Environmental Processing of Short-Chain Fatty Alcohols Induced by Photosensitized Chemistry of Brown Carbons

Fatty alcohols are known surfactants that are ubiquitous in the environment. They affect the physical, chemical, and optical properties of aerosols and air-water interfaces. However, little is known about their photochemistry. Brown carbon (BrC), as a photosensitizer, might initiate photochemical reactions of such nonphotoactive surfactants. Here, BrC was obtained by evaporating mixtures of methylglyoxal with ammonium sulfate or glycine (Gly). The photosensitization ability of BrC was evidenced through its reaction with 1-octanol under UV irradiation by means of attenuated total reflection-Fourier transform infrared spectrometry, which is a surface-sensitive technique. The condensed phase products were determined by gas chromatography-mass spectrometry. We found that a series of functionalized and unsaturated compounds were formed through photosensitized reaction, and more highly oxygenated compounds appeared after a long illumination time. This process contributes to the formation of reactive volatile and semi-volatile organic species and could further trigger secondary chemistry. The reaction is probably initiated by electronically excited BrC upon exposure to UV irradiation. A possible mechanism for the photosensitized degradation of fatty alcohols is proposed. We also observed that BrC can be easily photobleached and hence has a limited effect on the radiative forcing. However, BrC can play an important role in photochemistry on aerosols and air-water interfaces.