Abstract
With average mixing ratios (χ) around 550 ppb (nmole/mole), molecular hydrogen (H2) is the most abundant reduced gas in our atmosphere after methane (CH4), but considerably less studied. H2 is also a promising energy carrier that might replace fossil fuels in vehicles with great sustainability advantages, but there may be
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environmental side effects. Large-scale leakage of H2 into the atmosphere might affect the atmosphere’s oxidative capacity and stratospheric ozone chemistry. To assess these risks, a better understanding of the atmospheric H2 cycle is needed. Stable isotopic composition measurements can be used to constrain the source and sink terms in the budgets of atmospheric trace gases, as the different processes affect the stable isotopic composition of the gases in different ways. For H2, the effects are particularly large, due to the large relative mass difference between the isotopes (H and D). The largest source, hydrocarbon oxidation, yields D-enriched H2, whereas the smaller combustion-related sources and the minor microbial sources yield D-depleted and extremely D-depleted H2, respectively. Both sink processes, uptake in soils and reaction with hydroxyl radicals (OH), have a D-enriching effect, but the effect is much stronger for OH. Despite its usefulness, few environmental observations of H2 isotopic composition (δD(H2)) are available. We present three new χ(H2) and δD(H2) datasets to fill this gap. First, we present one- to five-year long time series from six globally distributed, predominantly background stations. As expected, average χ(H2) and δD(H2) values were larger in the southern hemisphere (SH) than in the northern hemisphere (NH). The minimum in δD(H2) was found at the NH midlatitude stations, likely a result of fossil fuel combustion. At the three NH coastal and island stations, seasonal δD(H2)-cycles were observed, which were five to six months out-of-phase with the χ(H2)-cycles. No δD(H2)-cycles were observed at the other sites. For the three coastal/island NH stations, a tentative analysis was made of the relative contribution of the two sink processes. This indicated that the relative contribution of soil uptake increases with latitude. In the next chapter, δD(H2) data are presented from samples collected by the CARIBIC passenger aircraft. This commercial aircraft flies in the upper troposphere (UT) but also regularly crosses into the lowermost stratosphere (LMS). In the LMS, tight correlations are found between δD(H2) and χ(CH4). This correlation has applications in global models of δD(H2). UT samples collected over India during the summer monsoon show a decrease in δD(H2) that is correlated with a CH4 increase, possibly indicating a previously unknown microbial H2-source. Lastly, we present a three-year long time series from the Cabauw tall tower in the Netherlands (200 m), which shows excursions to high χ(H2) and low δD(H2) values, especially in winter. These indicate that the local H2-cycle is under heavy anthropogenic influence, which is confirmed by an analysis of the apparent source signature. In addition, several height profiles (20, 60, 120 and 200 m) were measured. These show that the local soil uptake of H2 is weak
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