Modeling the impact of heterogeneous reactions of chlorine onsummertime nitrate formation in Beijing, China

Abstract. A comprehensive chlorine heterogeneous chemistry is incorporated into the Community Multiscale Air Quality (CMAQ) model to evaluate the impact of chlorine-related heterogeneous reaction on diurnal and nocturnal nitrate formation and quantify the nitrate formation from gas-to-particle partitioning of HNO 3 and from different heterogeneous pathways. The results show that these heterogeneous reactions increase the atmospheric Cl 2 and ClNO 2 level, leading to an increase of the nitrate concentration by ~ 10 % in the daytime. However, these reactions also lead to a decrease the nocturnal nitrate by ~ 20 %. Sensitivity analyses of uptake coefficients show that the empirical uptake coefficient for the O 3 heterogeneous reaction with chlorinated particles may lead to the large uncertainties in the predicted Cl 2 and nitrate concentrations. The N 2 O 5 uptake coefficient with particulate Cl − concentration dependence performs better to capture the concentration of ClNO 2 and nocturnal nitrate concentration. The reaction rate of OH and NO 2 in daytime increases by ~ 15 % when the heterogeneous chlorine chemistry is incorporated, resulting more nitrate formation from HNO 3 gas-to-particle partitioning. By contrast, the contribution of the heterogeneous reaction of N 2 O 5 to nitrate concentrations decreases by about 27 % in the nighttime when its reactions with chloriated particles are considered. However, the generated gas-phase ClNO 2 from the heterogeneous reaction of N 2 O 5 and chlorine-containing particles further decompose to increase the nitrate by 6 %. In general, this study highlights the potential of significant underestimation of daytime and overestimation of nighttime nitrate concentrations for chemical transport models without proper chlorine chemistry in the gas and particle phases.