Variations of chloride depletion and its impacts on ozone formation: Case study of a coastal area in Shanghai.

Chlorine plays a critical role in atmospheric chemistry. Marine chloride depletion, as a significant source of atmospheric chlorine, impacts coastal acid deposition, atmospheric oxidizing capacity, and global climate. Based on continuous monitoring data of PM_2.5 water soluble ions, criteria pollutants, and meteorological data at Chongming Dongtan supersite from 2019 to 2022, variations in chloride depletion and related impact factors were analyzed. Using trajectory analysis via Concentration Weighted Trajectory (CWT) method, the main source regions contributing to chloride depletion were identified. The influence of meteorological conditions on chloride depletion was examined, and the contribution of typical chloride depletion processes to ozone was analyzed using the community atmospheric chemistry box model Chemistry As A Box model Application/Module Efficiently Calculating the Chemistry of the Atmosphere (CAABA/MECCA). Results show that chloride depletion increases in summer and decreases in winter. Chloride depletion reaches to peak around noon and gradually decreases after 6 p.m. CWT analysis reveals that airflows predominantly originate from ocean during periods of chloride depletion. As a large coastal port, shipping NO_x emissions produce abundant N₂O₅ through oxidation processes. The liquid-phase reactions of N₂O₅ with sea-salt aerosol via liquid-phase reactions result in chloride depletion. Chlorine depletion follows the same trend as O₃ and temperature, while showing an inverse trend with NO₂, N₂O₅, NO₃, and pH. Modelling results indicate that oceanic chlorine depletion contributes approximately 8 ppb, 0.1 ppt, and 18 ppt to the enhancement of O₃, OH, and HO₂ concentrations. Therefore, attention should be paid to the contribution of ship emissions through chloride depletion mechanisms to O₃ pollution in coastal port cities. Chloride depletion serves as a typical indicator of the impact of chloride circulation on coastal ozone.