Measurements of hydrogen isotope ratios in atmospheric volatile organic compounds

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Measurements of stable isotope ratios in atmospheric volatile organic compounds (VOC) at ambient concentrations allow investigating sources, photochemical history, residence times, and atmospheric budgets of these compounds. However, up to now all studies concerning ambient VOC concentrated on carbon isotope ratios. In principle, the investigation of isotope ratios of other elements can further improve our understanding of atmospheric processes. Measurements of stable isotope ratios of hysdrogen would be especially promising. Hydrogen isotope effects will be more pronounced than carbon isotope effects because the mass ratio of hydrogen isotopes is larger than that of carbon isotopes. We propose to measure stable hydrogen isotope ratios of volatile organic compounds in the atmosphere using a gas chromatograph pyrolysis isotope ratio mass spectrometer (GC-P-IRMS). We developed a method for measurements of hydrogen isotope ratios of atmospheric VOC based on a modification of existing methods for the measurement of stable carbon isotope ratios in atmospheric VOC. To meet the necessary reproducibility and detection limits, high volume air samples are needed (depending on the VOC concentrations up to 200 L) to supply a sufficient amount of VOC. Therefore, we modified the existing preconcentration system accordingly. Meanwhile, we thoroughly characterised the system. We could show that the system is well suited for measurements of changes in hydrogen isotope ratios in ambient VOC due to chemical and physical processes in the atmosphere. First measurements of VOC in ambient air yielded promising results. Based on the hitherto experience we plan to improve the preconcentration system to further reduce water and carbon dioxide from the air samples, and test new adsorbents to improve the preconcentration and increase the spectrum of measurable compounds. In parallel, missing kinetic isotope effects will be measured as well as isotope ratios in VOC at the emission sources. These investigations will be followed by a longer-term measurement to investigate diurnal and seasonal variations. Additionally, new tools to interpret and apply the data will be developed. The proposed method is a sensitive tool to identify the origin of VOC, to study their atmospheric processing, to differentiate between impacts of mixing and chemical processes on their distribution as well as the residence times of the compounds. To our knowledge no measurements of stable hydrogen isotope ratios in atmospheric VOC are reported in the literature up to now. The new type of measurements will add another important piece of information to better understand chemical and physical processes in the atmosphere.