Contribution of fulvic acid to the photochemical formation of Fe(II) in acidic Suwannee River fulvic acid solutions

We investigated the contribution of fulvic acid to the photoformation of Fe(II) using aqueous Suwannee River fulvic acid (SRFA) as a surrogate for the humic-like substances (HULIS) found in atmospheric condensed phases. The effects of pH (3.2, 4.1, and 5.0) and wavelength (313, 334, 366, and 405 nm) on Fe(II) photoformation were studied using monochromatic radiation at 20 °C. We calculated the wavelength-dependent Fe(II) photoformation efficiency values (“E-value”), defined here as a weighted sum of the product of the quantum yield and molar absorptivity of each Fe(II)-forming chemical species, and found that the E-values of acidic SRFA solutions were similar to those of Fe(OH)2+. In addition, a comparison showed that the acidic SRFA solutions did not form Fe(II) fast enough to account for the observed Fe(II) formation efficiencies of the aqueous extracts of authentic aerosol samples. It was observed that 17–73% of Fe(III) had been reduced to Fe(II) in the dark in acidic SRFA solutions with added Fe(III) ranging from 0.5 to 10 μM. The results of this study suggest that HULIS is unlikely to be the major reducing ligand in the process of photochemical formation of Fe(II) in acidic atmospheric drops. However, HULIS could reduce Fe(III) to Fe(II) in the dark, which in turn, could be important for night-time OH formation via the reaction between Fe(II) and H2O2 (the Fenton reaction).