Impact of clean air action on the PM pollution in Beijing, China: Insights gained from two heating seasons measurements.

Comprehensive observations have been carried out in Beijing to investigate the impact of the Clean Air Action implemented in 2013 on changes in aerosol chemistry characteristics in heating seasons of 2016-2017 and 2017-2018. Results showed that PM, SO, NO, NH, O and CO concentrations decreased by 40.9%, 46.0%, 29.0%, 40.6%, 11.0% and 44.4%, respectively. Significant decreases were also observed for NO (32.5%), SO (52.9%), NH (56.0%), Cl (64.6%) and K (68.2%), on average. Enhanced PM pollution has changed from sulfate-driven to nitrate-driven. The decrease in SO was more significant than NO as a response to one reason of the larger decrease in SO concentration. The formation of sulfate was dominated by heterogeneous reactions in two heating seasons. Low pH could facilitate more efficient conversion of SO to sulfate. Photochemical reactions played a much more important role in the formation of nitrate in the second heating season, especially in the daytime. The major source regions for sulfate and nitrate were identified by back trajectories and the potential source function (PSCF). More nitrate was brought into Beijing when air masses coming from polluted regions in the southwest prevailed in 2017-2018 heating season. Thus, regional joint prevention and control are of great importance in the achievement of an effective reduction in PM pollution in the future.