PHOTOINDUCED DEGRADATION OF TRACER PHENOLS ADDED TO MARINE SURFACE MICROLAYERS

To determine photochemical reactivity of surface microlayer components and to estimate residence times of phenolic materials at the ocean surface, we evaluated photo-induced degradation of three tracer phenols (phloroglucinol [1,3,5-trihydroxybenzene), TMP (2,4,6-trimethylphenol), and cresol (3-methylphenol)) added to samples of surface microlayers and subsurface waters. When all samples were exposed to the same natural sunlight conditions, first-order degradation rates of phloroglucinol in microlayer samples from surface slicks were always faster than degradation rates in microlayer samples not from slicks or in samples from subsurface seawaters. TMP was less photo-reactive than phloroglucinol, probably because of its lower dissociation to phenoxide ion at seawater pH. Degradation rates of phloroglucinol and TMP in microlayer samples from surface slicks were depressed by the presence of 50-mu-M NaN3, a common singlet oxygen quencher, suggesting the involvement of photo-induced singlet oxygen. Generally, degradation of cresol was slower than either phloroglucinol or TMP, although it should have had a higher degree of dissociation than TMP at seawater pH; degradation of cresol in seawater may be not via singlet oxygen. Dark control experiments exhibited < 10% decreases in tracer phenol concentrations, indicating that microbial activities had little effect on degradation of these phenols. Because photoreactivity in samples from surface slicks was generally much greater than in other samples, microlayer photochemistry cannot be estimated by simply extrapolating near-surface reaction rates to surface light intensities.