The deuterium content of atmospheric molecular hydrogen: Method and initial measurements

Abstract

A new technique for determining the δD of atmospheric molecular hydrogen (H2) at background mixing ratios is described in detail. An air sample is compressed into a high-pressure cylinder, after which the cylinder is cooled to -196°C to condense the air and generate a headspace greatly enriched in H2. The H2-enriched headspace is collected and admitted into a high-vacuum extraction line, where the H2 is separated from the headspace gases by combustion to water followed by cryogenic collection. The H2-derived water is then reduced back to H2 for measurement of its D/H content via isotope-ratio mass spectrometry. The primary limitation of the method is contamination during compressor collection and subsequent storage of air samples in high-pressure cylinders. The δD of atmospheric H2 was measured on air samples collected between 46° N and 72° S during a research cruise in the Pacific Ocean during 1998 and at Cheeka Peak, Washington (48° N), during 1997-1999 and Point Barrow, Alaska (71° N), during 1996-1997. These measurements yield a global average δD of atmospheric H2 of +130±4‰ and average Northern and Southern Hemispheric δD values of +123±3‰ and +138±8‰, respectively (versus Vienna standard mean ocean water). Copyright 2000 by the American Geophysical Union.