On the Relevancy of Observed Ozone Increase during COVID-19 Lockdown to Summertime Ozone and PM2.5 Control Policies in China

In February 2020, China's strict lockdown policies led to significant reductions in anthropogenic nitrogen oxides (NOx) 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 NOx emission controls. On the basis of O-3 isopleths from a series of air quality simulations under different levels of NOx and volatile organic compound (VOC) emission reductions, we found that such concerns are not necessary. As NOx emissions have been reduced in recent years for particulate matter control, future NOx reductions are generally favorable for summertime maximum daily average 8-h (MDA8) O-3 reductions. Decreases in summertime O-3 due to NOx reductions will also lead to lower atmospheric oxidation capacity, characterized by decreased OH and NO3 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 NOx emission reduction policy has great potential in controlling O-3 and PM2.5 simultaneously. However, its effectiveness could be greatly reduced when applied on a limited spatial scale.

Automated summary

The study shows that recent reductions in NOx emissions in China are beneficial for reducing summer O3 and PM2.5, but the effectiveness could be diminished when applied in a limited spatial scale.