The impact of chlorine chemistry combined with heterogeneous N2O5 reactions on air quality in China

Abstract

The heterogeneous reaction of N2O5 on Cl-containing aerosols (heterogeneous N2O5ĝ€¯+ĝ€¯Cl chemistry) plays a key role in chlorine activation, NOx recycling, and consequently O3 and PM2.5 formation. In this study, we use the GEOS-Chem model with additional anthropogenic and biomass burning chlorine emissions combined with updated parameterizations for the heterogeneous N2O5ĝ€¯+ĝ€¯Cl chemistry (i.e., the uptake coefficient of N2O5 ( 3N2O5) and the ClNO2 yield ( †ClNO2)) to investigate the impacts of chlorine chemistry on air quality in China, the role of the heterogeneous N2O5ĝ€¯+ĝ€¯Cl chemistry, and the sensitivity of air pollution formation to chlorine emissions and parameterizations for 3N2O5 and †ClNO2. The model simulations are evaluated against multiple observational datasets across China and show significant improvement in reproducing observations of particulate chloride, N2O5, and ClNO2 when including anthropogenic chlorine emissions and updates to the parameterization of the heterogeneous N2O5ĝ€¯+ĝ€¯Cl chemistry relative to the default model. The simulations show that total tropospheric chlorine chemistry could increase annual mean maximum daily 8ĝ€¯h average (MDA8) O3 by up to 4.5ĝ€¯ppbv but decrease PM2.5 by up to 7.9ĝ€¯μgm-3 in China, 83ĝ€¯% and 90ĝ€¯% of which could be attributed to the effect of the heterogeneous N2O5ĝ€¯+ĝ€¯Cl chemistry. The heterogeneous uptake of N2O5 on chloride-containing aerosol surfaces is an important loss pathway of N2O5 as well as an important source of O3 and hence is particularly useful in elucidating the commonly seen ozone underestimations relative to observations. The importance of chlorine chemistry largely depends on both chlorine emissions and the parameterizations for the heterogeneous N2O5ĝ€¯+ĝ€¯Cl chemistry. With the additional chlorine emissions, the simulations show that annual MDA8 O3 in China could be increased by up to 3.5ĝ€¯ppbv. The corresponding effect on PM2.5 concentrations varies largely with regions, with an increase of up to 4.5ĝ€¯μgm-3 in the North China Plain but a decrease of up to 3.7ĝ€¯μgm-3 in the Sichuan Basin. On the other hand, even with the same chlorine emissions, the effects on MDA8 O3 and PM2.5 in China could differ by 48ĝ€¯% and 27ĝ€¯%, respectively, between different parameterizations.