Global scale observations of atmospheric molecular hydrogen and its stable isotopic composition (PhD thesis)
Creators: name: Batenburg, Anneke
Publisher: name: figshare
Year: 2014
Subjects: subject: Environmental Science
subject: Climate Science
subject: Atmospheric Sciences
subject: FOS: Earth and related environmental sciences
subject: FOS: Earth and related environmental sciences
subject: Environmental Chemistry

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