On the Relevancy of Observed Ozone Increase during COVID-19 Lockdown to Summertime Ozone and PM Control Policies in China.

In February 2020, China's strict lockdown policies led to significant reductions in anthropogenic nitrogen oxides (NO) emissions, and notable increases in surface ozone (O) followed in many urban areas, raising concerns about potential rises in summertime O due to NO emission controls. On the basis of O isopleths from a series of air quality simulations under different levels of NO and volatile organic compound (VOC) emission reductions, we found that such concerns are not necessary. As NO emissions have been reduced in recent years for particulate matter control, future NO reductions are generally favorable for summertime maximum daily average 8-h (MDA8) O reductions. Decreases in summertime O due to NO reductions will also lead to lower atmospheric oxidation capacity, characterized by decreased OH and NO concentrations, resulting in further reduction of secondary inorganic aerosols (nitrate, sulfate, and ammonium ion, NSA) formation. VOC emission reductions help to further reduce MDA8 O and are needed to control HCHO and primary air toxics simultaneously, but they are ineffective in reducing NSA. This study indicates that a nationwide NO emission reduction policy has great potential in controlling O and PM simultaneously. However, its effectiveness could be greatly reduced when applied on a limited spatial scale.