Atmospheric nitrous acid (HONO) is an important precursor of OH radical, accumulating during nighttime and photodissociating at daytime to form OH and NO. Despite its rapid photolysis, recent field studies have found daytime quasi-steady state concentration of HONO in the marine boundary layer (MBL), suggesting that there are unknown paths for light-induced HONO formation. While nighttime thermal formation of HONO is reasonably well understood, little is known about daytime HONO formation mechanisms. It is hypothesized that the presence of organic photosensitizers in dissolved organic matter (DOM) can convert NO2 into HONO and other nitrogen-containing gases. Here we present laboratory studies on the heterogeneous photolytic reduction of NO2 by humic acid films, a proxy for organic chromophoric compounds in sea spray aerosols. The effect of pH in the photosensitized formation of HONO and other nitrogen-containing gases from sea spray aerosols was also investigated. While HONO is preferentially formed at lower pHs, the presence of chlorine ions also leads to the formation of nitrosyl chloride (ClNO), a nighttime precursor of HONO. In general, film experiments show that organic acid photosensitizers can reduced adsorbed NO2 to form HONO, ClNO, and NO. In situ analysis of both gaseous and condensed phases is discussed for thermal and photolytic reaction pathways.

Effect of pH in the photosensitized reduction of NO2 adsorbed in humic acid films: Daytime formation of HONO, ClNO, N2O, and NOx

ACS Fall 2021

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