Abstract
Abstract. Nitrous acid (HONO) can strongly affect atmospheric photochemistry in polluted regions through the production of hydroxyl radicals (OHs). In January 2017, a severe pollution episode occurred in the Pearl River Delta (PRD) of China, with maximum hourly PM2.5, ozone, and HONO levels reaching 400 µg m−3, 150 ppb, and 8 ppb, respectively, at a suburban site. The present study investigated the sources and processes generating such high HONO concentrations and the role of HONO chemistry in this severe winter episode. Four recently reported HONO sources were added to the Community Multiscale Air Quality (CMAQ) model, including RH-dependent (relative humidity) and light-enhancing effects on heterogeneous reactions, photolysis of particulate nitrate in the atmosphere, and photolysis of HNO3 and nitrate on surfaces. The revised model reproduced the observed HONO and significantly improved its performance for O3 and PM2.5. The model simulations showed that the heterogeneous generation on surfaces (with RH and light effects) was the largest contributor (72 %) to the predicted HONO concentrations, with the RH-enhancing effects more significant at nighttime and the light-enhancing effects more important in the daytime. The photolysis of total nitrate in the atmosphere and deposited on surfaces was the dominant HONO source during noon and afternoon, contributing above 50 % of the simulated HONO. The HONO photolysis was the dominant contributor to HOx production in this episode. With all HONO sources, the daytime average O3 at the Heshan site was increased by 24 ppb (or 70 %), compared to the simulation results without any HONO sources. Moreover, the simulated mean concentrations of TNO3 (HNO3+ fine particle NO3-) at the Heshan site, which was the key species for this haze formation, increased by about 17 µg m−3 (67 %) due to the HONO chemistry, and the peak enhancement reached 55 µg m−3. This study highlights the key role of HONO chemistry in the formation of winter haze in a subtropical environment.
Highlights
Nitrous acid (HONO) can significantly affect atmospheric photochemistry through its photolysis producing hydroxyl radicals (OHs) (Reaction R1) and subsequent reactions of OH with other gases (Alicke et al, 2003; Kleffmann et al, 2005)
The hourly PM2.5 concentration peaked at 382 μg m−3, which was among the highest PM2.5 concentrations reported so far in the Pearl River Delta (PRD) region (Tan et al, 2009; Wang et al, 2012; Yue et al, 2015)
This study has identified the major contributors to the observed high HONO levels during a severe winter pollution episode and highlighted the importance of HONO chemistry in the combined photochemical and haze pollution in a subtropical region
Summary
Nitrous acid (HONO) can significantly affect atmospheric photochemistry through its photolysis producing hydroxyl radicals (OHs) (Reaction R1) and subsequent reactions of OH with other gases (Alicke et al, 2003; Kleffmann et al, 2005). OH radicals oxidize volatile organic compounds (VOCs) and convert nitric oxide (NO) into nitrogen dioxide (NO2) without consuming ozone (O3), leading to the generation of O3 (Reactions R2 to R6). The oxidation of oxides of nitrogen (NOx = NO+NO2), sulfur dioxide (SO2), and VOCs by OH and O3 produce secondary aerosols, which are the key components of haze
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