Abstract
Abstract. A new metric is introduced for representing the molecular signature of atmospherically relevant organic compounds, 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 mixtures. 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 atmospheric transformation mechanisms of organic species. The characteristics of trend lines for a variety of functionalities that are commonly present in the atmosphere 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.
Highlights
Organic species in the atmosphere – their chemical transformation, mass transport, and phase transitions – are essential for the interaction and coevolution of life and climate (Pöschl and Shiraiwa, 2015)
We propose a new metric, collision cross section ( ), for characterizing organic species of atmospheric interest
The collision cross section of individual molecular ions is calculated from the ion mobility measurements using an ion mobility spectrometer
Summary
Organic species in the atmosphere – their chemical transformation, mass transport, and phase transitions – are essential for the interaction and coevolution of life and climate (Pöschl and Shiraiwa, 2015). The degree of oxidation has been combined with the volatility (expressed as the effective saturation concentration, C∗), forming a 2-D volatility basis set to describe the coupled aging and phase partitioning of organic aerosol (Donahue et al, 2012) These three spaces are designed to represent fundamental properties of the organic mixtures and provide insight into their chemical evolution in the atmosphere. Species of similar volatility or elemental composition can differ vastly in structures and functionalities One weakness of these frameworks is that they do not provide information on the organic components at molecular level. The resulting IMS-MS plot provides separation of molecules according to two different properties: geometry (as reflected by the collision cross section) and mass (as reflected by the mass-to-charge ratio) (Kanu et al, 2008). Measured collision cross sections are shown to be consistent with theoretically predicted values from the trajectory method (Mesleh et al, 1996; Shvartsburg and Jarrold, 1996) and are used to identify isomers that are separated from an isomeric mixture
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