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<p>An important driver of climate change is stratospheric water vapor (SWV), which in turn is influenced by the oxidation of atmospheric methane (CH₄). In order to parameterize the production of water vapor (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. </p><p> 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 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 CH₄, H₂O and H₂, by being able to distinguish hydrogen atoms produced by CH₄ from H₂ from other sources.</p><p>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&ndash;1.7). Additionally, transport and subsequent photochemical processing of longer-lived intermediates (mostly H₂) raise the local yield values in the upper stratosphere and lower mesosphere above 2 (maximum&thinsp; &gt; &thinsp;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°&thinsp;S and 23°&thinsp;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.</p><p>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.</p>