Xing Chen

The absence of motor vehicle traffic and suspended human activities during the COVID-19 lockdown period in China produced a unique experiment to assess the efficiency of air pollution mitigation. Herein, we synthetically analyzed monitoring data of atmospheric pollutants together with meteorological parameters to investigate the impact of human activity pattern changes on air quality in Guiyang, southwestern China. The results show that the Air Quality Index (AQI) during the lockdown period decreased by 7.4% and 23.48% compared to pre-lockdown levels and the identical lunar period during the past 3 years, respectively, which exhibited optimal air quality due to reduced emissions. The sharp decrease in NO₂ concentration reduced the “titration” effect and elevated the O₃ concentration by 31.94% during the lockdown period. Meteorological conditions significantly impacted air quality, and serious pollution events might also occur under emission reductions. Falling wind speeds and increasing relative humidity were the direct causes of the pollution event on February 1st. The “first rain” increases the hygroscopicity of atmospheric particulate matter and then elevate its concentration, while continuous rainfall significantly impacted the removal of atmospheric particulate matter. As impacted by the lockdown, the spatial distribution of the NO₂ concentration sharply decreased on the whole, while the O₃ concentration increased significantly. The implications of this study are as follows: Measures should be formulated to prevent O₃ pollution when emission reduction measures are being adopted to improve air quality, and an emphasis should be placed on the impact of secondary aerosols formation by gas-particle conversion.