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| Titel |
Observations of molecular hydrogen (H2) mixing ratio and stable isotopic composition at the Cabauw tall tower; very depleted source signature suggests microbial H2 production in Dutch pasture soil. |
| VerfasserIn |
Anneke Batenburg, Elena Popa, Alex Vermeulen, Pim van den Bulk, Piet Jongejan, Rebecca Fisher, Dave Lowry, Euan Nisbet, Thomas Röckmann |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250144650
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| Publikation (Nr.) |
 EGU/EGU2017-8503.pdf |
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| Zusammenfassung |
| Molecular hydrogen (H2), though not toxic or a greenhouse gas itself, may influence air
quality and climate indirectly by affecting the atmosphere’s oxidative capacity. So as
increased use of hydrogen fuel is expected, a better understanding of the global, regional and
local atmospheric H2 cycles is needed. Studying the stable isotopic composition of H2
(δD(H2)) is a promising way to achieve this. Since the start of this century, the
isotope effects in H2 source and sink processes have been estimated, δD(H2) has
been incorporated into chemical transport models, and larger sets of environmental
observations of δD(H2) have appeared. The latter, however, were mostly obtained from
samples collected in remote regions of the atmosphere, which is not sufficient to fully
characterize the H2 cycle or to assess the possible environmental effects of H2 leakage in
urbanized regions. To address this gap, flask samples were collected at the Cabauw tall
tower at the CESAR site in the Netherlands. The air was sampled from inlets at 20,
60, 120, and 200 meter altitude for the analysis of H2 mixing ratio (χ(H2)) and
δD(H2). More than 250 samples were collected and analysed over a period of four
years.
The H2 mixing ratios in the samples show frequent excursions to high values above the
background. Previously published continuous χ(H2) observations at Cabauw and other
(sub)urban sites showed a similar pattern. With the isotope observations, we can now see
that these high χ(H2) excursions are accompanied by very low δD(H2) values;
probably at least partly a result of anthropogenic emissions of deuterium(D)-depleted
H2.
However, with a simple “Keeling plot” analysis, we obtained an apparent source
signature (-515 ± 26 ‰) that was much below the range of published values for H2
emissions from the combustion of fossil fuels. Since the result of the fit depended
markedly on the quality selection of the samples that were included, we applied a
bootstrap method to this fit to obtain a realistic picture of the uncertainty of the result.
This showed a wide distribution with more than 99 % of the values below -400 ‰,
suggesting that the H2 cycle at Cabauw is under the influence of a source mix that is
much more D-depleted than currently accepted values for fossil fuel combustion.
Since microbial production of very D-depleted H2 has been observed previously
at Cabauw, we consider it likely that this contributes to the low apparent source
signature.
A comparison of the samples from different sampling heights shows that there is a
significant shift to lower δD(H2) values at the lower sampling levels. This shows that the
uptake of H2 by the soil, which preferentially removes “light” H2, is relatively weak at the
site. It also points again to local to regional microbial production of H2, and possibly to
differences between national vehicle fleets. |
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