Evidence for dissolved organic matter as the primary source and sink of photochemically produced hydroxyl radical in arctic surface waters

Hydroxyl radical (OH) is an indiscriminate oxidant that reacts at near-diffusion-controlled rates with organic carbon. Thus, while OH is expected to be an important oxidant of dissolved organic matter (DOM) and other recalcitrant compounds, the role of OH in the oxidation of these compounds in aquatic ecosystems is not well known due to the poorly constrained sources and sinks of OH, especially in pristine (unpolluted) natural waters. We measured the rates of OH formation and quenching across a range of surface waters in the Arctic varying in concentrations of expected sources and sinks of OH. Photochemical formation of OH was observed in all waters tested, with rates of formation ranging from 2.6 ± 0.6 to 900 ± 100 × 10 ^-12 M s^-1. Steady-state concentrations ranged from 2 ± 1 to 290 ± 60 × 10^-17 M, and overlapped with previously reported values in surface waters. While iron-mediated photo-Fenton reactions likely contributed to the observed OH production, several lines of evidence suggest that DOM was the primary source and sink of photochemically produced OH in pristine arctic surface waters. DOM from first-order or headwater streams was more efficient in producing OH than what has previously been reported for DOM, and OH formation decreased with increasing residence time of DOM in sunlit surface waters. Despite the ubiquitous formation of OH in arctic surface waters observed in this study, photochemical OH formation was estimated to contribute ≤4% to the observed photo-oxidation of DOM; however, key uncertainties in this estimate must be addressed before ruling out the role of OH in the oxidation of DOM in these waters. This journal is