Impact of biomass burning on HONO chemistry and atmospheric oxidation capacity in the Yangtze River delta, China.

Extensive research confirms that biomass burning (BB) significantly contributes to particulate matter and gaseous pollutant emissions. Open burning of wheat straw is particularly common during the harvest season, directly affecting air quality and atmospheric oxidation capacity. In this study, nitrous acid (HONO) and associated atmospheric species were investigated during the BB season at a suburban station, as part of the 2018 EXPLORE-YRD campaign. Based on the enhancement ratio of acetonitrile (C2H3N) to CO, 10 nighttime BB events were identified, with the remaining nighttime periods defined as non-biomass burning (NBB) periods. Compared to the NBB period, the HONO levels during BB events increased from 0.49 ± 0.48 ppbv to 1.75 ± 0.98 ppbv. The average concentrations of other species also exhibited significant increases, ranging from 73 % to 192 %. The average HONO to CO normalized excess mixing ratio (NEMR) in the BB events ranged from 1.83 ± 0.42 to 3.41 ± 0.39 ppbv/ppmv, consistent with the range reported in previous laboratory and field burning studies. Smoldering combustion may contribute to the lower observed HONO to CO NEMR. Furthermore, the roles of different heterogeneous sources in HONO production were compared using correlation analysis and source strength calculations. The ground surface displayed significantly greater promotion of NO2 to HONO conversion than the aerosol surface during NBB periods. During BB periods, the significant increase in particle concentration and expanded aerosol surface area result in the contribution of aerosol surface being comparable to that of the ground surface. The NO2 heterogeneous conversion to HONO was influenced by relative humidity during nighttime. Following the BB events, HONO photolysis (1.76 ± 0.69 ppbv h-1) emerged as the primary hydroxyl radical (OH) source, exceeding contributions from formaldehyde photolysis (0.86 ± 0.36 ppbv h-1) and O3 photolysis (0.49 ± 0.32 ppbv h-1).