Characterizing nitrate radical budget trends in Beijing during 2013-2019

Nitrate (NO₃) radical is an important oxidant in the atmosphere as it regulates the NO_x budget and impacts secondary pollutant formation. Here, a long-term observational dataset of NO₃-related species at an urban site in Beijing was used to investigate changes in the NO₃ budget and their atmospheric impacts during 2013-2019, in this period the Clean Air Actions Plan was carried out in China. We found that (1) changes in NO₃ precursors (NO₂ and O₃) led to a significant increase in NO₃ formation in the surface layer in winter but a decrease in summer; (2) a reduction in NO_x promoted thermal equilibrium, favoring the formation of NO₃ rather than dinitrogen pentoxide (N₂O₅). The simultaneous decrease in PM_2.5, during these years, further weakened the N₂O₅ heterogeneous uptake; (3) a box model simulation revealed that both the reactions of NO₃ with volatile organic compounds (VOC) and N₂O₅ uptake were weakened in summer, implying that the policy actions implemented help to moderate secondary aerosol formation caused by NO₃ and N₂O₅ chemistry in summer; and (4) during winter, both NO₃ + VOC and N₂O₅ uptake were enhanced. Specifically, for the N₂O₅ uptake, the rapid increase in NO₃ production, or to some extent, NO₃ oxidation capacity, far outweighed the negative shift effect, leading to a net enhancement of N₂O₅ uptake in winter, which indicates that the action policy implemented led to an adverse effect on particulate nitrate formation via N₂O₅ uptake in winter. This may explain the persistent winter particulate nitrate pollution in recent years. Our results highlight the systematic changes in the NO₃ budget between 2013 and 2019 in Beijing, which subsequently affect secondary aerosol formation in different seasons.