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Investigating the yield of H2O and H2 from methane oxidation in the stratosphere.
- Source :
- Atmospheric Chemistry & Physics; 2018, Vol. 18 Issue 13, p9955-9973, 19p, 1 Diagram, 2 Charts, 13 Graphs
- Publication Year :
- 2018
-
Abstract
- An important driver of climate change is stratospheric water vapor (SWV), which in turn is influenced by the oxidation of atmospheric methane (CH<subscript>4</subscript>). In order to parameterize the production of water vapor (H<subscript>2</subscript>O) from CH<subscript>4</subscript> oxidation, it is often assumed that the oxidation of one CH<subscript>4</subscript> molecule yields exactly two molecules of H<subscript>2</subscript>O. 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 H<subscript>2</subscript>O and hydrogen gas (H<subscript>2</subscript>) from CH<subscript>4</subscript> oxidation. We thereby make use of the 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 CH<subscript>4</subscript>, H<subscript>2</subscript>O and H<subscript>2</subscript>, by being able to distinguish hydrogen atoms produced by CH<subscript>4</subscript> from H<subscript>2</subscript> from other sources. We apply three approaches, which all agree that assuming a yield of 2 overestimates the production of H<subscript>2</subscript>O in the lower stratosphere (calculated as 1.5-1.7). Additionally, transport and subsequent photochemical processing of longer-lived intermediates (mostly H<subscript>2</subscript>) 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 H<subscript>2</subscript>O increases, making it a crucial factor in the parameterization of the yield of H<subscript>2</subscript>O from CH<subscript>4</subscript> 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 and 23°N. It is found in the global approach that presented results are mostly valid for midlatitudes as well. During the polar night, the method is not applicable. Our conclusions question the use of a constant yield of H<subscript>2</subscript>O from CH<subscript>4</subscript> oxidation in climate modeling and encourage to apply comprehensive parameterizations that follow the vertical profiles of the H<subscript>2</subscript>O yield derived here and take the chemical H<subscript>2</subscript>O loss into account. [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 16807316
- Volume :
- 18
- Issue :
- 13
- Database :
- Complementary Index
- Journal :
- Atmospheric Chemistry & Physics
- Publication Type :
- Academic Journal
- Accession number :
- 130813465
- Full Text :
- https://doi.org/10.5194/acp-18-9955-2018