Date of Award

Spring 2022

Document Type

Restricted Thesis

Degree Name

Bachelor of Arts (BA)

Department

Chemistry

First Advisor

Dr. Juan G. Navea

Abstract

Nitrous acid (HONO) is an atmospheric trace gas known to accumulate during nighttime and undergo rapid photodissociation during the day to form NO and highly reactive hydroxyl radicals, making accurate HONO estimations important in understanding atmospheric reactions. However, HONO is often underestimated in global atmospheric models because its sources and sinks are not well understood. Despite its photolysis, field observations have found quasi-steady-state concentrations of HONO at midday, suggesting the presence of photoproduction pathways to replenish daytime atmospheric HONO. Recent studies suggest that the presence of complex organic photosensitizers in aerosols can convert atmospheric nitrogen dioxide (NO2) into HONO and other nitrogenous gases, but the influence of environmental conditions have not been well studied. To better understand the effect of environmental photosensitizers on daytime mechanisms of HONO formation, we present here laboratory studies on the heterogeneous photolytic reduction of NO2 by humic acid films, an environmental photosensitizer and proxy for marine chromophoric compounds. The effect of relevant environmental conditions like pH and the presence of chloride ions on the formation of HONO and other nitrogenous gases is investigated. A dual FTIR system is utilized to simultaneously perform in-situ analysis of condensed-phase reactants and gas-phase products. We find that HONO is preferentially formed in the presence of chloride ions and does not have a strong pH dependence. Reactive uptake of NO2 onto the humic acid is observed, especially in the absence of chloride ions, suggesting competing pathways which suppress daytime formation of nitrogenous gases. Significantly, the presence of chloride ions also leads to a previously undiscovered daytime production pathway of nitrosyl chloride (ClNO), a precursor to HONO and a source of atomic chlorine, through photosensitization of the organic chromophore. In addition to photosensitized products, NO­2 desorbs from the surface, nitic oxide (NO) accumulates from the dissociation of photosensitive gases, and nitrous oxide (N2O) is produced thermally. Overall, this work shows that organic photosensitizers can reduce adsorbed NO2 to form HONO, ClNO, NO, and N2O while simultaneously incorporating nitrogen into the organic chromophores present in aerosols.

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