<p>The heterogeneous reaction of N<sub>2</sub>O<sub>5</sub> on Cl-containing aerosols (N<sub>2</sub>O<sub>5</sub> − ClNO<sub>2</sub> chemistry) plays a key role in chlorine activation, NO<sub>x</sub> recycling and consequently O<sub>3</sub> formation. In this study, we use the GEOS-Chem model with additional anthropogenic and biomass burning chlorine emissions combined with updated parameterizations for N<sub>2</sub>O<sub>5</sub> − ClNO<sub>2</sub> chemistry (i.e. the uptake coefficient of N<sub>2</sub>O<sub>5</sub> (γ<sub>N2O5</sub>) and the ClNO<sub>2</sub> yield (φ<sub>ClNO2</sub>)) to investigate the impacts of chlorine chemistry on air quality in China, the role of N<sub>2</sub>O<sub>5</sub> − ClNO<sub>2</sub> chemistry, as well as their sensitivities to chlorine emissions and parameterizations for γ<sub>N2O5</sub> and φ<sub>ClNO2</sub>. The model evaluation with multiple data sets observed across China demonstrated significant improvement especially regarding the simulation of Cl<sup>−</sup>, N<sub>2</sub>O<sub>5</sub> and ClNO<sub>2</sub> with the updates in chlorine emissions and N<sub>2</sub>O<sub>5</sub> − ClNO<sub>2</sub> chemistry. Total tropospheric chlorine chemistry could increase annual mean MDA8 O<sub>3</sub> by up to 4.5 ppbv but decrease PM<sub>2.5</sub> by up to 7.9 μg m<sup>−3</sup> in China, 83 % and 90 % of which could be attributed to the effect of N<sub>2</sub>O<sub>5</sub> – ClNO<sub>2</sub> chemistry. The heterogeneous uptake of N<sub>2</sub>O<sub>5</sub> on chloride-containing aerosol surfaces is an important loss pathway of N<sub>2</sub>O<sub>5</sub> as well as a important source of O<sub>3</sub>, 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 N<sub>2</sub>O<sub>5</sub> – ClNO<sub>2</sub> chemistry. With the additional chlorine emissions annual mean maximum daily 8-hour average (MDA8) O<sub>3</sub> in China could be increased by up to 3.5 ppbv. The corresponding effect on PM<sub>2.5</sub> concentrations varies largely with regions, with an increase of up to 4.5 μg m<sup>−3</sup> in the North China Plain but a decrease of up to 3.7 μg m<sup>−3</sup> in the Sichuan Basin. On the other hand, even with the same chlorine emissions, the effects on MDA8 O<sub>3</sub> and PM<sub>2.5</sub> in China could differ by 48 % and 27 %, respectively between different parameterizations.</p>