Enhanced nocturnal oxidation chemistry in the upper mixing layer of megacities

The troposphere is considered an oxidizing atmospheric environment with radical chemical reactions. During the day, hydroxyl radicals (OH) are the primary oxidants, while at night, nitrate radicals (NO₃) take on this role. NO₃ radicals can react with volatile organic compounds (VOCs), especially alkenes, to form organic aerosols. In addition, NO₃ radicals can also generate nitrate aerosols through the heterogeneous reaction of N₂O₅, leading to severe air pollution issues. Nighttime atmospheric oxidizing capacity refers to the ability of oxidants to convert primary pollutants into secondary pollutants. Since NO₃ radicals are the main oxidants at night, the nitrate radical production rate (PNO₃) is often used as an indicator of nighttime atmospheric oxidizing capacity. However, unlike the well mixed during the day, nighttime air exhibits strong vertical stratification due to the cooling of the ground. As a result, there are significant differences in the concentration of pollutants and chemical reaction processes at different heights, leading to substantial variations in atmospheric oxidizing capacity with altitude. Therefore, ground-based observations cannot fully represent the entire nighttime boundary layer. To accurately describe the oxidative characteristics of the nighttime atmosphere, we combined vertical tower observation data to analyze the distribution characteristics of PNO₃ with altitude and classified the distribution patterns of PNO₃ under different environmental conditions.