[Photochemical Causes of Localized Ozone Pollution under Static and Stable Weather in Nanjing Area]

Based on the observational data of volatile organic compounds （VOCs）, conventional air pollutants, and ERA5 meteorological reanalysis data at three sites, namely, Caochangmen （CCM）, Pukou （PK）, and Xianlin University Town （XL）, in Nanjing from 2015 to 2021, the ozone generation and depletion mechanisms in ozone-polluted days under stable weather conditions were investigated using the observation-based model （OBM-MCM）. The results showed that ① Significant year-by-year differences exist in the frequency of stable weather on ozone-polluted days for the three sites. The maximum number of stable days occurred in 2019, with 46 d （66.7%）, 50 d （64.9%）, and 54 d （69.2%） at the CCM, PK, and XL sites, respectively. ② Significant differences exist between the net O₃ production rates for the CCM, PK, and XL sites during the polluted period, with the highest rate of 2.5×10^-9 h^-1 at the CCM site and the lowest rate of 1.4×10^-9 h^-1 at the XL site. Additionally, the O₃ production and depletion rate at the XL site were lower compared to those at the other two sites. ③ The reactions of HO₂·+NO and ·OH+NO₂, respectively, contributed the most to O₃ production and depletion. The HO₂·+NO reaction contributed to O₃ production by 69% （CCM）, 68% （PK）, and 71% （XL）, and the ·OH+NO₂ reaction contributed to O₃ depletion by 67% （CCM）, 63% （PK）, and 62% （XL）. ④ The modeling study observed that ozone pollution under stable weather conditions was mainly affected by local photochemistry processes； therefore, local emission reduction is very important for O₃ pollution mitigation.