Total hydrogen budget of the equatorial upper stratosphere

Abstract

Water vapor and methane mixing ratios 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 the hydrogen budget in the equatorial upper stratosphere. Multiyear 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 anticorrelated to CH4, are quasibiennial and seasonal in nature, and peak near 2 hPa with a magnitude of 3% of the H2O + 2*CH4 mixing ratio. We find that the ratio of the changes in water vapor to the changes in methane is consistently >2 over the entire equatorial upper stratosphere in HALOE, ACE, and MIPAS data. Ratios of the H2O to CH4 changes calculated from the CHEM2D model are similar, but slightly smaller, and the variations in H2O + 2*CH4 are balanced by nearly equivalent variations in molecular hydrogen. We use this relationship to infer molecular hydrogen mixing ratios from the observations which show that its mixing ratio decreases with altitude above 5 hPa. This net loss in molecular hydrogen drives additional water vapor production so that multiyear average profiles of H2O + 2*CH4 from HALOE, ACE, and MIPAS show an ∼0.4 ppmv increase between 5 hPa and 1.5 hPa, so that total hydrogen is conserved. Collectively, these results illustrate the importance of molecular hydrogen in the equatorial upper stratospheric hydrogen budget.