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

Hydroxyl radical (center dot OH) is an indiscriminate oxidant that reacts at near-diffusion-controlled rates with organic carbon. Thus, while center dot OH is expected to be an important oxidant of dissolved organic matter (DOM) and other recalcitrant compounds, the role of cOH in the oxidation of these compounds in aquatic ecosystems is not well known due to the poorly constrained sources and sinks of center dot OH, especially in pristine (unpolluted) natural waters. We measured the rates of center dot OH formation and quenching across a range of surface waters in the Arctic varying in concentrations of expected sources and sinks of center dot OH. Photochemical formation of cOH 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 photoFenton reactions likely contributed to the observed center dot OH production, several lines of evidence suggest that DOM was the primary source and sink of photochemically produced center dot OH in pristine arctic surface waters. DOM from first-order or headwater streams was more efficient in producing center dot OH than what has previously been reported for DOM, and cOH formation decreased with increasing residence time of DOM in sunlit surface waters. Despite the ubiquitous formation of cOH in arctic surface waters observed in this study, photochemical cOH formation was estimated to contribute # 4% to the observed photooxidation of DOM; however, key uncertainties in this estimate must be addressed before ruling out the role of cOH in the oxidation of DOM in these waters.