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

A new technique for determining the deltaD of atmospheric molecular hydrogen (H-2) 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 degreesC to condense the air and generate a headspace greatly enriched in H-2 The H-2-enriched headspace is collected and admitted into a high-vacuum extraction line, where the H-2 is separated from the headspace gases by combustion to water followed by cryogenic collection. The H-2-derived water is then reduced back to H-2 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 deltaD of atmospheric H-2 was measured on air samples collected between 46 degreesN and 72 degreesS during a research cruise in the Pacific Ocean during 1998 and at Cheeka Peak, Washington (48 degreesN), during 1997-1999 and Point Barrow, Alaska (71 degreesN), during 1996-1997. These measurements yield a global average FD of atmospheric H-2 Of +130+/-4 parts per thousand and average Northern and Southern Hemispheric deltaD values of +123+/-3 parts per thousand and +138+/-8 parts per thousand, respectively (versus Vienna standard mean ocean water).