Ground ozone rise caused by the larger emission reduction of nitrogen oxides than volatile organic components

Abstract

Ground-level ozone (O3) pollution has shifted from being a scientific topic to a governmental imperative in China. We analyze the mechanism for the O3 rise observed in Shanghai during the lockdown in the spread of COVID-19 in 2022 by combining utilizing ground-level observed data, an observation-based model, and a chemical transport model. We find that the increase in O3 can be mainly attributable to the larger emission reduction of nitrogen oxides (NOx) than volatile organic components (VOCs), of which the effect is amplified by the adverse meteorological conditions. The chemical transport modeling results suggest that a higher ratio of emission reduction of NOx to VOCs increases the daily maximum 8-hour moving average O3 concentration considerably relative to a counterfactual scenario assuming the same ratio of emission reduction of NOx to VOCs. This indicates that the concentration of O3 would depends not only on the strength of emission reduction but also the ratio of emission reduction between species. Our results highlight the importance of well-designed strategies with appropriate control of the VOCs to NOx ratio to mitigate O3 pollution in cities.