Isotopic Measurements of Water Vapour and Methane from the MIPAS Satellite Instrument

The isotopic composition of stratospheric water vapour depends on the sources of water vapour and on the temperature and precipitation history of the stratospheric air. Isotopic measurements of water vapour therefore have the potential to aid investigation of dehydration in the polar vortex, of stratospheric/tropospheric exchange, of upward transport in the tropics and of subsidence at polar latitudes. The isotopic composition of methane affects that of water vapour, since methane oxidation is an important source of water vapour in the stratosphere. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a high resolution limb sounder flying on the European Space Agency's Envisat satellite. MIPAS measures infrared atmospheric limb emission spectra from 685-2410cm^-1 over an altitude range of 6-68km. These spectra are acquired continuously during both day and night, providing a three--dimensional, almost global map of the atmospheric emission every 24 hours. After suitable ground processing, these spectra allow the retrieval of concentration profiles of numerous atmospheric trace gases. The MIPAS spectra contain enough information to distinguish minor isotopes of many of these gases, including water vapour (H_2¹⁸O, H_2¹⁷O and HDO). It is also possible to obtain measurements of CH₃D, giving the potential to investigate the deuterium budget of the stratosphere. Here we present global isotopic measurements of water vapour and methane from the MIPAS instrument, spanning the upper troposphere and lower stratosphere.