Identification of synergistic control for ozone and PM2.5 pollution during a large-scale emission reduction in China

A large-scale emission reduction during the COVID-19 outbreak provide a unique opportunity for exploring an effective ozone (O3) and fine particulates (PM2.5) co-control strategy. A notable spatiotemporal heterogeneities of the O3-precursor sensitivity (OPS) variations was noticed during the lockdown, which was collectively governed by the difference in emission structures and emission change tracks. Indicated by the close relationship between O3 and PM2.5 changes and the OPS, we found that shifting the OPS toward the NOx-limited regime is a pre-requisite for O3 and PM2.5 co-control to a large extent. By tracking O3 and PM2.5 changes response to the different emission reduction path, both the “NOx only” and “VOC only” path has inevitable limitations. Followed by the difference in OPS variations, the tipping-point between the VOCs control and NOx control strategy varied greatly in YRD, PRD, and HB, with values of 30%, 10%, and 20%, respectively. This study has important implications on the O3 and PM2.5 co-control philosophy which can helps policymakers to avoid the rebound of O3 and PM2.5 levels in response to the imbalanced control strategy in the future, and underscored the importance of strengthening NOx emission reduction to make OPS shifting into the NOx-limited regime as quickly as possible.