The role of molecular hydrogen and methane oxidation in the water vapour budget of the stratosphere

The detailed photochemistry of methane oxidation has been studied in a coupled chemical/dynamical model of the middle atmosphere. The photochemistry of formaldehyde plays an important role in determining the production of water vapour from methane oxidation. At high latitudes, the production and transport of molecular hydrogen is particularly important in determining the water vapour distribution. It is shown that the ratio of the methane vertical gradient to the water vapour vertical gradient at any particular latitude should not be expected to be precisely two, due both to photochemical and dynamical effects. Modelled H₂O profiles are compared with measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment at various latitudes. Molecular hydrogen is shown to be responsible for the formation of a secondary maximum displayed by the model water vapour profiles in high latitude summer, a feature also found in the LIMS data.