Increasing contribution of nighttime nitrogen chemistry to wintertime haze formation in Beijing observed during COVID-19 lockdowns

Nitrate comprises the largest fraction of fine particulate matter in China during severe haze. Consequently, strict control of nitrogen oxides (NO x ) emissions has been regarded as an effective measure to combat air pollution. However, this notion is challenged by the persistent severe haze pollution observed during the COVID-19 lockdown when NO x levels substantially declined. Here we present direct field evidence that diminished nitrogen monoxide (NO) during the lockdown activated nocturnal nitrogen chemistry, driving severe haze formation. First, dinitrogen pentoxide (N 2 O 5 ) heterogeneous reactions dominate particulate nitrate (pNO 3 − ) formation during severe pollution, explaining the higher-than-normal pNO 3 − fraction in fine particulate matter despite the substantial NO x reduction. Second, N 2 O 5 heterogeneous reactions provide a large source of chlorine radicals on the following day, contributing drastically to the oxidation of volatile organic compounds, and thus the formation of oxygenated organic molecules and secondary organic aerosol. Our findings highlight the increasing importance of such nocturnal nitrogen chemistry in haze formation caused by NO x reduction, motivating refinements to future air pollution control strategies.