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

Qiu, Xionghui; Ying, Qi; Wang, Shuxiao; Duan, Lei; Zhao, Jian; Xing, Jia; Ding, Dian; Sun, Yele; Liu, Baoxian; Shi, Aijun; Yan, Xiao; Xu, Qingcheng; Hao, Jiming

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 inline-formulaHNO3 and from different heterogeneous pathways. The results show that these heterogeneous reactions increase the atmospheric inline-formulaCl2 and inline-formulaClNO2 level (inline-formula∼ 100 %), which further affects the nitrate formation. Sensitivity analyses of uptake coefficients show that the empirical uptake coefficient for the inline-formulaO3 heterogeneous reaction with chlorinated particles may lead to the large uncertainties in the predicted inline-formulaCl2 and nitrate concentrations. The inline-formulaN2O5 uptake coefficient with particulate inline-formulaCl concentration dependence performs better in capturing the concentration of inline-formulaClNO2 and nocturnal nitrate concentration. The reaction of OH and inline-formulaNO2 in the daytime increases the nitrate by inline-formula∼15 % when the heterogeneous chlorine chemistry is incorporated, resulting in more nitrate formation from inline-formulaHNO3 gas-to-particle partitioning. By contrast, the contribution of the heterogeneous reaction of inline-formulaN2O5 to nitrate concentrations decreases by about 27 % in the nighttime, when its reactions with chlorinated particles are considered. However, the generated gas-phase inline-formulaClNO2 from the heterogeneous reaction of inline-formulaN2O5 and chlorine-containing particles further reacts with the particle surface to increase the nitrate by 6 %. In general, this study highlights the potential of significant underestimation of daytime concentrations and overestimation of nighttime nitrate concentrations for chemical transport models without proper chlorine chemistry in the gas and particle phases.

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Qiu, Xionghui / Ying, Qi / Wang, Shuxiao / et al: Modeling the impact of heterogeneous reactions of chlorine on summertime nitrate formation in Beijing, China. 2019. Copernicus Publications.

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