Long-term characterization of aerosol chemistry in cold season from 2013 to 2020 in Beijing, China

Severe haze episodes in cold season in Beijing have been mitigated greatly during the last decade. However, the changes in aerosol chemistry as responses to the large reductions in gaseous precursors during the two phases of clean air action, i.e., phase I (2013-2017) and phase II (2018-2020), are less understood. Here we characterized such changes in cold season (JanuaryeMarch) by using five-year realtime aerosol particle composition measurements. Our results showed consistently large reductions for all chemical species from 2013 to 2020 with the largest decreases being chloride (95%) and organics (74%) followed by sulfate (69%), while the decreases in nitrate were comparatively small (44%). However, the contributions of sulfate were fairly stable despite the increased nitrate contributions from 18% in 2013 to 30% in 2020. Organic aerosol (OA) composition also changed significantly since 2018 with large increases in the contributions of secondary OA and corresponding decreases in primary OA from fossil fuel combustion and cooking emissions. The changes in aerosol chemistry were closely related to the different reductions in gaseous precursors, e.g., SO2 vs. NO2, and the enhanced secondary processes, e.g., the increases in O-3, sulfur and nitrogen oxidation efficiency. Further, we found that the changes in aerosol chemistry in cold season during the phase II of clean air action (2018-2020) started to slow down with relatively small changes in PM2.5 and secondary inorganic species. Our results point towards a future challenge in mitigating air pollution in cold season, and the need of more stringent and scientific strategies to control secondary aerosol pollution in an environment with enhanced oxidation capacity and high precursors. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The study found significant changes in aerosol chemistry in Beijing during the cold season, with most chemical species showing reductions, especially chloride and organics. Starting from 2018, the changes in aerosol chemistry were closely related to different reductions in gaseous precursors and enhanced secondary processes.