Significant changes in autumn and winter aerosol composition and sources in Beijing from 2012 to 2018: Effects of clean air actions.

A seven-year long-term comprehensive measurement of non-refractory submicron particles (NR-PM1) in autumn and winter in Beijing from 2012 to 2018 was conducted to evaluate the effectiveness of the clean air actions implemented by the Chinese government in September 2013 on aerosols from different sources and chemical processes. Results showed that the NR-PM1 concentrations decreased by 44.1% in autumn and 73.2% in winter from 2012 to 2018. Sulfate showed a much larger reduction than nitrate and ammonium in both autumn (55%) and winter (86%) and that nitrate even slightly increased by 15.8% in autumn. As a result, aerosol pollution in winter gradually changed from sulfate-rich to nitrate-rich with a sudden change after 2016 and the dominant role of nitrate in autumn was also strengthened after 2016. Among primary organic aerosol (OA) types, biomass burning OA and coal combustion OA exhibited the largest decline in autumn and winter, with reductions of 87.5% and 77.3%, respectively, while hydrocarbon-like OA (HOA) exhibited the smallest decline in both autumn (24.4%) and winter (37.1%). These significant changes in aerosol compositions were highly consistent with the much faster reduction of SO2 (75-85%) than NOx (36-59%) and were mainly due to the clean air actions rather than the impact of meteorological conditions. What's more, the enhanced atmospheric oxidizing capacity, which was indicated by increased O3, altered the chemical processes of oxygenated OA (OOA), especially in autumn. Both of less-oxidized OOA (LO-OOA) and more-oxidized OOA showed elevated contributions in OA by 4% in autumn. The increased oxygen-to-carbon ratios of LO-OOA in autumn (from 0.42 to 0.58) and winter (from 0.44 to 0.52) indicated the enhanced atmospheric oxidizing capacity strengthened photochemical reactions and resulted in the increased oxidation degree of LO-OOA. This study demonstrates the effectiveness of the clean air actions for air quality improvement in Beijing.