Prevalence of Isomers in the Atmosphere and Their Impacts on Interpreting Laboratory Data

Recent advances in field-deployable mass spectrometry of gas- and particle-phase organic compounds have provided unprecedented characterization of atmospheric mixtures, both in the field and in the lab. However, while organic carbon across the entire range of atmospheric properties has become measurable by current state-of-the-art tools, many of these instruments identify analytes only by elemental formula with little or no structural information. Isomers with the same elemental formula may differ not only in their sources and formation chemistry, but also in their physicochemical properties and instrument sensitivity. Consequently, it is possible the presence of isomers in ambient and laboratory samples is a major source of uncertainty in data interpretation due to the lack of structural information provided by many instruments. We capitalize on isomer-resolved data from gas and liquid chromatography, ion mobility spectroscopy, and chemically-explicit models to understand the prevalence of isomers in atmospheric data and their diversity in properties. We find that both laboratory and ambient data typically contain several (and sometimes up to 10) isomers of a given formula, with wide ranges in estimated characteristics. We demonstrate that in real-world laboratory oxidation of biogenic precursors, the variability in isomer distributions with time and across different precursors may impact the calibration and interpretation of current state-of-the-art mass spectrometric data.