China has implemented "Air Pollution Prevention and Control Action Plan" (the "Clean Air Actions" for short) since September 2013 to tackle atmospheric fine particles in Beijing in autumn and winter.

To evaluate the impact of the "Clean Air Actions", Prof. WANG Yuesi and his team from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences observed the non-refractory submicron aerosol (NR-PM₁) in Beijing from 2012 to 2018, using the high-resolution aerosol mass spectrometer (HR-TOF-AMS).

"We found that the 'Clean Air Actions' played a major role in the reduction of NR-PM₁ concentration. NR-PM₁ concentrations decreased by 44.1% in autumn and 73.2% in winter from 2012 to 2018," said Prof. WANG.

According to their paper published in Environmental Pollution, sulfate showed a much larger reduction than nitrate and ammonium in both autumn and winter 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.

Biomass burning organic aerosol and coal combustion organic aerosol were the primary organic species with the greatest reductions in autumn and winter from 2012 to 2018, respectively, while hydrocarbon-like organic aerosol exhibited the smallest decline in both autumn and winter. These significant changes in aerosol compositions were highly consistent with the much faster reduction of SO₂ (75-85%) than NO_x (36-59%) and were mainly due to the "Clean Air Actions" rather than the impact of meteorological conditions.

"We also found the enhanced atmospheric oxidizing capacity strengthens photochemical reactions and results in the increased oxidation degree of less-oxidized oxygenated organic aerosol in autumn. We should note this phenomenon when we design emission reduction measures in the future," said Prof. WANG.