Absorption of sunlight by chromophoric dissolved natural organic matter (CDOM) is environmentally significant because it controls photic zone depth and causes photochemistry that affects elemental cycling and contaminant fate. Both the optics (absorbance and fluorescence) and photochemistry of CDOM display unusual properties that cannot easily be ascribed to a superposition of individual chromophores. These include (i) broad, unstructured absorbance that decreases monotonically well into the visible and near IR, (ii) fluorescence emission spectra that all fall into a single envelope regardless of the excitation wavelength, and (iii) photobleaching and photochemical quantum yields that decrease monotonically with increasing wavelength. In contrast to a simple superposition model, these phenomena and others can be reasonably well explained by a physical model in which charge-transfer interactions between electron donating and accepting chromophores within the CDOM control the optical and photophysical properties. This review summarizes current understanding of the processes underlying CDOM photophysics and photochemistry as well as their physical basis.

Various aquatic dissolved organic matter (DOM) samples produce singlet oxygen (1O2) and hydrogen peroxide (H2O2) with quantum yields of 0.59 to 4.5% (1O2 at 365 nm) and 0.017 to 0.053% (H2O2, 300-400 nm integrated). The two species' yields have opposite pH dependencies and strong, but opposite, correlations with the E2/E3 ratio (A254 divided by A365). Linear regressions allow prediction of both quantum yields from E2/E3 in natural water samples with errors ranging from -3% to 60%. Experimental evidence and kinetic calculations indicate that less than six percent of the H2O2 is produced by reaction between 1O2 and DOM. The inverse relationship between the 1O2 and H2O2 yields is thus best explained by a model in which precursors to these species are populated competitively. A model is presented, which proposes that important precursors to H2O2 may be either charge-transfer or triplet states of DOM.

Two aquatic fulvic acids and one soil humic acid were irradiated to examine the resulting changes in the redox and photochemical properties of the humic substances (HS), the relationship between these changes, and their relationship to changes in the optical properties. For all HS, irradiation caused photooxidation, as shown by decreasing electron donating capacities. Photooxidation was accompanied by decreases in specific UV absorbance and increases in the E2/E3 ratio (254 nm absorbance divided by that at 365 nm). In contrast, photooxidation had little effect on the samples' electron accepting capacities. The coupled changes in optical and redox properties for the different HS suggest that phenols are an important determinant of aquatic HS optical properties and that quinones may play a more important role in soil HS. Apparent quantum yields of H2O2, ·OH, and triplet HS decreased with photooxidation, thus demonstrating selective destruction of HS photosensitizing chromophores. In contrast, singlet oxygen ((1)O2) quantum yields increased, which is ascribed to either decreased (1)O2 quenching within the HS microenvironment or the presence of a pool of photostable sensitizers. The photochemical properties show clear trends with SUVA and E2/E3, but the trends differ substantially between aquatic and soil HS. Importantly, photooxidation produces a relationship between the (1)O2 quantum yield and E2/E3 that differs distinctly from that observed with untreated HS. This finding suggests that there may be watershed-specific correlations between HS chemical and optical properties that reflect the dominant processes controlling the HS character.