Abstract
Abstract. 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 HNO3 and from different heterogeneous pathways. The results show that these heterogeneous reactions increase the atmospheric Cl2 and ClNO2 level (∼ 100 %), which further affects the nitrate formation. Sensitivity analyses of uptake coefficients show that the empirical uptake coefficient for the O3 heterogeneous reaction with chlorinated particles may lead to the large uncertainties in the predicted Cl2 and nitrate concentrations. The N2O5 uptake coefficient with particulate Cl− concentration dependence performs better in capturing the concentration of ClNO2 and nocturnal nitrate concentration. The reaction of OH and NO2 in the daytime increases the nitrate by ∼15 % when the heterogeneous chlorine chemistry is incorporated, resulting in more nitrate formation from HNO3 gas-to-particle partitioning. By contrast, the contribution of the heterogeneous reaction of N2O5 to nitrate concentrations decreases by about 27 % in the nighttime, when its reactions with chlorinated particles are considered. However, the generated gas-phase ClNO2 from the heterogeneous reaction of N2O5 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.
Highlights
In recent years, nitrate has become the primary component of PM2.5 in Beijing, with sustained and rapid reduction of SO2 and primary particulate matter emissions (Ma et al, 2018; Li et al, 2018; Wen et al, 2018)
The overall impact of Reaction (R6) on nitrate remains to be investigated. Another related but unresolved issue is the sources of the high concentrations of Cl2, which could not be explained by the N2O5 heterogeneous reaction with Cl− and the subsequent reactions of ClNO2 in the gas phase
Predicted hourly Cl2, ClNO2 and N2O5 concentrations were compared with observations measured at the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (39.98◦ N, 116.37◦ E), using a highresolution time-of-flight chemical ionization mass spectrometer (CIMS) from 11 to 15 June 2017
Summary
Nitrate has become the primary component of PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 μm) in Beijing, with sustained and rapid reduction of SO2 and primary particulate matter emissions (Ma et al, 2018; Li et al, 2018; Wen et al, 2018). The overall impact of Reaction (R6) on nitrate remains to be investigated Another related but unresolved issue is the sources of the high concentrations of Cl2, which could not be explained by the N2O5 heterogeneous reaction with Cl− and the subsequent reactions of ClNO2 in the gas phase. WtoitphroCdPuscceanCpl qro(dKuncieppCiln2g, which et al, can subsequently photolyze 2000; George and Abbatt, 2010; Pratte and Rossi, 2006; Deiber et al, 2004; Faxon et al, 2015) These heterogeneous reactions in CPs are generally missing in most of the current CTMs, and it is unclear whether these reactions will be able to explain the observed Cl2 concentrations and the overall impact of these reactions on nitrate.
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