Significant contributions of sea spray aerosol to vertical ClNO2 formation over coastal South China

Nitryl chloride (ClNO₂), an important precursor of chlorine radicals (Cl•), significantly enhances atmospheric oxidative capacity (AOC) during early morning hours. Previous studies have shown that ClNO₂ exhibits distinct vertical characteristics, with concentrations typically higher in coastal areas, raising concerns about chlorine-induced pollution. This study investigates the vertical formation of ClNO₂ in the Pearl River Delta (PRD), focusing on the contribution of sea spray aerosols (SSA). Using field observations and WRF-CMAQ model simulations, we assess the impact of SSA on nocturnal heterogeneous reactions driving ClNO₂ formation. The observations show that ClNO₂ mixing ratios are significantly higher in the upper boundary layer (200 m) compared to surface measurements, with peak mixing ratios occurring in the early morning. Air mass trajectory analysis shows that the marine air masses are primarily responsible for elevated ClNO₂ levels aloft. The maximum contribution of SSA to ClNO₂ yield is found to be more than 97% of the total yield. Process analysis identifies the upper boundary layer as the critical region for ClNO₂ formation, with SSA playing a dominant role. Moreover, SSA not only enhances ClNO₂ production but also increases the mixing ratios of chlorine and hydroxyl radicals at higher altitudes the following day (400 m), significantly boosting AOC, with an increase in AOC of up to 10%. These findings highlight the pivotal role of SSA in modulating vertical ClNO₂ formation and its broader impacts on regional air quality, particularly in coastal areas.