Photochemical processes on ground and airborne surfaces have been suspected to lead to production of HONO in the sunlit lower troposphere, e.g. upon light activation of humic acids followed by reaction with adsorbed NO(2). Here, we used tannic and gentisic acids as proxies for atmospheric polyphenolic compounds to obtain further insights into the photoenhanced NO(2) conversion to HONO, which is a significant tropospheric hydroxyl radical (OH) source. The coated wall flow tube technique was used in combination with online detection of gas-phase HONO and NO(x) under different irradiation conditions. Photoenhanced HONO formation rates of 0.1 to 2 ppbv s(-1) were measured upon NO(2) (0-400 ppbv) uptake on tannic and gentisic acid coatings under irradiation with UV light. The data allow identification of three pathways of light-induced HONO formation: (I) photolysis of a nitroaromatic intermediate formed by a non-photochemical process in the dark, with a photolysis frequency of 10(4) s(-1) at 2 × 10(20) photons m(-2) photon flux; (II) direct photo-oxidation, presumably through electron or hydrogen transfer of the excited substrate; and (III) sensitized electron or hydrogen transfer as suggested before but also demonstrated for visible irradiation here. Aging of tannic acid under oxygen in the dark led to products which promoted light-induced HONO formation in the visible.
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