Abstract
Abstract. Although stringent emission mitigation strategies have been carried out since 2013 in Beijing–Tianjin–Hebei (BTH), China, heavy haze with high levels of fine particulate matter (PM2.5) still frequently engulfs the region during wintertime and the nitrate contribution to PM2.5 mass has progressively increased. N2O5 heterogeneous hydrolysis is the most important pathway of nitrate formation at nighttime. In the present study, the WRF-Chem model is applied to simulate a heavy haze episode from 10 to 27 February 2014 in BTH to evaluate contributions of N2O5 heterogeneous hydrolysis to nitrate formation and effects of organic coating. The model generally performs reasonably well in simulating meteorological parameters, air pollutants, and aerosol species against observations in BTH. N2O5 heterogeneous hydrolysis with all the secondary organic aerosol assumed to be involved in coating considerably improves the nitrate simulations compared to the measurements in Beijing. On average, organic coating decreases nitrate concentrations by 8.4 % in BTH during an episode, and N2O5 heterogeneous hydrolysis with organic coating contributes about 30.1 % of nitrate concentrations. Additionally, the reaction also plays a considerable role in the heavy haze formation, with a PM2.5 contribution of about 11.6 % in BTH. Sensitivity studies also reveal that future studies need to be conducted to predict the organic aerosol hygroscopicity for accurately representing the organic coating effect on N2O5 heterogeneous hydrolysis.
Highlights
Within recent decades, China has been suffering from pervasive and persistent haze pollution caused by elevated levels of fine particulate matters (PM2.5), in Beijing– Tianjin–Hebei (BTH) (Guo et al, 2014; Gao et al, 2016; Wang et al, 2016)
Organic layers may be formed by secondary organic aerosols, and such layers may consist of a single layer of molecules or of several molecule layers on the surface of the aqueous core
At 850 hPa, the southeast coastal areas of China are controlled by the anticyclone, whose center is located over the South China Sea (Fig. 2c)
Summary
China has been suffering from pervasive and persistent haze pollution caused by elevated levels of fine particulate matters (PM2.5), in Beijing– Tianjin–Hebei (BTH) (Guo et al, 2014; Gao et al, 2016; Wang et al, 2016). Wang et al (2017) evaluated the potential particulate nitrate formation through the N2O5 hydrolysis reaction without considering the organic coating effect during a haze pollution episode in Beijing, and found that the observed nitrate concentration (20.6 μg m−3 on average) is lower than the assessment (57.0 μg m−3 on average). Considering the high organic aerosol concentration and increasing trend of particulate nitrate during haze days in BTH, it is imperative to assess the effect of organic coating on N2O5 hydrolysis and its consequent contribution to nitrate formation. In the present study, based on Riemer parameterization, the contribution of the organic coating effect on N2O5 hydrolysis to nitrate formation is investigated using the WRFChem model.
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