The total hydrogen budget of the equatorial upper stratosphere

Water vapor and methane data measured by the Halogen Occultation Experiment (HALOE), the Atmospheric Chemistry Experiment (ACE), and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) along with simulations from the NRL CHEM2D middle atmosphere model are used to study time series of H₂O and CH₄ in the equatorial upper stratosphere. Multi-year time series of equatorial upper stratospheric H₂O+2*CH₄ show temporal variations during periods of relative long-term stability in water vapor and methane entering the stratosphere. These variations, which are anti-correlated to CH₄, are quasi-biennial and seasonal in nature, and peak near 2 hPa with a magnitude of 3 % of the H₂O+2*CH₄ mixing ratio. We find that β, or the ratio of the change in water vapor to the change in methane, is consistently >2 over the entire equatorial upper stratosphere in HALOE, ACE and MIPAS data. β values calculated from the CHEM2D model are similar, but slightly smaller, and this model shows that the quantity H₂O+2*CH₄+H₂ is nearly conserved. We use this relationship to calculate molecular hydrogen mixing ratios from the observations. Multi-year average profiles of H₂O+2*CH₄ from HALOE, ACE, and MIPAS show an ~ 0.4 ppmv increase between 7 hPa and 1.5 hPa in the equatorial upper stratosphere, which we conclude must be offset by a decrease in molecular hydrogen, so that total hydrogen is nearly conserved. Collectively, these results illustrate the importance of the inclusion of molecular hydrogen in the hydrogen budget of the equatorial upper stratosphere.