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

In coastal regions, particulate chloride (Cl−) in sea-salt can react with gaseous dinitrogen pentoxide (N 2 O 5) and produces nitryl chloride (ClNO 2) after the heterogeneous uptake of N 2 O 5. Photolysis of ClNO 2 by sunlight enhances atmospheric oxidation capacity and contributes to the formation of ozone (O 3). 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 O 3 formation over the Hong Kong-Pearl River Delta (HK-PRD) and surrounding maritime regions. Two typical O 3 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 ClNO 2 by a factor of 2 in the 2018 case when N 2 O 5 and ClNO 2 were measured. The temporal and spatial distributions of chloride loss and ClNO 2 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 N 2 O 5 heterogeneous reaction which lead to elevated ClNO 2 mixing ratios (up to 0.6 ppb) produced at night. During the phase of continental outflow, the heterogeneous reaction of N 2 O 5 contributed 18–33% to the depletion of particulate sea-salt Cl− in the coastal areas, leading to an increase in ClNO 2 mixing ratio up to 0.8 ppb in the residual layer (~300 m). The ClNO 2 from sea-salt chloride increased the O 3 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 O 3 formation in coastal regions. • The emission and chemical processes of sea-salt chloride were reasonably well reproduced by using the WRF-chem model. • The particulate Cl− in sea salt was depleted by 40% in inland area of South China through a heterogeneous reaction with N 2 O 5. • The heterogeneous reaction of N 2 O 5 with sea-salt chloride increased O 3 mixing ratios by up to 6.5 ppb. [ABSTRACT FROM AUTHOR]