Investigating the yield of H<sub>2</sub>O and H<sub>2</sub> from methane oxidation in the stratosphere

Abstract

Abstract. An important driver of climate change is stratospheric water vapour (SWV), which in turn is influenced by the oxidation of atmospheric methane (CH 4 ). In order to parameterize the production of water vapour (H 2 O) from CH 4 oxidation, it is often assumed that the oxidation of one CH 4 molecule yields exactly two molecules of H 2 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- 5Climate model (CCM), namely the ECHAM/MESSy Atmospheric Chemistry (EMAC) model, and present three approaches to investigate the yield of H 2 O and hydrogen gas (H 2 ) from CH 4 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 CH 4 , H 2 O and H 2 , by being able to distinguish hydrogen atoms stemming from CH 4 and other sources. 10We apply three approaches, which all agree that assuming a yield of 2 overestimates the production of H 2 O 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 H 2 O increases, making it a crucial factor in the parameterization of the yield of H 2 O from CH 4 oxidation. An additional sensitivity study with the Chemistry As A Boxmodel Application (CAABA) shows