Mitigated PM2.5 Changes by the Regional Transport During the COVID‐19 Lockdown in Shanghai, China

Intensive observations and WRF‐Chem simulations are applied in this study to investigate the adverse impacts of regional transport on the PM2.5 (fine particulate matter; diameter ≤2.5 μm) changes in Shanghai during the Coronavirus Disease 2019 lockdown. As the local atmospheric oxidation capacity was observed to be generally weakened, strong regional transport carried by the frequent westerly winds is suggested to be the main driver of the unexpected pollution episodes, increasing the input of both primary and secondary aerosols. Contributing 40%–80% to the PM2.5, the transport contributed aerosols are simulated to exhibit less decreases (13.2%–21.8%) than the local particles (37.1%–64.8%) in urban Shanghai due to the lockdown, which largely results from the less decreased industrial and residential emissions in surrounding provinces. To reduce the influence of the transport, synergetic emission control, especially synergetic ammonia control, measures are proved to be effective strategies, which need to be considered in future regulations. Plain Language Summary The anthropogenic emissions were sharply reduced in China due to the lockdown measures during the Coronavirus Disease 2019 pandemic. However, the reduced emissions did not lead to expected decreases in fine particulate matter in Shanghai. To understand why haze pollution still occurred during the lockdown, intensive observations and a variety of simulations using the regional chemical transport model were conducted in this study. Contributing 40−80% to the fine particles in Shanghai, the strong transport of air pollutants carried by the frequent westerly winds is found to be the main driver of the observed pollution episodes, which brings substantial primary and secondary aerosols to the city. Compared to those originated from the local sources, the transport contributed aerosols are simulated to be less affected by the intervention measures, which can be largely attributed to the less decreased industrial and residential emissions in Jiangsu, Zhejiang, and Anhui and enhanced aerosol formation during the transport. Synergetic emission control measures, including reductions in both the traditional regulated emissions and agricultural emissions, in the Yangtze River Delta region are proved to be effective in mitigating the adverse impacts of the transport, which provide practical implications for future pollution control strategies in cities like Shanghai. Key Points PM2.5 pollution in Shanghai were