The significant reduction in PM2.5 mass concentration after the outbreak of
COVID-19 provided a unique opportunity further to study the formation mechanism of secondary inorganic
aerosols. Hourly data of chemical components in PM2.5, gaseous
pollutants, and meteorological data were obtained from January 1 to 23, 2020 (pre-lockdown) and January 24 to February 17, 2020 (COVID-lockdown) in Zhengzhou, China.
Sulfate,
nitrate, and
ammonium were the main components of PM2.5 during both the pre-lockdown and COVID-lockdown periods. Compared with the pre-lockdown period, even though the concentration and proportion of
nitrate decreased,
nitrate was the dominant component in PM2.5 during the COVID-lockdown period. Moreover,
nitrate production was enhanced by the elevated O3 concentration, which was favorable for the homogeneous and hydrolysis
nitrate formation despite the drastic decrease of NO2. The proportion of
sulfate during the COVID-lockdown period was higher than that before. Aqueous-phase reactions of H2O2 and transition
metal (TMI) catalyzed oxidations were the major pathways for
sulfate formation. During the COVID-lockdown period, TMI-catalyzed oxidation became the dominant pathway for aqueous-phase
sulfate formation because the elevated acidity favored the dissolution of TMI. Therefore, the enhanced TMI-catalyzed oxidation affected by the elevated particle acidity dominated the
sulfate formation, resulting in the slight increase of
sulfate concentration during the COVID-lockdown period in Zhengzhou.