Contributions of Anthropogenic Chlorine Emissions to secondary pollutions in China

Chlorine species play a crucial role as precursors to Cl radicals, which can significantly impact the atmospheric oxidation capacity and influence the levels of trace gases related to climate and air quality. We developed The Anthropogenic Chlorine Emission Inventory for China (ACEIC), which was the first chlorine emission inventory for China based on local data, and explored the impact of chlorine species emissions on secondary pollutants in 2019 in China using the CMAQ model. Considering chlorine emissions, the concentration of chlorine radicals (Cl·) in China increased by about 1000 molecules/cm³ on average, with a maximum increase of more than 6000 molecules/cm³ in major cities. Cl₂ and HOCl emissions were the most important contributors to the increase of Cl·, with both Cl₂ and HOCl emissions originating mainly from the residential sector. Regarding monthly variation, the increase in Cl· was most significant in summer due to intensified human activities. Regarding daily variation, the increase in Cl· peaked around 9 am and decreased to zero at night. Process analyses showed that the main reactions affecting the change in Cl· were the photolysis reactions of Cl₂ and HOCl and the consumption reactions of Cl· and VOC_s. As an important precursor of Cl·, the concentration of Nitryl chloride (ClNO₂) in China increased by about 50 ppt on average, with a maximum increase of more than 150 ppt in major cities. HCl and fine particulate Cl^- emissions were the most important contributors to the increase of ClNO₂. The increase in ClNO₂ was most significant in winter, peaked around 6 am and decreased to zero at daytime. Process analysis identified the upper boundary layer as the critical region for ClNO₂ formation. Chlorine emissions caused some increase in O₃ concentration. Maximum Daily 8-hour Average O₃ (MDA8 O₃) concentration increased by about 1 ppb, with O₃ increasing much higher in winter than in summer. In the daily variation, the ozone increase was most significant at 12 am, with a maximum increase of more than 1.5 ppb. These findings highlight the significant contribution of chlorine emissions to secondary pollutants and can aid in the formulation of emission control strategies to mitigate secondary pollution in China.