Determination of D/H in atmospheric H₂ at nano-molar quantities

Abstract

Molecular hydrogen is the second most abundant reduced gas in the Earth's atmosphere (after methane) with a mixing ratio of ~500 ppbv. Variations in its isotopic composition have the potential to constrain problems in atmospheric chemistry, pollution, and atmosphere-biosphere-lithosphere interactions, however, these variations are not well characterized largely because of the difficulties of established methods. Previous measurements involve liquification of air using mechanical pumps, oxidation of residual, non-condensible molecular hydrogen to water, reduction of that water over a metal catalyst, and conventional mass spectrometry of the evolved H_2 [1-4]. These methods require thousands of liters of air and involve significant corrections for blanks and fractionations associated with sample processing. Studies using this technique in association with investigations of bomb produced tritium established that atmospheric H2 is enriched in D relative to Standard Mean Ocean Water (SMOW) by ~60-180 ‰ and that urban and industrial pollution shifts this ratio to values significantly depleted in D relative to SMOW [1-4]. Gerst and Quay [5] refined these methods and found an average δD value for H_2 in unpolluted air from widely dispersed locations of 126±20 ‰ (n=36); they also suggest that δD differs between the northern and southern hemispheres and varies with time in any one location. However, the rate at which samples can be processed by even this refined method precludes highly resolved time-series or spatial coverage in a reasonable time frame.

Files

Additional details