A Novel Framework for Molecular Characterization of Atmospheric Organic Aerosol Based on Collision Cross Section and Mass-to-Charge Ratio
Journal Article
Abstract. A new metric is introduced for representing the molecular signature of atmospheric organic aerosols, the collision cross section (Ω), a quantity that is related to the structure and geometry of molecules and is derived from ion mobility measurements. By combination with the mass-to-charge ratio (m/z), a two-dimensional Ω–m/z space is developed to facilitate the comprehensive investigation of the complex organic aerosol mixture. A unique distribution pattern of chemical classes, characterized by functional groups including amine, alcohol, carbonyl, carboxylic acid, ester, and organic sulfate, is developed on the 2-D Ω–m/z space. Species of the same chemical class, despite variations in the molecular structures, tend to situate as a narrow band on the space and follow a trend line. Reactions involving changes in functionalization and fragmentation can be represented by the directionalities along or across these trend lines, thus allowing for the interpretation of mechanisms associated with the formation and evolution of atmospheric organic aerosol. The characteristics of trend lines for a variety of functionalities that are commonly present in ambient aerosols can be predicted by the core model simulations, which provide a useful tool to identify the chemical class to which an unknown species belongs on the Ω–m/z space. Within the band produced by each chemical class on the space, molecular structural assignment can be achieved by utilizing collision induced dissociation as well as by comparing the measured collision cross sections in the context of those obtained via molecular dynamics simulations.;
