The significant contribution of HONO to secondary pollutants during a severe winter pollution event in southern China

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 PM _2.5 , ozone, and HONO levels reaching 400 μg mg ^-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 HNO ₃ and nitrate on surfaces. The revised model reproduced the observed HONO and significantly improved its performance for O ₃ and PM _2.5 . 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 O ₃ 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 TNO ₃ (HNO ³⁺ fine particle NO ₃ g ^- ) at the Heshan site, which was the key species for this haze formation, increased by about 17 μg mg ^-3 (67 %) due to the HONO chemistry, and the peak enhancement reached 55 μg mg-3. This study highlights the key role of HONO chemistry in the formation of winter haze in a subtropical environment.