The lockdown due to COVID-19 created a rare opportunity to examine the nonlinear responses of secondary aerosols, which are formed through atmospheric oxidation of gaseous precursors, to intensive precursor emission reductions. Based on unique observational data sets from six supersites in eastern China during 2019−2021, we found that the lockdown caused considerable decreases (32−61%) in different secondary aerosol components in the study region because of similar-degree precursor reductions. However, due to insufficient combustionrelated volatile organic compound (VOC) reduction, odd oxygen (O x = O 3 + NO 2 ) concentration, an indicator of the extent of photochemical processing, showed little change and did not promote more decreases in secondary aerosols. We also found that the Chinese provinces and international cities that experienced reduced O x during the lockdown usually gained a greater simultaneous PM 2.5 decrease than other provinces and cities with an increased O x . Therefore, we argue that strict VOC control in winter, which has been largely ignored so far, is critical in future policies to mitigate winter haze more efficiently by reducing O x simultaneously.

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