Industrial-Related Emissions May Be an Overlooked Source of Atmospheric Nitrous Acid

Gaseous nitrous acid (HONO), as a key hydroxyl radical reservoir, is essential for atmospheric chemistry. However, potential HONO sources from direct or secondary pathways remain unclear, especially in industrial environments where HONO may be formed accompanied by large industrial NOx emissions. Herein, by combining field observations in a densely industrial site during summertime of 2021 and modeling results, we found that besides the currently recommended HONO formation mechanisms, including photolysis of adsorbed nitric acid and particulate nitrate, (photoenhanced) NO2 heterogeneous conversions, and direct emissions, a considerable unidentified HONO source (i.e., Pmissing) emerged in the industrial area with an increasing significance at night, whereas it did not appear in the urban and rural areas although the same parametrization was considered. When the missing gap of HONO was incorporated into a positive matrix factorization model, a significant industrial-related source dominated by the tracers of Fe, Mn, Zn, and Cu with Pmissing (ca. 76%) was first revealed, accounting for ca. 34% of HONO production in this industrial area. In addition, a good correlation (rs = 0.64, p < 0.1) between the industrial NOx emissions and atmospheric HONO abundance during the summertime of 2022–2023 also highlighted the significance of industrial HONO emissions. Therefore, it is anticipated that paying more attention to industrial-related HONO emissions would be a new perspective to further improve regional air quality.