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 levels increased from 35.8 to 44.7 μg m, and PM increased from 51.4 to 69.0 μg m from 2019 to 2020. The higher PM was caused by two sandstorm events on May 11 and June 3, 2020, while the higher PM 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 distribution showed that severe haze occurred when particles within Bin2 (size ranging 1-2.5 μm) dominated. SO and SO remained stable under different periods at 0.5 and 0.8, respectively, indicating that SO existed mainly in smaller particles. Decreases in NO and increases in NO from clean to polluted conditions, similar to the variations in PM distribution, suggest that NO played a role in the worsening of pollution. O concentrations were higher in 2020 (108.6 μg m) than in 2019 (96.8 μg m). Marked decreases in fresh NO alleviated the titration of O. Furthermore, the oxidation reaction of NO that produces NO was dominant over the photochemical reaction of NO that produces O, making NO less important for O pollution. In comparison, a lower VOC/NO ratio (less than 10) meant that Beijing is a VOC-limited area; this indicates that in order to alleviate O pollution in Beijing, emissions of VOCs should be controlled.