Optical properties of dissolved organic matter in Japanese rivers and contributions to photoformation of reactive oxygen species.

The optical properties of dissolved organic matter (DOM) from five rivers (Kokubu, Kurose, Ohta, Yamato, and Yodo) in Japan were investigated and contributions of DOM to photoformation of three reactive oxygen species (ROS) (hydroxyl radicals (OH), nitric oxide radicals (NO), and singlet oxygen (1O2)) were assessed. The lowest and highest mean dissolved organic carbon concentrations were for the Ohta River (0.95 (mg C) L-1) and Yamato River (2.85 (mg C) L-1), respectively, and the concentrations correlated with some optical parameters. Absorption ratios (e.g., the E2:E3 and A280/A350 ratios) and the spectral slope S275-295 indicated that DOM from the Yodo and Kokubu rivers had the lowest and highest molecular weights, respectively. PARAFAC models and DOM excitation-emission matrices were used to assess the sources and fates of DOM in the rivers. The PARAFAC model indicated that the main types of fluorescent DOM in the rivers were terrestrial humic-like (TH-L) and tryptophan-like (TP-L) substances. The Kokubu River contained other compounds such as fluorescent whitening agents, autochthonous humic-like substances, and extracellular polymeric substances. Statistically significant relationships between the dissolved organic carbon and TH-L, TP-L, and extracellular polymeric substance concentrations suggested that TH-L, TP-L, and extracellular polymeric substances are important contributors to total DOM in the rivers. TH-L and TP-L substances strongly contribute to ROS photoformation, but TH-L substances play roles in both ROS generation and scavenging. Comprehensive models for estimating the photoformation rates of different ROS (in M s-1) were established by integrating the contributions of the relevant major and minor sources. Examples are ROH (10-12) = 21.0 [NO2-]_μM + 0.460 [TH-L]_QSU + 10.9, RNO (10-12) = 67.9 [NO2-]_μM + 35.2 [a300]_m-1 - 2.51 [TH-L]_QSU - 0.765 [TP-L]_QSU - 8.14, and R1O2 (10-9) = 3.81 [a300]_m-1 - 0.101 [TP-L]_QSU + 11.1.