Numerical modeling on the mechanisms of chlorine chemistry in snowpack and their impact on secondary atmospheric pollution

Abstract. Snow with high albedo enhances atmospheric photochemical reactions, influencing key oxidative processes. Nitryl chloride (ClNO₂), as a strong oxidizing species, is generated by the heterogeneous reaction between dinitrogen pentoxide (N₂O₅) and chloride adsorbed on aerosol and the ground surfaces. After sunrise, the photolysis of ClNO₂ rapidly releases highly reactive chlorine radicals (Cl·), which contributes to the formation of secondary pollutants. However, the pollution mechanisms in high-latitude, snow-covered regions associated with increased chlorine emissions remain unclear. In this study, we employed the WRF-CAMx model (Weather Research and Forecasting Model-Comprehensive Air Quality Model with extensions) with a modified chemical mechanism (CB6r2h_lts, Carbon Bond 6 revision 2 with heterogeneous chemistry for low-temperature and snow-covered conditions) that incorporated heterogeneous N₂O₅ reactions and ClNO₂ photolysis on ground surfaces to assess their impact on regional atmosphere under snow-covered conditions in Northeast China. Our findings reveal that under snow-covered conditions, the YU20 aerosol scheme (from study by YU et al., 2020) outperforms the BT09 scheme (from study by Bertram et al., 2009) in simulating N₂O₅ and ClNO₂ concentrations within the CAMx model. Incorporating anthropogenic chlorine emissions and ground surface chemistry significantly improved model performance for ClNO₂, reducing the mean bias (MB) from -105.78 pptv to 2.66 pptv and increasing the index of agreement (IOA) from 0.39 to 0.86. These processes resulted in a maximum hourly increase of 3.65 µg/m³ in PM_2.5 (relative contribution: 15.34 %) and 3.41 ppbv in MDA8 O₃ (5.68 %). Notably, ground surface chemical processes were identified as the dominant source of nocturnal ClNO₂, contributing approximately 28.36 % to nighttime accumulation across Northeast China. These findings not only highlight the pivotal role of chlorine chemistry in atmospheric processes under snow-covered conditions, but also provide crucial support for the refinement of the mechanisms governing the flux exchange of chemical substances between the atmosphere and the cryosphere.