D/H ratios of atmospheric H_2 in urban air: Results using new methods for analysis of nano-molar H_2 samples

Abstract

We present the results of a study of the concentration and D/H ratio of molecular hydrogen from air in the Los Angeles Basin and adjacent San Gabriel Mountains. These data define a mixing relationship in dimensions of D/H ratio vs. 1/(H_2) which constrains the δD_(VSMOW) of unpolluted winter air in this region to be ca. +100 to +125 ‰ and that of urban H2 sources to be ca. −270 ‰. This study makes use of a new method for measuring the deuterium content of molecular hydrogen in small samples (∼100 to 500 cc) of air, which we describe in detail. The method consists of an off-line separation of H_2 followed by introduction to the mass spectrometer in a continuous flow of He. Off-line, all components of an atmospheric gas sample, with the exception of He, H_2, and Ne are condensed by exposure to a cold-trap held at 30 Kelvin. This separation is followed by cryo-transfer of non-condensable gases to a small volume molecular sieve finger, with assist from a mercury piston pump. At the mass spectrometer, the sample is put in line with a continuous flow of He where it is focused on to an additional column of molecular sieve before subsequent introduction into the ion source. Analyses of DH/H_2 ratio have accuracy and precision of ±4 to 7 per mil. Comparison of sample peak area to peak areas of standards of known size allows for determination of H_2 concentration with accuracy and precision of ∼±5%, relative. The method reduces sample size and processing time by several orders of magnitude compared to previous methods, allowing for sampling at proportionately higher spatial and temporal resolution.

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