Impacts of chlorine emissions on secondary pollutants in China

Abstract The identified components of fine particulate matter with dynamic diameters less than 2.5 μm (PM2.5) suggests the remarkable increase in Cl- in PM2.5 during autumn and winter over North China. Anthropogenic chlorine emissions, including those from biomass burning, coal combustion and biofuel burning, are the main sources of Cl-, which could potentially affect the formation of secondary pollutants in the troposphere. In this study, we resolve the effect of chlorine emissions on PM2.5 and O3 by using the Weather Research and Forecasting (WRF) model coupled with Chemistry (WRF-Chem model) with an updated chemistry mechanism that includes the gaseous chemistry of chlorine, N2O5 heterogeneous reactions and detailed aqueous chemistry. Additionally, the biomass burning emissions of chlorine are derived from the Fire Inventory of the National Center for Atmospheric Research (NCAR) (FINN), which are considered the main contributor to atmospheric chloride over inland areas. The improved model is able to simulate the atmospheric Cl- in PM2.5 with reasonable agreement with the available observations in China. In addition, this model reduces the mean bias of ground-level PM2.5 concentrations from -7.5% to 3.2% over the BTH area and from -11.6% to -0.4% over the Northeast China Plain (NEP). The model also slightly improves O3 simulations by reducing the mean bias from 25.3% to 23.9% over mainland China. The anthropogenic chlorine emissions are estimated to increase the monthly average PM2.5 by 7.5 μg/m3 (9.1%), whereas ozone decreases slightly by up to 2.1 ppbv (4.3%) in November 2014. The influence of ozone was more significant in summer, reaching 2-8 ppbv over the biomass burning regions in 2014. The increase in Cl- and NH4+ accounted for the impacts on PM2.5, and the decline in O3 could be explained by the cycle between chlorine radicals and gases and changes in precursor concentrations. Our results suggest the importance of chlorine emissions and their chemical processes in the formation of PM2.5 and O3 over high-emission regions in China.