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

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.

Significant production of ClNO<sub>2</sub> and possible source of Cl<sub>2</sub> from N<sub>2</sub>O<sub>5</sub> uptake at a suburban site in eastern China

“…Besides ClNO 2 , Cl 2 is another important chlorine radical precursor that is present in the lower troposphere (Spicer et al, 1998;Custard et al, 2016;Priestley et al, 2018). Elevated levels of Cl 2 (up to ∼ 400 pptv) have been observed during the daytime in polar and continental environments (Liao et al, 2014;Liu et al, 2017), whereas other studies found nocturnal peaks of Cl 2 mixing ratios in polar, coastal, and continental sites (Mielke et al, 2011;Riedel et al, 2012Riedel et al, , 2013McNamara et al, 2019). Several potential sources of Cl 2 have been proposed, such as direct emissions from power plants (Riedel et al, 2013) and water treatment facilities (Mielke et al, 2011), photochemical formation associated with O 3 (Liao et al, 2014), photoinduced production by TiO 2 (Li et al, 2020), and heterogeneous conversion from chlorinated compounds (Reactions R5 and R6) (Deiber et al, 2004;Pratte and Rossi, 2006;McNamara et al, 2019).…”

Modeled changes in source contributions of particulate matter during the COVID-19 pandemic in the Yangtze River Delta, China

“…Fine particulate matter (PM 2.5 , an aerodynamic diameter of fewer than 2.5 µm) has been a great concern in China since 2013 due to its high levels and related health risks (Lelieveld et al, 2015;Huang et al, 2014;He and Christakos, 2018;Shang et al, 2018;Song et al, 2017Song et al, , 2016Yan et al, 2018;Du and Li, 2016;Liu et al, 2016;Shen et al, 2020a). To improve air quality, China has promulgated stringent emission control plans such as the Air Pollution Prevention and Control Action Plan, and PM 2.5 concentrations have been reduced significantly in different regions (Zheng et al, 2018;Cai et al, 2017;Zhang et al, 2016;Zheng et al, 2017).…”

Changes in source contributions of particulate matter during COVID-19 pandemic in the Yangtze River Delta, China

“…The daily simulated trends of SIA generally agreed with the observations, although the model slightly overpredicted SIA concentrations with the MFB values of 0.19-0.37 and the MFE values of 0.41-0.68 (Table S3). The overestimation of nitrate has been reported in the previous studies (Chang et al, 2018;Shen et al, 2020b;Choi et al, 2019) and the possible reason was the lack of chlorine heterogeneous chemistry in the model (Qiu et al, 2019). Compared to the predictions before the lockdown, the MFB and MFE values of PM2.5 and its components during the lockdown were slightly lower than the values before the lockdown (Table S4), indicating the model performance during the lockdown was better.…”