Stronger secondary pollution processes despite decrease in gaseous precursors: A comparative analysis of summer 2020 and 2019 in Beijing

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; PM₂.₅ levels increased from 35.8 to 44.7 μg m⁻³, and PM₁₀ increased from 51.4 to 69.0 μg m⁻³ from 2019 to 2020. The higher PM₁₀ was caused by two sandstorm events on May 11 and June 3, 2020, while the higher PM₂.₅ 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 PMₓ distribution showed that severe haze occurred when particles within Bin2 (size ranging 1–2.5 μm) dominated. SO₄²⁻₍₁/₂.₅₎ and SO₄²⁻₍₂.₅/₁₀₎ remained stable under different periods at 0.5 and 0.8, respectively, indicating that SO₄²⁻ existed mainly in smaller particles. Decreases in NO₃⁻₍₁/₂.₅₎ and increases in NO₃⁻₍₂.₅/₁₀₎ from clean to polluted conditions, similar to the variations in PMₓ distribution, suggest that NO₃⁻ played a role in the worsening of pollution. O₃ 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 O₃. Furthermore, the oxidation reaction of NO₂ that produces NO₃⁻ was dominant over the photochemical reaction of NO₂ that produces O₃, making NO₂ less important for O₃ pollution. In comparison, a lower VOC/NOₓ ratio (less than 10) meant that Beijing is a VOC-limited area; this indicates that in order to alleviate O₃ pollution in Beijing, emissions of VOCs should be controlled.