To control the spread of COVID-19, China implemented a series of lockdowns, limiting various offline interactions . This provided an opportunity to study the response of air quality to emissions control . By comparing the characteristics of pollution in the summers of 2019 and 2020, we found a significant decrease in gaseous pollutants in 2020 . However, particle pollution in the summer of 2020 was more severe; PM 2.5 levels increased from 35.8 to 44.7 μg m -3, and PM 10 increased from 51.4 to 69.0 μg m -3 from 2019 to 2020 . The higher PM 10 was caused by two sandstorm events on May 11 and June 3 , 2020, while the higher PM 2.5 was the result of enhanced secondary formation processes indicated by the higher sulfate oxidation rate (SOR) and nitrate oxidation rate (NOR) in 2020 . Higher SOR and NOR were attributed mainly to higher relative humidity and stronger oxidizing capacity . Analysis of PM x distribution showed that severe haze occurred when particles within Bin2 (size ranging 1-2.5 μm) dominated . SO 4 2- (1/2.5) and SO 4 2- (2.5/10) remained stable under different periods at 0.5 and 0.8, respectively, indicating that SO 4 2- existed mainly in smaller particles . Decreases in NO 3 - (1/2.5) and increases in NO 3 - (2.5/10) from clean to polluted conditions, similar to the variations in PM x distribution, suggest that NO 3 - played a role in the worsening of pollution . O 3 concentrations were higher in 2020 (108.6 μg m -3) than in 2019 (96.8 μg m -3). Marked decreases in fresh NO alleviated the titration of O 3 . Furthermore, the oxidation reaction of NO 2 that produces NO 3 - was dominant over the photochemical reaction of NO 2 that produces O 3, making NO 2 less important for O 3 pollution . In comparison, a lower VOC/NO x ratio (less than 10) meant that Beijing is a VOC-limited area; this indicates that in order to alleviate O 3 pollution in Beijing, emissions of VOCs should be controlled.
Index: COVID-19, O(3) pollution, Oxidizing capacity, PM(x) distribution, Secondary formation