Response of fine aerosol nitrate chemistry to Clean Air Action in winter Beijing: Insights from the oxygen isotope signatures

摘要

The implementation of Clean Air Actions improved air quality in Beijing, and the mass loadings of PM_(2.5) and numerous gaseous precursors decreased considerably. However, concentrations of particulate nitrate (p-NO_3) changed slightly in wintertime aerosol and its mass fraction increased from 11.3% in 2013 to 29.8% in 2018. Therefore, understanding the characteristics of nitrate chemistry is imperative for controlling the aerosol pollution in the context of emission reductions in China. To this end, the properties of aerosol, chemistry of nitrate during the past winter seasons of Beijing (2013-2017) were investigated. Results indicated that nitrate remained entirely in the particle phase. The weak response of p-NO~-_3 to NOx emission reduction (～17.9%) was attributed to the enhanced gaseous HNO_3 formation. The oxygen isotopic signatures of p-NO~-_3 (δ~(18)O-NO~-_3) in winter 2017 of Beijing (75.0 ± 12.6‰) was significantly lower than that in winter 2014 (82.6 ± 12.7‰), but comparable with that in Puding (77.3 ± 4.6‰, background site of Southwest China) and Nanning (72.9 ± 4.9‰, urban site in South China) of winter 2017. We inferred an increased photochemistry activity in the formation of winter p-NO~-_3 in Beijing recently. The enhanced photochemistry was mainly due to the increased photolysis of HONO, a major source of atmospheric hydroxyl radical (•OH) in polluted urban environment. The considerable generated HONO was attributed to the photolysis of p-NO~-_3 according to δ~(18)O isotopic evidence. The present study implied that the response of aerosol species to emission reductions and their feedbacks associated with the atmospheric oxidants and aerosol properties were complex and needed further investigations.