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Hydroxyl radical ( • OH) photoproduction in 25 authentic acidic ( p H = 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 m M ) 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.
Arakaki et al. (Sun,) studied this question.