Evaluating the impact of control measures on sulfate-nitrate-ammonium aerosol variations and their formation mechanism in northern China during 2022 Winter Olympic Games

Sulfate, nitrate, and ammonium (SNA) significantly contribute to fine particulate matter (PM2.5). To ensure good air quality during 2022 Winter Olympic Games (WOG), stringent control measures were implemented in northern China. In this study, comprehensive atmospheric observations were conducted from 2022 January to March in four representative cities to investigate the changes in SNA and their formation mechanism in response to these control measures. The SNA concentrations obviously decreased (29.45%-42.83%) during the control periods in the four cities, but their proportion of PM2.5 remained relatively stable (>50%). SNA was the key factor driving the decrease and increase in the PM2.5 concentration during WOG, with nitrate making the most prominent contribution. The formation of SNA was largely dominated by aqueous heterogeneous chemistry instead of gas-phase oxidation. However, the aerosol liquid water content (ALWC) decreased while odd oxygen (O-X) increased during the control periods. Such a variation influenced the role of aqueous heterogeneous and gas-phase reactions in SNA formation, with the former's dominant role being replaced by the latter. After the control periods, the aqueous heterogeneous reactions were enhanced again due to the increased ALWC. Among the cities in the study region, SNA formation became more intense from north to south with increased air temperature and relative humidity (RH). Moreover, ISORROPIA-II thermodynamic modelling revealed notable impacts of sulfate, ammonium, and nonvolatile cations (NVCs) on variations in particle acidity throughout WOG. These results provide a more comprehensive understanding of SNA responses to pollution control on a regional scale and insight into future conventional control.