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

Abstract

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.

Effect of restricted emissions during COVID-19 on atmospheric aerosol chemistry in a Greater Cairo suburb: Characterization and enhancement of secondary inorganic aerosol production

“…As a result, the concentrations of both NO 3 − and SO 4 2− in PM 2.5 still decreased, although their secondary production rates increased. Consistent with previous studies ( Chang et al, 2020 ; Huang et al, 2021 ; Yang et al, 2022 ), the NO 3 − formation was enhanced relative to the reduction of NO 2 emission during the COVID-lockdown period in Beijing, Zhengzhou, and Shanghai. In China (Yangtze River Delta region), nitrogen oxide (NOx) reductions enhanced O 3 accumulation and therefore increased secondary aerosol formation ( Wang et al, 2021 ).…”

“…The oxidation rates of SO 2 to SO 4 2− and NO 2 to NO 3 − during the COVID lockdown were higher than those during the pre-COVID period ( Liu et al, 2021 ). Sulfur and nitrogen conversion ratios during the COVID lockdown period were higher than those during the pre-lockdown period ( Yang et al, 2022 ). In the present study, the relatively higher values of F SCR and F NCR in the COVID partial lockdown suggest that the oxidizing capacity of the atmosphere was significantly enhanced during the COVID partial lockdown period, and consequently, the oxidation of SO 2 to SO 4 2− and NO 2 to NO 3 − increased.…”

“…NOx reductions can increase O 3 levels by decreasing the NOx titration (through NO + O 3 and also via nocturnal NO 2 + O 3 reactions) ( Leung et al, 2020 ). The elevated O 3 concentration was favorable for the formation of NO 3 − despite the drastic decrease of NO 2 during the COVID lockdown period ( Chang et al, 2020 ; Huang et al, 2021 ; Lv et al, 2020 ; Xu et al, 2020 ; Yang et al, 2022 ). An increase in the oxidation capacity (O 3 ) of the atmosphere led to the enhancement of sulfur oxidation ratio, and then SO 4 2− formation during the COVID lockdown ( Duan et al, 2021 ).…”

“…This means that the transformations of NO 2 to NO 3 − and SO 2 to SO 4 2− were improved during the COVID partial lockdown. Yang et al (2022) reported that the disproportionate drop of particulate NO 3 − compared to NO 2 gas concentration (60%) implied the improved conversion of NO 2 during the COVID lockdown period. Compared with those during the pre-COVID period, Cl − , NH 4 + , Ca 2+ , K + , Mg 2+ , and Na + concentration reductions were 28.79%, 4.24%, 47.93%, 21.03%.…”

“…Previous research articles were focused on the status of air pollution, aerosol chemical composition, sources and secondary pollutants like O 3 before and during the COVID-19 lockdown period in different countries around the world (e.g. Liu et al, 2021 ; Chang et al, 2020 ; Huang et al, 2021 ; Yang et al, 2022 ; Shrestha et al, 2020 ; Tobías et al, 2020 ; Wang and Su, 2020 ; Zhang et al, 2020 ; Muhammad et al, 2020 ; Dutheil et al, 2020 ; Collivignarelli et al, 2020 ; Venter et al, 2020 ; Parra and Espinoza, 2020 ; Zambrano-Monserrate and Ruano, 2020 ; Hashim et al, 2020 ; He et al, 2021a , He et al, 2021b ; He et al, 2021a , He et al, 2021b ). However, there are no data concerning the effect of partial lockdown on the chemical characterization of atmospheric aerosol and the secondary inorganic aerosol production in a semi-desert climate area like Egypt.…”