Seasonal variations in chemical characterization of submicron aerosol

Seasonal variations in chemical characterization of submicron aerosol particles in Shanghai, China: Insights from a highresolution aerosol mass spectrometry Wenfei Zhu 1, 2, Shengrong Lou 2*, Zhen Cheng 3, Naiqiang Yan 3, Song Guo 1 1 State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China; 2 State Environmental Protection Key Laboratory of Formation of Urban Air Pollution Complex, Shanghai Academy of Environmental Sciences, Shanghai 200233, China; 3 School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Introduction Submicron particulate matter PM 1 is an important component of urban atmospheric fine particulate matter PM 2. 5, accounting for about 80% to 90% of the total mass of PM 2. 5 (Hu et al. , 2016 b; Zhang et al. , 2018). Yet the study comprehensively characterized the seasonal variations in PM 1 under different meteorological conditions and pollutant sources in megacity Shanghai is lacking, especially based on high-mass-resolution measurements. For these reasons, we report in this work the real-time measurement results at urban sites in Shanghai using a high-resolution time-of-flight AMS (HR-To. F-AMS) and other online instruments from 2016 to 2017. Here we have a detailed seasonal characterization of submicron aerosol species including mass concentrations, chemical composition, diurnal variations. This study is of great significance for further understanding complex air pollution and providing scientific support for model simulations of atmospheric aerosols, and also serves as a theoretical basis for fine particulate pollution control. Fig. 2. The van Krevelen diagram of OA during seasonal observations in Shanghai Fig. 3. Diurnal variations of mass concentrations and average fractions in OA components in (a) spring, (b) summer, (c) autumn and (d) winter observations in Shanghai ü Main experimental instruments üSource apportionment via PMF on OA mass spectra resolved four components, i. e. , HOA, COA, LO-OOA, and MO-OOA during seasonal observations. ü The mass fractions of COA remained relatively stable in four seasons (24% in summer and 32% in winter), which is an important source of OA in four seasons in Shanghai. ü OOA accounts for a higher fraction (61%) of OA during summertime than other seasons in Shanghai due to strong photochemical oxidation. üSecondary particles (SNA+SOA) accounted for ~70% of total PM 1 in Shanghai, stressing the importance of control making for reducing emissions of gaseous precursors such as NOx, SO 2 and VOCs. üLocation of observation sites of four seasons in Shanghai Results and Discussion üThe average mass concentrations of submicron particulate matter during spring, summer, autumn and winter observations in Shanghai are 23. 9 ± 20. 7 μg/m 3, 28. 5 ± 17. 6 μg/m 3, 22. 0 ± 17. 2 μg/m 3 and 31. 9 ± 22. 7 μg/m 3, respectively. The major chemical components in submicron particulate matter showed obvious seasonal and daily variations. üOA was the most important component in PM 1 in Shanghai (39%-49%). Sulfate and nitrate accounted for a high fraction of PM 1 in summer and winter in Shanghai, respectively. ü The increase of submicron particulate matter is mainly due to the contribution of nitrate in spring, autumn and winter, while the photochemical reaction promotes the rapid growth of sulfate in summer. Fig. 1. Fractions of main chemical compositions in submicron aerosols at different PM 1 levels during the (a) spring, (b) summer, (c) autumn and winter (d) observations in Shanghai Fig. 4. The ratio of MO-OOA to LO-OOA and O/CSOA at different RH and Ox levels during the (a) spring, (b) summer (c) autumn and (d) winter observations in Shanghai ü The liquid phase oxidation have a significant contribution to the formation of more oxidized oxygenated organic aerosol, in addition, the strong photochemical reaction have a significant contribution to less oxidized oxygenated organic aerosol in the spring, summer and winter observations. üO/CSOA showed remarkably similar RH/Ox dependency to MO-OOA / LO-OOA during the four seasons. This may be due to the fact that O/CSOA is mainly contributed by MO-OOA having a higher oxidation degree than LO-OOA. These results suggest the idea that aqueousphase processing also played an important role in enhancing oxidation degree of OA substantially in spring, summer and winter. Acknowledgement This work is supported by the National Key Research and Development Program of China (2016 YFC 0208700), National Natural Science Foundation of China (21607100) and National Natural Science Foundation of Shanghai (15 ZR 1434700).