Spatial and Temporal Distribution of Singlet Oxygen in Lake Superior

A multiyear field study was undertaken on Lake Superior to investigate singlet oxygen (¹O₂) photoproduction. Specifically, trends within the lake were examined, along with an assessment of whether correlations existed between chromophoric dissolved organic matter (CDOM) characteristics and ¹O₂ production rates and quantum yields. Quantum yield values were determined and used to estimate noontime surface ¹O₂ steady-state concentrations ([¹O₂]_ss). Samples were subdivided into three categories based on their absorbance properties (<i>a</i>300): <i>riverine</i>, <i>river</i>-<i>impacted</i>, or <i>open lake</i> sites. Using calculated surface [¹O₂]_ss, photochemical half-lives under continuous summer sunlight were calculated for cimetidine, a pharmaceutical whose reaction with ¹O₂ has been established, to be on the order of hours, days, and a week for the <i>riverine</i>, <i>river-impacted</i>, and <i>open lake</i> waters, respectively. Of the CDOM properties investigated, it was found that dissolved organic carbon (DOC) and <i>a</i>300 were the best parameters for predicting production rates of [¹O₂]_ss. For example, given the correlations found, one could predict [¹O₂]_ss within a factor of 4 using <i>a</i>300 alone. Changes in the quantum efficiency of ¹O₂ production upon dilution of river water samples with lake water samples demonstrated that the CDOM found in the open lake is not simply diluted riverine organic matter. The <i>open lake</i> pool was characterized by low absorption coefficient, low fluorescence, and low DOC, but more highly efficient ¹O₂ production and predominates the Lake Superior system spatially. This study establishes that parameters that reflect the quantity of CDOM (e.g., <i>a</i>300 and DOC) correlate with ¹O₂ production rates, while parameters that characterize the absorbance spectrum (e.g., spectral slope coefficient and E2:E3) correlate with ¹O₂ production quantum yields

Spatial and Temporal Distribution of Singlet Oxygen in Lake Superior

A multiyear field study was undertaken on Lake Superior to investigate singlet oxygen (¹O₂) photoproduction. Specifically, trends within the lake were examined, along with an assessment of whether correlations existed between chromophoric dissolved organic matter (CDOM) characteristics and ¹O₂ production rates and quantum yields. Quantum yield values were determined and used to estimate noontime surface ¹O₂ steady-state concentrations ([¹O₂]_ss). Samples were subdivided into three categories based on their absorbance properties (<i>a</i>300): <i>riverine</i>, <i>river</i>-<i>impacted</i>, or <i>open lake</i> sites. Using calculated surface [¹O₂]_ss, photochemical half-lives under continuous summer sunlight were calculated for cimetidine, a pharmaceutical whose reaction with ¹O₂ has been established, to be on the order of hours, days, and a week for the <i>riverine</i>, <i>river-impacted</i>, and <i>open lake</i> waters, respectively. Of the CDOM properties investigated, it was found that dissolved organic carbon (DOC) and <i>a</i>300 were the best parameters for predicting production rates of [¹O₂]_ss. For example, given the correlations found, one could predict [¹O₂]_ss within a factor of 4 using <i>a</i>300 alone. Changes in the quantum efficiency of ¹O₂ production upon dilution of river water samples with lake water samples demonstrated that the CDOM found in the open lake is not simply diluted riverine organic matter. The <i>open lake</i> pool was characterized by low absorption coefficient, low fluorescence, and low DOC, but more highly efficient ¹O₂ production and predominates the Lake Superior system spatially. This study establishes that parameters that reflect the quantity of CDOM (e.g., <i>a</i>300 and DOC) correlate with ¹O₂ production rates, while parameters that characterize the absorbance spectrum (e.g., spectral slope coefficient and E2:E3) correlate with ¹O₂ production quantum yields