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 vapor (SWV), which in turn is influenced by the oxidation of atmospheric methane (CH 4 ). In order to parameterize the production of water vapor (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 stateof-the-art chemistry-climate 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 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 4 , H 2 O and H 2 , by being able to distinguish hydrogen atoms produced by CH 4 from H 2 from other sources.We 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 (mostly H 2 ) 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 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 2 O from CH 4 oxidation in climate modeling and encourage to apply comprehensive parameterizations that follow the vertical profiles of the H 2 O yield derived here and take the chemical H 2 O loss into account.

Implication of strongly increased atmospheric methane concentrations for chemistry–climate connections

“…Oxidation of CH 4 in the stratosphere produces additional H 2 O and is therefore an important source for SWV (Hein et al, 2001;Rohs et al, 2006;Frank et al, 2018). The enhanced CH 4 mixing ratios in the stratosphere cause a steady increase in SWV with height in both sensitivity simulations as indicated by Fig.…”

“…In the sensitivity simulation with 2-fold (5-fold) CH 4 , lower CH 4 values of about 5 % (10 %) are found between 50 and 1 hPa compared to the corresponding n-folded reference. Identically prescribed SST in all three model simulations determines the forcing of atmospheric dynamics to a large extent and also constrains the stratosphere to a large part (see Garny, 2010). Therefore, modified atmospheric circulation patterns are unlikely the cause of these changes in stratospheric CH 4 mixing ratios.…”

“…A change in tropospheric CH 4 concentration affects the oxidizing capacity of the atmosphere, modifies ozone in the troposphere and influences the CH 4 lifetime itself (e.g. Saunois et al, 2016;Frank, 2018;Holmes, 2018). Additionally, it affects the stratosphere.…”

The community atmospheric chemistry box model CAABA/MECCA-4.0

“…Photodissociation and photoionization due to the absorption of solar X-ray, EUV (extreme ultraviolet), and UV radiation are calculated using fluxes from the SOLAR2000 empirical model (Tobiska et al, 2000), the GLOW model (Solomon et al, 1988), and data presented by Henke et al (1993) and Fennelly and Torr (1992). Relative partitioning between the possible products of the ionization process are based on the model of Strickland and Meier (1982) and Fuller-Rowell (1993).…”

Multifractal Detrended Cross-Correlation Analysis of Global Methane and Temperature

“…On the other hand, the reaction with the hydroxyl radical (OH) is considered to be the basic sink of atmospheric CH 4 , a process that is subject to interannual fluctuations [20]. Apart from destroying CH 4 , this reaction is one of the main sources of water vapor in the stratosphere [21]. Atmospheric CH 4 concentrations have been systematically monitored through a global network of surface air-sampling stations [22].…”