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)-O-.) 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)-O-.-mediated oxidation of benzene (1.2 mM) that was added as an (OH)-O-. scavenger, Based on the effect of added bisulfite (HSO3-/HOSO2-), an HOOH sink, the (OH)-O-. 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)-O-. source is due to direct photolysis (313 nm) of HOOH. Nearly all of the HOOH-dependent (OH)-O-. source is accounted for by an iron(II)-HOOH photo-Fenton reaction mechanism (Fe(II) + HOOH --> Fe(III) + (OH)-O-. + 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(LII) 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)-O-.. The Fe(r) photochemical cycle, which drives the aqueous phase photoformation of (OH)-O-., is analogous to the classical NOx photochemical cycle, which drives the gas phase formation of O-3 and thus (OH)-O-.. Based on the cloud waters studied here, the iron(II)HOOH photo-Fenton reaction is a significant source of (OH)-O-. to acidic continental cloud waters in comparison to gas-to-drop partitioning processes. Filtering (0.5 mu m Teflon) cloud water samples had little effect on the (OH)-O-. photoformation kinetics. Measured lifetimes of aqueous (OH)-O-. ranged from 2.4 to 10.6 mu s in these cloud waters, and decreased with increasing concentration of dissolved organic carbon. In acidic atmospheric water drops, the principal aqueous sinks for (OH)-O-. will be reactions with dissolved organic compounds, bisulfite, and Cl-. Given such short chemical reaction lifetimes, little of the aqueous phase photoformed (OH)-O-. is likely to escape to the gas phase.