Characterization of Light-Absorbing Oligomers from Reactions of Phenolic Compounds and Fe(III)

Abstract

Phenolic compounds are common constituents of atmospheric aerosols. They form by pyrolysis of lignin and by biodegradation of plant material and are commonly found in biomass burning plumes, resuspended soil dust, and in anthropogenic secondary organic aerosols (SOA). In this study, we show that reactions of Fe(III), a major constituent of mineral dust, with several phenolic compounds (guaiacol, catechol, syringol, o- and p-cresol) that are common in atmospheric aerosols, result in the formation of water insoluble light-absorbing compounds and reduced Fe(II). The study was conducted under acidic conditions (pH = 1–2), relevant for areas impacted by biomass burning, anthropogenic emissions, and mineral dust. The reaction products have been characterized using a high-performance liquid chromatography coupled to photodiode array and high-resolution mass spectrometry detectors, UV–visible spectroscopy, X-ray photoelectron spectroscopy, and thermal gravimetric analysis. The major identified chromophores are oligomers of the reaction precursors that efficiently absorb light between 300 and 500 nm. The amounts of oligomers vary significantly between the systems studied. The highest amount was observed for guaiacol and catechol, and the least were detected in the syringol experiments, suggesting that the oligomerization proceeds through carbon–carbon coupling preferred at para- and ortho- positions, coupled to the reduction of Fe(III) to Fe(II). The results suggest that aqueous-phase radical reactions of phenolic compounds may be an efficient source of light-absorbing atmospheric organic compounds (brown carbon) that play important roles in Earth’s radiative forcing on global and regional scales and of quinones that can affect health.