Potential deterioration of ozone pollution in coastal areas caused by marine- emitted halogens: A case study in the Guangdong-Hong Kong-Macao Greater Bay Area

Ozone (O3) is one of the most important air pollutants worldwide in terms of its great damage to human health and agriculture. Previous studies show that marine-emitted halogens significantly influence O3 concentrations, mainly through the consumption of O3 by bromine and iodine atoms. In this study, we investigate the temporal variation at finer time scales (daily and hourly) than previous studies (annual or monthly) to better characterize the influence of marine-emitted halogens on coastal O3. In contrast to previous studies that mainly reported a decrease in O3, our re-sults show significant temporal variations in halogen-induced O3 changes. More specifically, the halogen-induced de-crease in coastal O3 in southern China is concentrated on clean days, while an unexpected increase in some regions of up to >10 ppbv could occur on polluted days. On polluted days, the activation of particulate chloride (Cl-) in sea salt aerosol (SSA) is effective due to the high level of dinitrogen pentoxide (N2O5) that is formed from the reactions of O3 and nitrogen dioxide (NO2). In addition, the wind fields are unfavorable for the transport of marine air masses with large O3 depletion inland. These two factors together result in the increase in hourly and MDA8 O3 on polluted days in some regions in the GBA. The locations of O3 increases are controlled by the distribution of nitryl chloride (ClNO2) at sunrise, which is influenced by O3 and NO2 during the previous night. As a result, the increase in O3 is a continuation of the O3 pollution from the previous day, and the whole area is under potential threat of this worsening pollution.