Effects of Anthropogenic Chlorine on PM_2.5 and Ozone Air Quality in China

China has large anthropogenic chlorine emissions from agricultural fires, residential biofuel, waste incineration, coal combustion, and industrial processes. Here we quantify the effects of chlorine on fine particulate matter (PM_2.5) and ozone air quality across China by using the GEOS-Chem chemical transport model with comprehensive anthropogenic emissions and detailed representation of gas-phase and heterogeneous chlorine chemistry. Comparison of the model to observed ClNO₂, HCl, and particulate Cl^- concentrations shows that reactive chlorine in China is mainly anthropogenic, unlike in other continental regions where it is mostly of marine origin. The model is successful in reproducing observed concentrations and their distributions, lending confidence in the anthropogenic chlorine emission estimates and the resulting chemistry. We find that anthropogenic chlorine emissions increase total inorganic PM_2.5 by as much as 3.2 μg m^-3 on an annual mean basis through the formation of ammonium chloride, partly compensated by a decrease of nitrate because ClNO₂ formation competes with N₂O₅ hydrolysis. Annual mean MDA8 surface ozone increases by up to 1.9 ppb, mainly from ClNO₂ chemistry, while reactivities of volatile organic compounds increase (by up to 48% for ethane). We find that a sufficient representation of chlorine chemistry in air quality models can be obtained from consideration of HCl/Cl^- thermodynamics and ClNO₂ chemistry, because other more complicated aspects of chlorine chemistry have a relatively minor effect.