Abstract
Abstract. The photolysis of HONO is important for the atmospheric HOx (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/NOx ratio is a good proxy predictor for HONO mixing ratios under different atmospheric conditions. From the robust relationship between HONO and NOx, a representative mean HONO/NOx ratio of 0.02 has been derived. Using a global chemistry-climate model and employing this HONO/NOx 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 HOx 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.
Highlights
Nitrous acid (HONO) photolysis has been shown to be an important source of hydroxyl (OH) radicals, especially during the early morning when other sources are of minor importance (Perner and Platt, 1979; Harris et al, 1982)
Elshorbany et al.: Impact of HONO on global atmospheric chemistry have shown that additional HONO sources are required to match measured HONO mixing ratios; for some regions tenfold mismatches have been found based on the known gas phase HONO formation only (i.e., OH + NO → HONO)
The applied modelling system is based on the ECHAM5 general circulation model (Roeckner et al, 2006) and the Modular Earth Submodel System (MESSy, Jockel et al, 2005) to simulate the meteorology and atmospheric chemistry
Summary
Nitrous acid (HONO) photolysis has been shown to be an important source of hydroxyl (OH) radicals, especially during the early morning when other sources are of minor importance (Perner and Platt, 1979; Harris et al, 1982). Elshorbany et al.: Impact of HONO on global atmospheric chemistry have shown that additional HONO sources are required to match measured HONO mixing ratios; for some regions tenfold mismatches have been found based on the known gas phase HONO formation only (i.e., OH + NO → HONO). A common factor between the known major day- and nighttime sources is that they are generally surface-based (except the gas phase photolysis of nitro-aromatic compounds) Such HONO sources contribute a major fraction to the measured HONO levels (Harrison and Kitto, 1994; Stutz et al, 2002; Veitel, 2002; Kleffmann et al, 2003; Zhang et al, 2009; Sorgel et al, 2011b; Wong et al, 2011) and the dispersion into the atmosphere is a function of turbulent mixing (Sorgel et al 2011b).
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