The initial focus of this work was to look at basic photochemical mechanisms impacting the production of sulfur species in the surface ocean. Specifically addressed were photochemical reactions that could potentially influence the chemistry and consequently the sea/air transfer of OCS, H2S, and CO. DOM participation in the potential pathways of formation of these compounds was also addressed.

This thesis describes experiments designed to further the understanding of mechanisms and pathways of free radicals, specifically sulfur-centered radicals in natural water. In this study, the mechanisms for the photoproduction of carbonyl sulfide (OCS) and carbon monoxide (CO) in natural waters were studied by evaluating the results from different aqueous systems.

Experimental evidence indicates a coupled mechanism operating on CO and OCS photoproduction. For CO photoproduction the presence of a carbonyl group is necessary while for OCS a source of reduced sulfur in addition to the carbonyl is also required. An acyl radical is postulated to be the key intermediary for OCS and CO photoproduction while a sulfur-centered radical (thiyl radical) is likely to be the key species that reacts with acyl radicals to produce OCS. Addition of reduced sulfur to seawater and subsequent irradiation leads to a decrease in CO and an increase in OCS photoproduction rates. This anti-correlation is likely to be seen in natural waters with high biological productivity in which reduced sulfur compounds and dissolved organic matter are abundant.

Furthermore, this thesis describes experiments performed to check the participation of light on the photooxidation of sulfide in seawater. The kinetics of H2S photosensitized reaction in seawater was studied to determine the potential of this reaction as a sink for H2S. Information concerning the competition between oxidation by oxygen and photosensitized reaction by DOM was obtained. Photokinetic studies demonstrate that sulfide is consumed in seawater upon light exposure. The half-life (t1/2) of sulfide (10 mM) added to Biscayne Bay water (BBSW) and Gulf Stream water (GSSW) are 49(±15) minutes and 147(±10) minutes respectively. This as yet unaccounted sink of sulfide in seawater could be responsible for the sulfide day time low and night time high concentration values observed by several investigators. This finding further unbalances the sulfide budget and reinforces the need for systematic research on the role of photochemical processes on sulfur species in seawater.