Enhanced PM2.5 Decreases and O3 Increases in China During COVID-19 Lockdown by Aerosol-Radiation Feedback

We apply an online-coupled meteorology-chemistry model (WRF-Chem) embedded with an improved process analysis to examine aerosol-radiation feedback (ARF) impacts on effectiveness of emission control due to Coronavirus Disease 2019 (COVID-19) lockdown over North China Plain. Emission reduction alone induces PM2.5 decrease by 16.3 mu g m(-3) and O-3 increase by 10.2 ppbv during COVID-19 lockdown. The ARF enhances PM2.5 decrease by 2.7 mu g m(-3) (16.6%) and O-3 increase by 0.8 ppbv (7.8%). The ARF-induced enhancement of PM2.5 decline is mostly attributed to aerosol chemistry process, while enhancement of O-3 rise is ascribed to physical advection and vertical mixing processes. A set of sensitivity experiments with emission reductions in different degrees indicate that the ARF-induced enhancements of PM2.5 declines (O-3 rises) follow a robust linear relationship with the emission-reduction-induced PM2.5 decreases. The fitted relationship has an important implication for assessing the effectiveness of emission abatement at any extent. Plain Language Summary Efforts to inhibit the spread of Coronavirus Disease 2019 (COVID-19) remarkably lowered social-economic activities over China during January-February 2020. Observational evidence indicated moderate PM2.5 decreases and undesirable O-3 increases during COVID-19 lockdown. It is known that aerosols can influence air quality via aerosol-radiation feedback (ARF). Therefore, it is of great interest to explore how ARF affects the responses of PM2.5 and O-3 air quality to emission reduction due to COVID-19 lockdown. Our study shows that ARF enhances emission-reduction-induced PM2.5 decrease by 16.6% and O-3 increase by 7.8% during COVID-19 lockdown. The fitted linear relationship between emission-reduction-induced PM2.5 decreases and ARF-induced enhancements of PM2.5 declines (O-3 rises) has an important implication for evaluating the effectiveness of emission mitigation in any degree. Key Points . Emission reduction due to COVID-19 lockdown induces PM2.5 decrease by 16.3 mu g m(-3) (O-3 increase by 10.2 ppbv) over North China Plain Aerosol-radiation feedback enhances emission-reduction-induced PM2.5 decrease by 16.6% (O-3 increase by 7.8%) during COVID-19 lockdown The enhancement of PM2.5 decline (O-3 rise) is ascribed to aerosol chemistry process (physical advection and vertical mixing processes)