Abstract

Co-occurrences of high concentrations of PM2.5 and ozone (O3) have been frequently observed in haze aggravating processes in the North China Plain (NCP) over the past few years, and higher O3 concentrations during hazy days were supposed to be related to nitrous acid (HONO), but the key sources of HONO enhancing O3 during haze aggravating processes remain unclear, and will be explored in this study by using the WRF-Chem model, which is improved to include ground-based (traffic, soil, and indoor emissions, and the NO2 heterogeneous reaction on ground surface (Hetground)) and aerosol-related (the NO2 heterogeneous reaction on aerosol surfaces (Hetaerosol) and nitrate photolysis (Photnitrate)) potential HONO sources. The results indicate that ground-based HONO sources producing HONO enhancements showed a rapid decrease with height, while the NO+OH reaction and aerosol-related HONO sources decreased slowly with height. Photnitrate contributions to HONO concentrations enhanced with aggravated pollution levels, the enhanced HONO due to Photnitrate in hazy days was about one order of magnitude larger than in clean days and Photnitrate dominated HONO sources (~30–70 % when the ratio of the photolysis frequency of nitrate (Jnitrate) to gas nitric acid (JHNO3) equals 30) at higher layers (> 800 m). Compared with that in clean days, the Photnitrate contribution to the enhanced daily maximum 8-h averaged O3 was increased by over one magnitude during the haze aggravating process. Photnitrate contributed only ~5 % of the surface HONO in daytime with a Jnitrate/JHNO3 ratio of 30 but contributed ~30–50 % of the enhanced O3 near the surface in NCP in hazy days. Surface O3 was dominated by volatile organic compounds-sensitive chemistry, while O3 at higher altitude (> 800 m) was dominated by NOx-sensitive chemistry. Photnitrate had a limited impact on nitrate concentrations (< 15 %) even with a Jnitrate/JHNO3 ratio of 120. The above results suggest that more field studies of Jnitrate in the atmosphere are still needed.

Highlights

  • IntroductionHONO sources can be generally classified into three categories, i.e., direct emissions, homogeneous and heterogeneous reactions

  • Including air temperature (T), relative humidity (RH) and wind speed (WS) were comparable with the previous modelling results of other researchers (Table 3), the simulated wind direction (WD) bias within 45° accounted for ~56%, and the bias within

  • We found that the overall impact of nitrate photolysis to OH and O3 would be severely underestimated when the contribution of nitrate photolysis to vertical HONO was excluded

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Summary

Introduction

HONO sources can be generally classified into three categories, i.e., direct emissions, homogeneous and heterogeneous reactions. The reaction of nitric oxide (NO) with OH is usually thought as the dominant homogeneous reaction and is important during daytime but could be neglected at night due to low OH concentrations. The heterogeneous reactions mainly include nitrogen dioxide (NO2) hydrolysis and reduction reactions on various humid surfaces (Finlayson-Pitts et al, 2003; Ge et al, 2019; Gómez Alvarez et al, 2014; Ma et al., 2013; Marion et al, 2021; Sakamaki et al, 1983; Tang et al, 2017; Yang et al, 2021b) and nitrate photolysis (Romer et al, 2018; Zhou et al, 2003), and are usually thought as the main contributor to HONO concentrations in the atmosphere

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Discussion
Conclusion

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