Chemistry of Atmospheric Fine Particles During the COVID‐19 Pandemic in a Megacity of Eastern China

Air pollution in megacities represents one of the greatest environmental challenges. Our observed results show that the dramatic NO(x) decrease (77%) led to significant O(3) increases (a factor of 2) during the COVID‐19 lockdown in megacity Hangzhou, China. Model simulations further demonstrate large increases of daytime OH and HO(2) radicals and nighttime NO(3) radical, which can promote the gas‐phase reaction and nocturnal multiphase chemistry. Therefore, enhanced NO(3) (−) and SO(4) (2−) formation was observed during the COVID‐19 lockdown because of the enhanced oxidizing capacity. The PM(2.5) decrease was only partially offset by enhanced aerosol formation with its reduction reaching 50%. In particular, NO(3) (−) decreased largely by 68%. PM(2.5) chemical analysis reveals that vehicular emissions mainly contributed to PM(2.5) under normal conditions in Hangzhou. Whereas, stationary sources dominated the residual PM(2.5) during the COVID‐19 lockdown. This study provides evidence that large reductions in vehicular emissions can effectively mitigate air pollution in megacities.