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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2018-170
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
05 Mar 2018
Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).
Investigating the yield of H2O and H2 from methane oxidation in the stratosphere
Franziska Frank1, Patrick Jöckel1, Sergey Gromov2,3, and Martin Dameris1 1Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
2Max-Planck-Institute for Chemistry, Air Chemistry Departement, Mainz, Germany
3Institute of Global Climate and Ecology Roshydromet & RAS (IGCE), Moscow, Russia
Abstract. An important driver of climate change is stratospheric water vapour (SWV), which in turn is influenced by the oxidation of atmospheric methane (CH4). In order to parameterize the production of water vapour (H2O) from CH4 oxidation, it is often assumed that the oxidation of one CH4 molecule yields exactly two molecules of H2O. However, this assumption is based on an early study, which also gives evidence, that this is not true at all altitudes.

In the current study we re-evaluate this assumption with a comprehensive systematic analysis using a state-of-the art Chemistry-Climate model (CCM), namely the ECHAM/MESSy Atmospheric Chemistry (EMAC) model, and present three approaches to investigate the yield of H2O and hydrogen gas (H2) from CH4 oxidation. We thereby make use of Module Efficiently Calculating the Chemistry of the Atmosphere (MECCA) in a box model and global model configuration. Furthermore, we use the kinetic chemistry tagging technique (MECCA-TAG) to investigate the chemical pathways between CH4, H2O and H2, by being able to distinguish hydrogen atoms stemming from CH4 and other sources.

We apply three approaches, which all agree that assuming a yield of 2 overestimates the production of H2O in the lower stratosphere (calculated as 1.5–1.7). Additionally, transport and subsequent photochemical processing of longer-lived intermediates raise the local yield values in the upper stratosphere and lower mesosphere above 2 (maximum > 2.2). In the middle and upper mesosphere, the influence of loss and recycling of H2O increases, making it a crucial factor in the parameterization of the yield of H2O from CH4 oxidation. An additional sensitivity study with the Chemistry As A Boxmodel Application (CAABA) shows a dependence of the yield on the hydroxyl radical (OH) abundance. No significant temperature dependence is found. We focus representatively on the tropical zone between 23° S–23° N, where seasonal variations are negligible. It is found in the global approach that presented results are mostly valid for mid latitudes as well.

Our conclusions question the use of a constant yield of H2O from CH4 oxidation in climate modeling and encourage to apply comprehensive parameterizations that follow the vertical profiles of the H2O yield derived here and take the chemical H2O loss into account.
Citation: Frank, F., Jöckel, P., Gromov, S., and Dameris, M.: Investigating the yield of H2O and H2 from methane oxidation in the stratosphere, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-170, in review, 2018.

Franziska Frank et al.
Franziska Frank et al.
Franziska Frank et al.

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Short summary
Previously, it was assumed that one molecule of methane produces two water molecules. Applying various modeling concepts, we find that this overestimates the production in some areas of the atmosphere and underestimate it in other. Additionally, the chemical loss of water influences the actual abundance of it, especially in the upper atmosphere. We question the assumption of a vertically constant chemical production of water vapor and rather encourage to apply a sophisticated vertical profile.
Previously, it was assumed that one molecule of methane produces two water molecules. Applying...
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