Spilled oil is highly susceptible to sunlight-induced transformations, both as films on the surface of water and material dissolved or dispersed in the water column. We utilized ultrahigh-resolution mass spectrometry and optical spectroscopy to understand shifts in oil photoproduct distributions as a function of photo-oxygenation. Oxygenation of oil produces compounds that have increased polarity, resulting in greater partitioning to the oil–water interface and eventually greater partitioning into the aqueous phase. Such partitioning was shown to be dependent on the carbon number and oxygen content of the photoproducts, providing an empirical basis for predicting the partitioning of oil photodegradation products between the oil phase, the interfacial region, and into the aqueous phase to form petroleum-derived dissolved organic matter. While such photochemical transformations have been predicted for many years, there has not been direct evidence previously for the photodissolution process. Furthermore, the relationship of carbon number and oxygen content with progression from the oil phase to the interfacial phase to the aqueous phase has not been demonstrated. This paper details this progression and observable properties that can be used to understand oil behavior after a spill during sunlight exposure, thus providing greater predictability of oil fate, transport, impact, and effective remediation strategies.
To examine the molecular-level composition and acute toxicity per unit carbon of the petroleum-derived dissolved organic matter (DOMHC) produced via photo-oxidation, heavy and light oils were irradiated over seawater with simulated sunlight. Increases in dissolved organic carbon concentrations as a function of time were associated with changes in the DOMHC composition and acute toxicity per unit carbon. Parallel factor analysis showed that the fluorescent dissolved organic matter (FDOM) composition produced from the heavy oil became more blue-shifted over time, while the light oil produced a mixture of blue- and red-shifted components similar to FDOM signatures. Ultrahigh-resolution mass spectrometry reveals that the composition of the DOMHC produced from both heavy and light oils was initially relatively reduced, with low O/C. With time, the composition of the DOMHC produced from the heavy oil shifted to unsaturated, high-oxygen compounds, while that produced from the light oil comprised a range of high O/C aliphatic, unsaturated, and aromatic compounds. Microtox assays suggest that the DOMHC initially produced is the most toxic (62% inhibition); however, after 24 h, a rapid decrease in toxicity decreased linearly to 0% inhibition for the heavy DOMHC and 12% inhibition for the light DOMHC at extended exposure periods.
Soil‐atmosphere fluxes of carbon monoxide (CO) were investigated during BOREAS 1994 (June to September 1994) in forest sites near the northern study area (NSA) of the Boreal Ecosystem‐Atmosphere Study (BOREAS). Fluxes and related ancillary data were measured for both upland black spruce (located on poorly drained clay‐textured soils) and jack pine sites (well‐drained sandy soils) that were in early stages of succession following stand replacement fires that occurred within 7 years of BOREAS 1994. Nearby control stands that had not burned in the past 80 years were studied for comparison. Net fluxes measured by using transparent closed chambers were generally positive at the warmer, sunlit burn sites but negative (sink activity) in the shaded, cooler control sites. Carbon monoxide uptake in controls, which was first order with respect to CO concentration, was little affected by covering the sampling chambers to exclude light. Median deposition velocities calculated from the uptake fluxes were 0.015 cm s −1 at the black spruce control site and 0.0085 cm s −1 at the jack pine control site, at the lower end of the range of values observed by others in tropical and temperate ecosystems. Daytime CO fluxes at the burn sites were generally positive (10 11 ‐10 12 molecules cm −2 s −1 ) and were lowered when solar irradiance was excluded from the chambers by covering or when cloudiness or smoke reduced the light intensity. Net fluxes at the burn sites were controlled by competition between abiotic production, mainly at the surface, and by oxidation deeper in the soil. Abiotic production, which was attributable to photoproduction and thermal decomposition of the surface organic layer and charcoal, strongly correlated with incident solar irradiance, and thus the greatest fluxes were observed during midday. Results of these studies indicate that the locally dependent changes in boreal fire return intervals that are linked to global climate change represent an important biospheric/physical feedback that is likely to alter the biosphere‐atmosphere exchange of CO.
