Comment on acp-2021-850

Abstract

The heterogeneous reaction of N2O5 on Cl-containing aerosols (N2O5 − ClNO2 chemistry) plays a key role in chlorine activation, NOx recycling and consequently O3 formation. In this study, we use the GEOS-Chem model with additional anthropogenic and biomass burning chlorine emissions combined with updated parameterizations for N2O5 − ClNO2 chemistry (i.e. the uptake coefficient of N2O5 (γN2O5) and the ClNO2 yield (φClNO2)) to investigate the impacts of chlorine chemistry on air quality in China, the role of N2O5 − ClNO2 chemistry, as well as their sensitivities to chlorine emissions and parameterizations for γN2O5 and φClNO2. The model evaluation with multiple data sets observed across China demonstrated significant improvement especially regarding the simulation of Cl−, N2O5 and ClNO2 with the updates in chlorine emissions and N2O5 − ClNO2 chemistry. Total tropospheric chlorine chemistry could increase annual mean MDA8 O3 by up to 4.5 ppbv but decrease PM2.5 by up to 7.9 μg m−3 in China, 83 % and 90 % of which could be attributed to the effect of N2O5 – ClNO2 chemistry. The heterogeneous uptake of N2O5 on chloride-containing aerosol surfaces is an important loss pathway of N2O5 as well as a important source of O3, and hence is particularly useful in elucidating the commonly seen ozone underestimations. The importance of chlorine chemistry largely depends on both chlorine emissions and the parameterizations for N2O5 – ClNO2 chemistry. With the additional chlorine emissions annual mean maximum daily 8-hour average (MDA8) O3 in China could be increased by up to 3.5 ppbv. The corresponding effect on PM2.5 concentrations varies largely with regions, with an increase of up to 4.5 μg m−3 in the North China Plain but a decrease of up to 3.7 μg m−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 48 % and 27 %, respectively between different parameterizations.