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 decrease (77%) led to significant O 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 radicals and nighttime NO radical, which can promote the gas-phase reaction and nocturnal multiphase chemistry. Therefore, enhanced NO and SO formation was observed during the COVID-19 lockdown because of the enhanced oxidizing capacity. The PM decrease was only partially offset by enhanced aerosol formation with its reduction reaching 50%. In particular, NO decreased largely by 68%. PM chemical analysis reveals that vehicular emissions mainly contributed to PM under normal conditions in Hangzhou. Whereas, stationary sources dominated the residual PM during the COVID-19 lockdown. This study provides evidence that large reductions in vehicular emissions can effectively mitigate air pollution in megacities.