Atmospheric nitrous acid (HONO), a trace atmospheric gas, is often underestimated in global atmospheric models due to the poor understanding of its sources and sinks, especially in the marine boundary layer (MBL). HONO is known to accumulate during nighttime and undergo rapid photodissociation during the day to form NO and highly reactive OH radical, making it important to have accurate atmospheric HONO estimations. Despite its rapid photolysis, recent field observations have found quasi-steady-state concentrations of HONO in the MBL at midday, suggesting photolytic HONO formation pathways to replenish daytime atmospheric HONO. While nighttime thermal formation of HONO is reasonably well understood, little is known about daytime HONO formation mechanisms. Recent studies suggest that the presence of complex organic photosensitizers in atmospheric aerosols convert atmospheric NO2 into HONO and other nitrogen-containing gases. Sea spray aerosols, originating from oceanic wave breaking and bubble bursting, are rich in chloride ions (Cl-) and have been shown to quickly acidify. To better understand the daytime mechanisms of HONO formation in the MBL under environmentally relevant conditions, we present laboratory studies on the heterogeneous photolytic reduction of NO2 by humic acid films as a proxy for organic chromophoric compounds in sea spray aerosols. The effect of pH and Cl- in the photosensitized formation of HONO and other nitrogen-containing gases from sea spray aerosols is also investigated. A dual FT-IR system is utilized to simultaneously perform in situ analysis of condensed-phase reactants and gas-phase products. We find that HONO is preferentially formed at a lower pH and in the presence of Cl-. In the absence of Cl-, nitration of the complex organic chromophores is observed in the aerosol phase, suggesting a competing pathway which results in suppressed daytime formation of nitrogenous gases. Significantly, the presence of chloride ions also leads to the organic-mediated photolytic formation of nitrosyl chloride (ClNO), a precursor of HONO. Overall, this work shows that organic acid photosensitizers can reduce adsorbed NO2 to form HONO, ClNO, and NO while simultaneously incorporating nitrogen into the organic chromophores present in the aerosol.

Photosensitization of adsorbed NO2 on humic acid films as a daytime source of HONO, ClNO, and NO: Effect of pH and chloride ions

ACS Spring 2022

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