Lifetimes of Triplet Dissolved Natural Organic Matter (DOM) and the Effect of NaBH4 Reduction on Singlet Oxygen Quantum Yields: Implications for DOM Photophysics

The natural lifetimes of triplet dissolved organic matter ((3)DOM) were determined by an O(2) saturation kinetics study of singlet oxygen quantum yields (Φ(1O2)) in buffered D(2)O. At least two distinct (3)DOM pools are present, and the observed lifetime range (∼20 to 80 μs) leads to a dependence of Φ(1O2) on O(2) concentrations between 29 and 290 μM. Thus, steady-state (1)O(2) concentrations will depend on [O(2)] in natural waters. The lifetimes are essentially identical for DOM samples of different origins and do not vary with excitation wavelength. However, Φ(1O2) varies greatly between samples and decreases with excitation wavelength. These data strongly suggest that (3)DOM quantum yields decrease with excitation wavelength, which gives rise to the Φ(1O2) variation. Borohydride reduction of several samples in both D(2)O and H(2)O lowers the absorbance and (1)O(2) production rates, but it does not alter Φ(1O2). This is consistent with a model in which (1)O(2) sensitizing chromophores are borohydride reducible groups in DOM, such as aromatic ketones. Interpreted in the framework of a charge transfer (CT) model for DOM optical properties, the collective data suggest a model in which electron acceptor moieties are important (1)O(2) sensitizers and where CT interactions of these moieties disrupt their ability to produce (1)O(2).