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

Abstract

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.

Key Points

• Air pollutants PM ₁₀, PM _2.5, NO _x, CO, and SO ₂ decreased whereas O ₃ increased during the COVID‐19 lockdown in megacity Hangzhou

• Enhanced NO ₃ ⁻ and SO ₄ ²⁻ formation caused by the enhanced oxidizing capacity partially offset the decrease of PM _2.5 in megacity Hangzhou

• The contribution of vehicular emissions to PM _2.5 was over stationary sources under normal conditions in megacity Hangzhou