[Atmospheric HONO Concentration Levels and Generation Mechanisms in Nanjing in Winter and Summer]

HONO, or gaseous nitrous acid, has a significant impact on air quality and climate. An atmospheric gaseous nitrous acid （HONO） observation study was carried out in December 2021 （winter） and July 2022 （summer） in the Jiangbei New District of Nanjing City, respectively. Using the HONO concentration observation data, combined with the atmospheric NO_x and O₃ concentrations during the sampling period as well as the reaction rate of the HONO generation process and other parameters, we compared and analyzed the change rule of the atmospheric HONO concentration during winter and summer in the Nanjing area, quantitatively investigated the sources of diurnal atmospheric HONO and its generation mechanism, and explored the unknown sources of atmospheric HONO during the daytime. The results showed that the mean value of atmospheric ρ（HONO） in the Nanjing area during winter was （2.2±1.1） μg·m^-3, which was 3.0 times the average concentration [（0.72±0.45） μg·m^-3] during summer. The atmospheric HONO concentrations during winter and summer showed a significant daily pattern of low during the daytime and high at nighttime. Based on the reaction rates of different HONO generation pathways, it was calculated that the non-homogeneous reaction of NO₂ was the main pathway of atmospheric HONO generation at night in Nanjing. The analysis of the atmospheric HONO balance during the daytime showed that there existed an important unknown source of HONO during the daytime （P_unknown）, and its contribution to the daytime atmospheric HONO concentration was 69% during winter and 47% during summer, respectively. The results of correlation analysis showed that P_unknown was related to the light-catalyzed reaction of NO₂ during the daytime.