The impact of sea-salt chloride on ozone through heterogeneous reaction with N2O5 in a coastal region of south China

Abstract

In coastal regions, particulate chloride (Cl−) in sea-salt can react with gaseous dinitrogen pentoxide (N2O5) and produces nitryl chloride (ClNO2) after the heterogeneous uptake of N2O5. Photolysis of ClNO2 by sunlight enhances atmospheric oxidation capacity and contributes to the formation of ozone (O3). Using a regional chemical model (WRF-Chem), we evaluated the emission of particulate Cl− from the South China Sea, the chlorine loss in sea-salt aerosols, and the impact of sea-salt chloride on O3 formation over the Hong Kong-Pearl River Delta (HK-PRD) and surrounding maritime regions. Two typical O3 episodes in early autumn (September 2017 and 2018) were analyzed. The modeled results of particulate Cl− agreed well with the observations at a coastal site in both two cases, but the model underestimated ClNO2 by a factor of 2 in the 2018 case when N2O5 and ClNO2 were measured. The temporal and spatial distributions of chloride loss and ClNO2 production were simulated for the 2017 case which contained maritime inflow and continental outflow. During maritime winds, the oceanic fine particulate Cl− penetrated deep inland and was depleted by up to 40% by N2O5 heterogeneous reaction which lead to elevated ClNO2 mixing ratios (up to 0.6 ppb) produced at night. During the phase of continental outflow, the heterogeneous reaction of N2O5 contributed 18–33% to the depletion of particulate sea-salt Cl− in the coastal areas, leading to an increase in ClNO2 mixing ratio up to 0.8 ppb in the residual layer (~300 m). The ClNO2 from sea-salt chloride increased the O3 mixing ratios by up to 2.0 ppb (4%) over the inland areas during marine winds and up to 3.8 ppb (5.5%) and 6.5 ppb (7.6%) over the South China Sea. This study highlights the considerable impact of the heterogeneous reaction of reactive nitrogen on chlorine loss of sea-salt and O3 formation in coastal regions.