compounds serving as key precursors (Fang et al., 2021). 320 The semi-volatile nature of HOA means it could be evaporated to gas phase and further oxidized to recondense as SOA 321 (Robinson et al., 2007). The decrease rate of HOA with increased photochemical age was also found in urban environment 322 (Zhu et al., 2021), generally consistent with the reacted rate in this study. Here we linked the declining rate as a function of 323 photochemical activities for both reacted aerosol and gas phases for traffic sources. The gases evaporated from aerosol phase 324 (especially under higher temperature) and primary VOCs may be simultaneously involved in the photooxidation, further 325 contributing to the SOA formation. 326 For nocturnal oxidation shown in Fig. 5b, the reacted species are cooking VOCs and COA (decrease by 35 % and 77 %, 327 respectively), producing SecVOC1 and large-MW oxygenated VOCs, with an increase of night SOA formation by a factor 328 of 1.7. The nocturnal processes may have largely involved aqueous reactions, because the variations of reacted or produced 329 species were highly correlated with RH (Fig. 5b). The large-MW VOCs (mostly IVOCs) increased by 50% and reached 330 maxima when highest RH. This suggests the moisture may have been involved in converting some primary VOCs to IVOCs, 331 which further contribute to the SOA production during nighttime. Previous studies also found the oxidation of IVOCs from 332 cooking sources can be an important source of SOA (Zhang et al., 2020b). The evidence is given here that organic aerosols 333 and gases from cooking emission had been reacted and contributed to SOA. The production rate of 0.2 µg m -3 h -1 was 334 generally consistent with previous laboratory work using gas precursors from cooking sources (0.07-0.5 µg m -3 h -1 ) (Liu et 335 al., 2017). Notably, daytime SOA had a higher oxidation state, implying the importance of photooxidation in producing 336 highly oxidized OA. This may be because of the high temperature at daytime and a species may require a lower volatility 337 (hereby more oxygenated) to be in condensed phase than at night.

Conclusion 345
In this study, organic gases and aerosols were concurrently characterized through online mass spectrometers at a 346 megacity in central China. Through the factorization analysis on the organic mass spectra, two principal sources -the traffic 347 and cooking sources were identified for both aerosol and gas phases, hereby the reacted and produced species between 348 phases were interlinked. We observed clear evidence of daytime and nighttime oxidation of source-attributed OA and VOCs. 349 Daytime photooxidation caused 60 % decrease of primary aerosol and 40 % of primary VOCs reduction for traffic sources, 350 producing oxygenated SecVOCs by a factor of 1.7 and OOA by a factor of 2.5, in a 6 hours photochemical ageing. 351 Nocturnal ageing caused a reduction of primary OA (by 77%) and primary VOCs (by 35%) from cooking sources, producing 352 oxygenated VOCs and OOA by a factor of 1.4 and 1.7, respectively. In particular, larger molecular IVOCs produced (by a 353 factor of 1.7) at night may importantly contribute to the OOA. This implies primary species in aerosol and gas phases both 354 contribute to the production of OOA. A higher oxidation state of OOA from daytime photooxidation was found than 355 nighttime, suggesting different compositions of produced OOA modulated by solar radiation and moisture, respectively. As 356 vehicle and cooking emissions are the major contributors of organic aerosols in urban areas, especially in megacities. These 357 results provide direct observations about the reaction rate for primary precursors and production rate for secondary aerosols, 358 as influenced by primary sources and meteorological conditions. The environmental policy making should therefore consider 359 the respective primary sources and ageing mechanisms for local and regional atmospheric environmental problems. 360

Data availability 361
The data in this study are available from the corresponding author upon request. 362