Impact of HONO on global atmospheric chemistry calculated with an empirical parameterization in the EMAC model

Abstract. The photolysis of HONO is important for the atmospheric HO_x (OH + HO₂) radical budget and ozone formation, especially in polluted air. Nevertheless, owing to the incomplete knowledge of HONO sources, realistic HONO mechanisms have not yet been implemented in global models. We investigated measurement data sets from 15 field measurement campaigns conducted in different countries worldwide. It appears that the HONO/NO_x ratio is a good proxy predictor for HONO mixing ratios under different atmospheric conditions. From the robust relationship between HONO and NO_x, a representative mean HONO/NO_x ratio of 0.02 has been derived. Using a global chemistry-climate model and employing this HONO/NO_x ratio, realistic HONO levels are simulated, being about one order of magnitude higher than the reference calculations that only consider the reaction OH + NO → HONO. The resulting enhancement of HONO significantly impacts HO_x levels and photo-oxidation products (e.g, O₃, PAN), mainly in polluted regions. Furthermore, the relative enhancements in OH and secondary products are higher in winter than in summer, thus enhancing the oxidation capacity in polluted regions, especially in winter when other photolytic OH sources are of minor importance. Our results underscore the need to improve the understanding of HONO chemistry and its representation in atmospheric models.