ABSTRACT

Reactive oxygen species (ROS) are photochemically generated in sunlit natural water and are involved in degradation of organic matter, redox reactions, and biological processes. Hydroxyl radicals (·OH), nitric oxide radicals (NO·), and singlet oxygen (¹O₂) are some of the dominant ROS in natural water. In this study, these three ROS were measured in samples collected from nine rivers across 65 stations along the west to east axis of Japan. Quantification of ·OH, NO·, and ¹O₂ was performed by High-Performance Liquid Chromatography using benzene, 4, 5-diaminofluorescein-2, and furfuryl alcohol as chemical probes, respectively. The absorption coefficient at 300 nm (a₃₀₀, m^-1), which ranged from 2.44 to 36.2 m^-1, was used to investigate the chromophoric dissolved organic matter (CDOM) properties of the rivers. The photoformation rate ranges were (13.9-944) × 10^-12 M s^-1 for ·OH, (2.76-2610) × 10^-12 M s^-1 for NO·, and (9.48-133) × 10^-9 M s^-1 for ¹O₂. The steady-state concentration ranges were (1.53-16) × 10^-16 M for ·OH, (10.2-1520) × 10^-12 M for NO·, and (3.79-53.4) × 10^-14 M for ¹O₂. The results showed that nitrite was a major source for both ·OH and NO·, and CDOM was a major source for ¹O₂ across all the rivers. According to significant relationships with these sources, models were generated to predict the formation rates of the ROS (in M s^-1) from known concentrations of source compounds using the equations R·OH (10^-12) = 19.2 [NO₂^-1]-μM + 36.9, RNO· (10^-12) = 41.4 [NO₂^-1]-μM + 44, and R_¹O2)(10^-9) = 3.52 (a₃₀₀)-m^-1 + 1.61. Dissolved organic matter, escape to the atmosphere, and water molecules were the major sinks for river ·OH, NO·, and ¹O₂, respectively. A general scavenging rate constant of ·OH as a function of the dissolved organic carbon concentration was obtained [k_C,OH = [(7.5 ± 6.8) × 10⁸ L (mol C)^-1 s^-1]. These models will allow for easy prediction of ROS concentrations on a large-scale.