Photolysis of Fe (III)-hydroxy complexes as sources of OH radicals in clouds, fog and rain

Photolysis of the monohydroxy complex of Fe(III), Fe(OH) 2+ , has been proposed as a major source of OH radicals in rain. It is also a significant source of OH radicals in clouds and fog, and probably in some acidic surface waters. Fe(OH) 2+ is the dominant monomeric Fe(III)-hydroxy complex between pH 2.5 and 5, based on currently available equilibrium constants for Fe(III)-hydroxy complexes. Quantum efficiencies for the photolysis of Fe(OH) 2+ are 0.04±0.04 at 313 nm and 0.017±0.003 at 360 nm (293 K, ionic strength = 0.03 M). The measured rate constant for midday June sunlight photolysis of Fe(OH) 2+ is 6.3 × 10 −4 s −1 (half life = 18 min). Model calculations based on measured quantum yields and absorption spectra are in satisfactory agreement with measured sunlight photolysis rates of monomeric Fe(III).