Characterization of nitrous acid and its potential effects on secondary pollution in the warm season in Beijing urban areas

As a key source of hydroxyl (OH) radicals, nitrous acid (HONO) has attracted much attention for its important role in the atmospheric oxidant capacity (AOC) increase. In this study, we made a comparison of the ambient levels, variation patterns, sources, and formation pathway in the warm season (from June to October in 2021) on the basis of a continuous intensive observation in an urban site of Beijing. The monthly average mixing ratios of HONO were 1.3, 1.3, 1.0, 0.96, and 0.89 ppb, respectively, showing a larger contribution to OH radicals relative to ozone in the daytime. The emission factor (EF) relative to NOx from the vehicle emissions was estimated to be 0.017, higher than most studies conducted in Beijing. The average nocturnal NO2/HONO conversion frequency kHONO was 0.008 h-1. In the warm season, the missing source of HONO, Punknown, around noontime was 0.29-2.7 ppbh-1. According to the OH production from HONO, the OH production rate from the missing HONO was also very important to the AOC. This work highlights the importance of HONO for the AOC in the warm season, while encouraging long-term HONO observation to assess the contribution of HONO sources over time compared to the capture of pollution processes.