This work shows how activated nocturnal nitrogen chemistry drives severe haze formation in polluted environments when NO emissions suddenly reduce.

 Nitrate comprises the largest fraction of fine particulate matter in China during severe haze. Consequently, strict control of nitrogen oxides (NOx) 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 NOx levels substantially declined. The study presents present direct field evidence that diminished nitrogen monoxide (NO) during the lockdown activated nocturnal nitrogen chemistry, driving severe haze formation. Dinitrogen pentoxide (N2O5) undergoes heterogeneous uptake on aerosol during nighttime to form ClNO2, Cl2, and particulate nitrate. At sunrise, these chlorine reservoirs will be photolyzed to the Cl radical, which can oxidize volatile organic compounds to produce oxygenated organic molecules - the essential precursors of secondary organic aerosol. Typically, the NO concentration is high in the pre-lockdown period, restricting the formation of N2O5 and its heterogeneous processes. During the lockdown period, NO concentration was significantly decreased, enhancing the formation of N2O5, ultimately leading to an increase in fine particulate matter. The results are published in Nature Geoscience: