Sources, sinks, and mechanisms of hydroxyl radical (•OH) photoproduction and consumption in authentic acidic continental cloud waters from Whiteface Mountain, New York: The role of the Fe(r) (r = II, III) photochemical cycle

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Abstract
Hydroxyl radical (•OH) photoproduction in 25 authentic acidic (pH = 2.9–4.4) continental cloud waters from Whiteface Mountain, New York was quantified by phenol formed from the •OH‐mediated oxidation of benzene (1.2 mM) that was added as an •OH scavenger. Based on the effect of added bisulfite (HSO3−/HOSO2−), an HOOH sink, the •OH photoproduction in these samples was apportioned into two categories: HOOH‐dependent sources (dominant), and HOOH‐independent sources (minor). On average only a small percentage (median = 9.4%, mean±standard deviation = 16±12%) of the HOOH‐dependent •OH source is due to direct photolysis (313 nm) of HOOH. Nearly all of the HOOH‐dependent •OH source is accounted for by an iron(II)‐HOOH photo‐Fenton reaction mechanism (Fe(II) + HOOH → Fe(III) + •OH + OH−) that is initiated by photoreduction of Fe(III) to Fe(II) in the presence of HOOH. A photostationary state is established, involving rapid photolysis of Fe(III) to form Fe(II), and rapid reoxidation of Fe(II) to Fe(III). Consequently, a new term is introduced, Fe(r) (r = II, III), to represent the family of labile Fe(III) and Fe(II) species whose rapid photoredox cycling drives the Fenton production of •OH. The Fe(r) photochemical cycle, which drives the aqueous phase photoformation of •OH, is analogous to the classical NOx photochemical cycle, which drives the gas phase formation of O3 and thus •OH. Based on the cloud waters studied here, the iron(II)‐HOOH photo‐Fenton reaction is a significant source of •OH to acidic continental cloud waters in comparison to gas‐to‐drop partitioning processes. Filtering (0.5 μm Teflon) cloud water samples had little effect on the •OH photoformation kinetics. Measured lifetimes of aqueous •OH ranged from 2.4 to 10.6 μs in these cloud waters, and decreased with increasing concentration of dissolved organic carbon. In acidic atmospheric water drops, the principal aqueous sinks for •OH will be reactions with dissolved organic compounds, bisulfite, and Cl−. Given such short chemical reaction lifetimes, little of the aqueous phase photoformed •OH is likely to escape to the gas phase.