title: Evidence for dissolved organic matter as the primary source and sink of photochemically produced hydroxyl radical in arctic surface waters.
abstract: 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.
authors.affiliation: Institute of Biogeochemistry and Pollutant Dynamics, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland. kris.mcneill@env.ethz.ch.
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Page
authors.forename: J Robert
authors.initials: JR
authors.lastname: Logan
authors.forename: Rose M
authors.initials: RM
authors.lastname: Cory
authors.forename: Kristopher
authors.initials: K
authors.lastname: McNeill
mesh_terms: D001110:Arctic Regions; D004784:Environmental Monitoring; D005618:Fresh Water / Q000737:chemistry*; D006812:Humic Substances; D017665:Hydroxyl Radical / Q000032:analysis / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D055668:Photochemical Processes
chemical_list: D006812:Humic Substances; D017665:Hydroxyl Radical