Analysis of the southern hemispheric H2 Cycle using D/H measurements of H2 and CH4 in Cape Grim air archive

Abstract

Understanding the hydrogen cycle has gained additional importance because of the acknowledgement of the potential role of hydrogen as a major energy carrier in the future. We investigate the southern hemispheric H2 cycle using D/H stable isotope analyses of H2 and CH4 in 29 samples of air collected from 1992 and 2004 at the Australian Cape Grim baseline air monitoring station and stored in the unique air archive in Aspendale. The D/H ratios were determined using the fairly new technique of continuous-flow isotope ratio mass spectrometry. Quality control tests for the mixing ratios and isotope ratios of H2 in archived air confirm the absence of artifacts that could occur during the collection and subsequent storage. A harmonic-polynomial function was applied to isolate seasonal variation and long term trends during the period of observation. Mean seasonal variations of the D/H ratio of H2 reveal a similar in-phase variation as observed in the southern hemispheric free troposphere (CARIBIC flights). The long-term trends for the mixing ratios and the D/H ratios of H2 and CH4 show a remarkable feature of the SH H2 cycle, namely, the rate of increase of CH4 is about 3.8 times faster than that of H2, whereas the D/H ratio of CH4 increases about 1.8 times slower than that for H2. This paradox cannot be explained by the photochemical oxidation of CH4 and H2 in the southern hemisphere as the major process for these long-term trends. We suspect a strong external influences acting like source or sink for H2 in the form of inter-hemispheric exchange of air masses.