The remarkable but varied photosensitized oxidation by atmospheric humic-like substances at the air-water interface

Humic-like substances (HULIS), a class of macromolecular photosensitizers, are major components of light-absorbing aerosols in the atmosphere. Due to the amphiphilic nature of HULIS, they tend to appear at the air-water interface of atmospheric water. However, there have been limited studies on the photosensitized chemistry of HULIS at the air-water interface owing to the lack of methods with high interface selectivity. Here, a unique field-induced droplet ionization mass spectrometry (FIDI-MS) technique was used to investigate the photosensitized chemistry of several samples at the air-water interface. These samples include commercially available humic acid and the PM_2.5 sample collected during Beijing’s severe winter haze in December 2016. Ultrahigh-resolution Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) was also applied to analyze the atomic compositions, the degree of unsaturation and the aromaticity of these samples. Both Type I and Type II photosensitized oxidation pathways were observed, which were postulated to be dictated by the packing density of the molecules at the air-water interface. These findings reveal that the photosensitization of HULIS at the air-water interface contributes greatly to the formation of atmospheric aerosols, further advancing our understanding of the important roles played by photosensitized chemistry in the atmosphere.