Analysis of chemical composition , source and process 1 characteristics of submicron aerosol in the Summer of 2 Beijing , China

11 12 In this study, aerosol chemical speciation monitor (ACSM) and various collocated 13 instruments are used to observe and analyze the chemical compositions, source and 14 extinction characteristics of submicron aerosol (PM1, aerodynamic diameter less than 15 1 μm) in Beijing from July to September, 2012. The results show that the average 16 mass concentration of PM1 during the whole observation period is 53.8μg m, 17 accounting for 70-85% of PM2.5 on average. From July to September, the average 18 mass concentration of non-refractory submicron aerosol (NR-PM1) declines monthly 19 with the increasing fraction of OA in NR-PM1. The organics aerosol (OA) contributes 20 the major mass fraction of PM1 during the cleaning days, and the fraction of 21 inorganics shows a significant increasing trend with the accumulation of pollutants. 22 The effects of meteorology on PM pollution and aerosol processing are also explored. 23 In particular, SOR increase significantly at elevated relative humidity (RH) periods 24 which suggested that the conversion of SO2 to SO4 in pollution episodes is more 25 effective through the aqueous-phase oxidation of SO2 instead of the gas-phase 26 oxidation. In addition, the effect of wind speed on the primary species (PPM) is 27 significantly weaker than that on the secondary species (SPM). In addition, the mass 28 concentration of SPM (or organics) is more sensitive to wind speed changes, 29 * Corresponding Author: , E-mail address: sunyele@mail.iap.ac.cn AC CE PT ED M AN US CR IP T 2 compared with PPM (or inorganics). The proportion of oxygenated OA (OOA) in OA 30 is significantly higher than that of hydrocarbon-like OA (HOA), and as the proportion 31 of OA in PM1 increases, the mass fraction of OOA in OA decreases gradually. 32 Moreover, the particulate matter (PM) in Beijing shows essentially neutral during the 33 observation period. The total extinction coefficient of PM tracks well with the PM1 34 (r= 0.72), and the extinction efficiency of the secondary particulate matter (SPM) 35 (r= 0.92) is significantly higher than that of the primary particulate matter (PPM) 36 (r= 0.58). Meanwhile, the correlation between OA and extinction coefficient (r = 37 0.56) is weaker than that between inorganics and extinction coefficient (r = 0.86). 38 39

. Among them, organic aerosol is 69 the major fraction, accounting for 45% of PM 1 on average, and its sources include 70 both primary organics discharged directly and secondary organics generated by 71 photochemical or liquid-phase chemical reactions (Ci et al., 2013;Zhang et al., 2007). 72 The sources of PM 1 depend on sites and seasons, mainly including secondary 73 inorganics species (sulfate + nitrate + ammonium salt), mineral dust, motor vehicle  82 and the results showed that there were significant differences in aerosols chemical 83 composition between pollution days and clean days. The average contribution of OA 84 to PM 1 was about 70% during clean days, while the proportion of secondary 85 inorganics species increased significantly during pollution days, more than ~50%. 86

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4 impact of PM 1 on regional and global climate change is mainly reflected in both direct 89 and indirect radiation effects. The indirect effect is of the most uncertainties on 90 radiative forcing, and one reason is that previous studies could not accurately measure  The positive matrix factorization (PMF) method is performed on ACSM organic 170 aerosol mass spectra for source apportionment, following the procedures described by  Table 2. 215 During the whole observation period, the variation trend of NR-PM 1 measured by 216 ACSM is in good agreement with the variation of PM 2.5 measured by TEOM, which 217 indicates the good operation state of the instrument and the feasibility of the data. periods (PM 1 < 20 μg m -3 ), which usually shows a "sawtooth" structure variation of 231 the asymmetric structure: slow fluctuation accumulation and rapid clearance 232 mechanism. At the same time, the duration of clean periods is usually short (generally 233 less than 3 days), while the days of pollution often last longer, sometimes even more

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9 than a week (e.g., the pollution episode on July 16-21). The switch of clean and 235 polluted days is closely related to meteorological elements in Beijing. Comparatively, 236 the serious pollution episodes are usually associated with higher relative humidity 237 (RH > 60%) and easterly or southerly winds less than ~1.5 m·s -1 . 238 Organics is one of the most important components of submicron aerosols in 239 Beijing during observation, accounting for ~39% of PM 1 on average, followed by 240 nitrate (~21%), sulfate (~17%), ammonium (~14%), BC (~8%), and chloride (~1%). pollution process from September 7th to 12th, the SOR is always higher than 0.65, and

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10 the contribution of sulfate to NR-PM 1 has increased by ~10%-15%. At the same time, 264 pollutants continue to accumulate in this period due to the higher relative humidity. The formula for estimating the concentration of ammonium particles is as follows: . 289 A ratio of NH 4 + measured /NH 4 + Predicted less than 1 indicates that the atmospheric 290 aerosol is acidic and lower ratio suggests higher acidity. On the contrary, the 291 atmospheric aerosol is considered to be alkaline. If the ratio is closed to 1, the

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11 atmospheric aerosol is considered to be basically neutral.  In July and August, the sulfate presents an enhanced noon peak, and the peak in 396 August is significantly lower than that in July. However, such peak is not observed in 397 September. One of the reasons for the peak of SO 4 2is the photochemical production 398 of H 2 SO 4 from SO 2 + OH, which is enhanced with the solar light intensity at noon. In temperature-dependent gas-particle partitioning of NH 4 Cl and the change of the PBL. 423 The diurnal variation of chloride shows a small peak at ~20:00 LT in July, which is 424 supposed to be caused by local emissions. 425 The diurnal variation of the ammonium is not significant, which shows a small  (Fig. 7) 438

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16 440 The extinction of the particles includes both light scattering and light absorption 441 of the particles, and scattering is dominant. Figure 8 shows  Fig. 9 shows that the diurnal cycles of extinction coefficient reach a maximum at 479 about 10:00, which corresponds well with the trend of nitrate and ammonium (Fig.7). 480 Therefore, the peak may be mainly caused by the extinction of ammonium nitrate. In  organics when the wind speed is less than 2.5 m s -1 . 511 To further explore the effects of wind on PM species, the wind speed is separated 512 into two groups with relative humidity (RH) higher than 60% and lower than 60%. 513 The average mass concentration of each PM 1 species at high RH is higher than that at