Elevated particle acidity enhanced the sulfate formation during the COVID-19 pandemic in Zhengzhou, China()

The significant reduction in PM(2.5) 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 PM(2.5), 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 PM(2.5) 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 PM(2.5) during the COVID-lockdown period. Moreover, nitrate production was enhanced by the elevated O(3) concentration, which was favorable for the homogeneous and hydrolysis nitrate formation despite the drastic decrease of NO(2). The proportion of sulfate during the COVID-lockdown period was higher than that before. Aqueous-phase reactions of H(2)O(2) 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.