Effects of chlorine chemistry combined with Heterogeneous N2O5 reactions on PM2.5 and Ozone formation in China

Heterogeneous reactivity of N₂O₅ on Cl-containing aerosols can produce nitric acid (HNO₃) and nitryl chloride (ClNO₂), which is a critical parameter in assessing O₃ variation, nitrate production, and chloride activation. In this study, we used the GEOS-Chem to quantify the effects of chlorine chemistry on fine particulate matter (PM2.5) and O₃ formation across China, with comprehensive anthropogenic chlorine emissions (HCl + Cl₂ + particulate Cl^-). We extended GEOS-Chem to include the heterogeneous reactions of N₂O₅ and assess the impact of different parameterizations of uptake coefficient of N₂O₅(γ(N₂O₅)), and ClNO₂ yield (Φ(ClNO₂)). Observation from three representative sites in the north, east and south China were selected to assess the model performance with regard to particulate chloride. With the addition of anthropogenic chlorine emissions, model bias in particulate chloride decreased from -79.10% to -39.64% (Dongying), -60.55% to -34.14% (Shenzhen), and -77.53% to -39.97% (Gucheng), respectively. The results show that N₂O₅-ClNO₂ chemistry can reduce the concentration of NO₃^- and NH₄⁺, but increase the concentration of SO₄^2- slightly, consequently leading to a reduction in the concentration of PM2.5 in China(0.5 μg/m³ on average and 1.8 μg/m³ on haze days). On the other hand, the monthly average O₃ MDA8 concentration in China increased by up to 2 ppbv(8 ppbv on haze days), which is mainly due to the increase of OH concentration associated with the photolysis of ClNO_2.