A New Structural Classification Scheme for Dissolved Organic Sulfur in Urban Snow from North China

The chemical composition of dissolved organic sulfur in snow is important in understanding the sources and scavenging processes of atmospheric organic matter. Snow samples collected simultaneously from four megacities in North China were analyzed using ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry. The modified oxygen and redefined aromaticity index help in the interpretation of the possible structural information and evaluation of the aromaticity of sulfur-containing molecules. By extending these parameters, we provide a new structural classification for organic sulfur species in the atmosphere. With the new classification, the oxidized (O/S > 3) and less oxidized (O/S ≤ 3) sulfur-containing molecules can be easily distinguished. Typical known secondary organosulfates and sulfonates and anthropogenically derived anionic surfactants verified the validity of this new classification. The new classification was applied to the molecular characterization of dissolved organic sulfur in snow samples. More than one hundred (138–150) of the molecules with medium O/S ratios of 5–11 and a low to medium mass range of <500 Da are related with typical known secondary organosulfates and anthropogenically derived anionic surfactants. Our study provides new insights into the molecular compositions of organic sulfur species in ambient air, although their atmospheric behaviors between the snow–aerosol interfaces warrant further examination.