Decade-long trends in chemical component properties of PM_2.5 in Beijing, China (2011-2020)

A 10-year-long measurement of water-soluble inorganic ions in PM_2.5 was made in Beijing from June 2011 to December 2020, to investigate the interannual trends of chemical characteristics of PM_2.5 and to provide insights into the future prevention and control of PM_2.5 pollution. From 2011 to 2020, with the implementation of strict air pollution control strategies, significant changes of PM_2.5 have been observed in Beijing, with NO₃^-, SO₄^2- and NH₄⁺ decreasing at rates of 5.10, 8.80 and 7.64% yr^-1 respectively. The percentages of NO₃^- and SO₄^2- under elevated pollution levels were investigated. When PM_2.5 values fell in the range of 0-400 μg m^-3, NO₃^-/ SO₄^2- values were mostly higher than 1 and showed upward trends from 2011 to 2020. However, under extremely heavy haze conditions, SO₄^2- dominated PM_2.5 formation. This result was closely related to the change characteristics of the oxidation ratio of sulfate (SOR), the oxidation ratio of nitrate (NOR) and gaseous precursors under different pollution levels. The change characteristics of NOR and SOR under elevated PM_2.5 levels indicated that the aqueous phase oxidation was the key process driving SO₄^2- formation; while as for NO₃^-, in addition to the availability of NH₄⁺, the atmospheric oxidation capacity made crucial roles. The analysis of typical haze episodes during the past decade indicated that the emission reduction of gaseous pollutants, especially SO₂, made great contributions to the improved PM_2.5 air quality in Beijing. We highlighted that future efforts should focus on enhanced reduction of NO₂ emission and control of atmospheric oxidation capacity to further reduce particulate nitrate formation.