The role of anthropogenic chlorine emission in surface ozone formation during different seasons over eastern China

Anthropogenic chlorine emission is an important source of Cl radicals, which plays an important role in the oxidative chemistry of the troposphere. However, its seasonal impacts on surface ozone levels in China have yet been comprehensively explored. In this study, we conducted numerical simulations for January, April, July and October 2015 by using the Community Multiscale Air Quality (CMAQ) modeling system with updated heterogeneous reactions of nitrogen oxides with particulate chlorine and updated Anthropogenic Chlorine Emission Inventory for China (ACEIC). Two experiments with and without ACEIC in the model were established, and their results were compared with each other. The model can faithfully reproduce the magnitudes and variations of meteorological parameters and air pollutant concentrations. Cl radicals were generated by the photolysis of ClNO₂, ClNO and Cl₂, HCl oxidation by OH radicals, and the heterogeneous reactions of NO₃ with particulate Cl^-. ClNO₂ and ClNO were mainly produced from the heterogeneous reactions of N₂O₅ and NO₂ with particulate Cl^-, respectively. The spatial and seasonal variations ofz these chlorinated species and their responses to the implementation of ACEIC were revealed in this study. Our results suggested that besides N₂O₅, the heterogeneous reactions of NO₂ and NO₃ with particulate Cl^- could be important sources of Cl radicals. Anthropogenic chlorine emission increased the Cl radical concentration through enhancing the photolysis of ClNO, Cl₂, and ClNO₂. The implementation of ACEIC in the model increased the degradation of Volatile Organic Compounds (VOCs) not only by Cl radicals but also by OH radicals. Although the seasonal variation of AECIE was insignificant, the larger formation of Cl radicals caused by higher levels of NO_x in January was counteracted by the larger loss of them due to more VOCs degradations, resulting in a lower increase in Cl radicals due to the implementation of ACEIC compared with other months. The anthropogenic chlorine emissions increased the monthly mean maximum daily 8-hour average (MDA8) O₃ mixing ratio by up to 4.9 ppbv, and increased the 1-hour O₃ mixing ratio by up to 34.3 ppbv. The impact of ACEIC was the most significant in January and the least in July due to the high emissions of NO_x and VOCs and adverse meteorological conditions in winter. It indicated that although the ozone concentration was low, the anthropogenic chlorine emission significantly contributed to the atmospheric oxidation capacity and increase ozone concentrations in winter.