A modeling approach to dynamically estimating local photochemistry process and its contribution to surface O3 pollution.

Ozone (O3) pollution in city level is a complex issue that arises not only from local photochemistry process but also involves mid- or long-range O3 transport. In this study, we developed a modeling approach to dynamically quantifying local photochemical process (indicated as Chem_O3) and estimating its role in surface O3 pollution in city level. The work was conducted on North BTH of China for summer of 2022 and mainly focused on the urban areas, in which surface O3 usually as the most dominant air pollutants to harm population health. The method was constructed via establishing the hourly response of locally-formed O3 to locally-released NOx (RO3-NO2, ppb·ppb-1) based on ISAM simulations and then combining RO3-NO2 and ambient NO2 levels to quantify time-varying Chem_O3. The results showed that the monthly mean of Chem_O3 and its proportion to actual O3 (Chem%) was 17.9-26.0 ppb and 46.7%-62.6% in major urban areas of North BTH, following the order of mega-city > industrialized city > normal city > forest city. Moreover, daily Chem% presented the different trend with daily O3 in these study areas, slight-positive for mega-cities, but moderate or strong-negative for most other cities. Specially, our developed method could additionally disentangling O3 physical transport among the studied cities, and we found the inflow of O3 was much lower than the outflow of O3 for two mega-cities, while it was opposite in other cities. We think this method could clearly point out the role of local photochemistry control in O3 reduction, which could help city environment managers to develop scientific and effective policy strategies to cope with ozone-related problems.