Factors affecting the mixed-layer concentrations of singlet oxygen in sunlit lakes

The steady-state concentration of singlet oxygen within a lake ([¹O₂]_SS) is an important parameter that can affect the environmental half-life of pollutants and environmental fate modelling. However, values of [¹O₂]_SS are often determined for the near-surface of a lake, and these values typically do not represent the average over the epilimnia of lakes. In this work, the environmental and physical factors that have the largest impact on [¹O₂]_SS within lake epilimnia were identified. It was found that the depth of the epilimnion has the largest impact on depth-averaged [¹O₂]_SS, with a factor of 8.8 decrease in [¹O₂]_SS when epilimnion depth increases from 2 m to 20 m. The next most important factors are the wavelength-dependent singlet oxygen quantum yield relationship and the latitude of the lake, causing variations in [¹O₂]_SS by factors of 3.2 and 2.5 respectively, over ranges of representative values. For a set of representative parameters, the depth-averaged value of [¹O₂]_SS within an average epilimnion depth of 9.0 m was found to be 5.8 × 10^-16 M and the near-surface value of [¹O₂]_SS was found to be 1.9 × 10^-14 M. We recommend a range of 6 × 10^-17 to 5 × 10^-15 M as being more representative of [¹O₂]_SS values within the epilimnia of lakes globally and potentially more useful for estimating pollutant lifetimes than those calculated using [¹O₂]_SS values that correspond to near-surface, summer midday values. This work advances our understanding of [¹O₂]_SS inter-lake variability in the environment, and provides estimates of [¹O₂]_SS for practitioners and researchers to assess environmental half-lives of pollutants due to reaction with singlet oxygen.