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

Abstract. Benefiting from a series of pollution prevention initiatives, fine particle (PM_2.5) pollution was effectively controlled in 2018–2020, but ground-level ozone pollution during warm season has become a major issue for the continuous air quality improvement in Beijing. As a key source of hydroxyl (OH) radical, nitrous acid (HONO) has attracted much attention for its important role in the atmospheric oxidant capacity (AOC) increase; the elucidation of the pollution characteristics, unknown sources and the contribution to secondary pollution of HONO has become a research hotspot. In this study, we made a comparative study on the ambient levels, variation patterns, sources and formation pathway in warm season (from June to October in 2021) on a basis of a continuous intensive observation in an urban site of Beijing. The monthly average mixing ratio of HONO were 1.26 ppb, 1.28 ppb, 1.01 ppb, 0.96 ppb, 0.89 ppb, respectively, showing a larger contribution to OH radical relative to ozone at daytime with a mean OH production rate of 2.70, 2.91, 2.00, 2.25, and 0.93 ppb/h, respectively. The emission factor from the vehicle emissions, P_emis, was estimated to be 0.017, higher than most studies conducted in Beijing, the observation site and traffic control policies could affect this phenomenon. The homogeneous production of HONO via reaction of NO + OH, P_netOH+NO, in each month were 0.050, 0.045, 0.033, 0.052, and 0.17 ppb/h, respectively. The average nocturnal NO₂ to HONO conversion frequency C_HONO in each month were 0.011 h⁻¹, 0.0096 h⁻¹, 0.013% h⁻¹, 0.0081 h⁻¹, and 0.0017 h⁻¹, respectively.

In warm seasons, the missing source of HONO, P_unknown, around noontime were 0.29–2.37 ppb/hr. P_unknown in each month might be various. According to the observation results, relatively low humidity and strong solar illumination were conductive to HONO formation in June, which might be due to light-induced heterogeneous reactions of NO₂. In July in Beijing, high humidity condition was beneficial to the heterogeneous reaction of NO₂, and due to the increase of precipitation, more HONO would enter the liquid phase (the high Henry coefficient of HONO). For days with high humidity and strong sunlight in June and August, photolysis of nitrate was also one important HONO source. For August and September, light-induced reactions of NO₂ on non-aerosol surfaces under relatively low humidity and strong light conditions could be an important HONO source. In addition, the presence of Cl ions and sulfate could enhance the photolysis of nitrate, and this was obvious in July and October; the presence of organic compounds also could have this effect, which was obvious in June and October. Not only the HONO concentration but also the HONO source has temporal patterns, even within a season, it varies from month to month. This work highlights the importance of HONO for AOC in warm season, while encouraging long-term HONO observation to assess the contribution of HONO sources over time compared to the capture of pollution processes.