On the Relevancy of Observed Ozone Increase during COVID-19 Lockdown to Summertime Ozone and PM 2.5 Control Policies in China.

In February 2020, China's strict lockdown policies led to significant reductions in anthropogenic nitrogen oxides (NO x ) emissions, and notable increases in surface ozone (O 3 ) followed in many urban areas, raising concerns about potential rises in summertime O 3 due to NO x emission controls. On the basis of O 3 isopleths from a series of air quality simulations under different levels of NO x and volatile organic compound (VOC) emission reductions, we found that such concerns are not necessary. As NO x emissions have been reduced in recent years for particulate matter control, future NO x reductions are generally favorable for summertime maximum daily average 8-h (MDA8) O 3 reductions. Decreases in summertime O 3 due to NO x reductions will also lead to lower atmospheric oxidation capacity, characterized by decreased OH and NO 3 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 3 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 x emission reduction policy has great potential in controlling O 3 and PM 2.5 simultaneously. However, its effectiveness could be greatly reduced when applied on a limited spatial scale.