Modeling the impact of chlorine emissions from coal combustion and prescribed waste incineration on tropospheric ozone formation in China

Abstract. Chlorine radicals can enhance atmospheric oxidation, which potentially increases tropospheric ozone concentration. However, few studies have been done to quantify the impact of chlorine emissions on ozone formation in China due to the lack of a chlorine emission inventory used in air quality models with sufficient resolution. In this study, the Anthropogenic Chlorine Emissions Inventory for China (ACEIC) was developed for the first time, including emissions of hydrogen chloride (HCl) and molecular chlorine (Cl₂) from coal combustion and prescribed waste incineration (waste incineration plant). The HCl and Cl₂ emissions from coal combustion in China in 2012 were estimated to be 232.9 and 9.4 Gg, respectively, while HCl emission from prescribed waste incineration was estimated to be 2.9 Gg. Spatially the highest emissions of HCl and Cl₂ were found in the North China Plain, the Yangtze River Delta, and the Sichuan Basin. Air quality model simulations with the Community Multiscale Air Quality (CMAQ) modeling system were performed for November 2011, and the modeling results derived with and without chlorine emissions were compared. The magnitude of the simulated HCl, Cl₂ and ClNO₂ agreed reasonably with the observation when anthropogenic chlorine emissions were included in the model. The inclusion of the ACEIC increased the concentration of fine particulate Cl⁻, leading to enhanced heterogeneous reactions between Cl⁻ and N₂O₅, which resulted in the higher production of ClNO₂. Photolysis of ClNO₂ and Cl₂ in the morning and the reaction of HCl with OH in the afternoon produced chlorine radicals which accelerated tropospheric oxidation. When anthropogenic chlorine emissions were included in the model, the monthly mean concentrations of fine particulate Cl⁻, daily maximum 1 h ClNO₂, and Cl radicals were estimated to increase by up to about 2.0 µg m⁻³, 773 pptv, and 1.5  ×  10³ molecule cm⁻³ in China, respectively. Meanwhile, the monthly mean daily maximum 8 h O₃ concentration was found to increase by up to 2.0 ppbv (4.1 %), while the monthly mean NO_x concentration decreased by up to 0.5 ppbv (6.1 %). The anthropogenic chlorine emissions potentially increased the 1 h O₃ concentration by up to 7.7 ppbv in China. This study highlights the need for the inclusion of anthropogenic chlorine emission in air quality modeling and demonstrated its importance in tropospheric ozone formation.