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

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 (HSO 3 - /HOSO 2 - ), 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 NO x photochemical cycle, which drives the gas phase formation of O 3 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.

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