Photochemical Smog in Southern China: A Synthesis of Observations and Model Investigations of the Sources and Effects of Nitrous Acid

Recent studies have revealed potentially important effects of additional source(s) of hydroxyl radicals on the atmosphere’s oxidative capacity and, in turn, the production of secondary air pollutants. In this paper, we give an overview of our recent efforts in investigating the sources and effects of nitrous acid (HONO) on ozone and some secondary aerosols in southern China by combining field measurements and model simulations. Beginning in 2011, a series of field measurements of HONO were conducted at five sites, with diverse land use and different effects of emission sources. We observed the seasonal characteristics, emission ratios, heterogeneous production, and made simulations with a chemical transport model for the photochemical effects of HONO. The key findings are as follows. The derived emission ratios from vehicles exhibited wide variability and were mostly higher than the more uniform value of 0.8% reported in the literature. Larger nocturnal heterogeneous conversion rates of NO2 to HONO were observed when air masses were passing over sea surfaces, compared with land surfaces. Widely reported daytime sources of HONO also exist in Hong Kong. Moreover, the revised WRF-Chem model with comprehensive HONO sources significantly improved the simulations of the observed HONO, which enhanced regional hydroxyl radicals, O3, and PM2.5 by 10–20, 8–15, and 10–15% over urban areas in the Pearl River Delta region, respectively. Our studies highlight the importance of considering HONO sources when simulating secondary pollutants in polluted atmospheres.