Soluble iron is a key reactant in the atmospheric aqueous phase, undergoing photochemical and redox reactions and forming complexes. Its reaction with dihydroxybenzenes drives the formation of light-absorbing products. This study systematically investigates the formation of colored organic compounds from catechol precursors catalyzed by iron, with a focus on the role of iron oxidation state, UV light, and the presence of hydrogen peroxide (H2O2) at acidic conditions (pH 3). In the presence of Fe(III) in the dark, light-absorbing, greyish-black, insoluble particles were formed. UV light reduced their formation, while H2O2 inhibited the formation completely. However, the presence of H2O2 led to a colored, strongly light-absorbing solution in both Fe(II) and Fe(III) experiments. Fe(II)-based experiments required UV light for the formation of those greyish-black particles. These findings emphasize the significant impact of soluble iron in forming light-absorbing compounds under varying conditions, highlighting its potential influence on the Earth’s radiation budget.
Lüchtrath et al. (Sun,) studied this question.