Microwave observations and modeling of O₃, H₂O, and HO₂ in the mesosphere

Abstract

Microwave (wave ∼1.2 mm) spectral line observations of HO 2 , H 2 18 O, and O 3 were obtained at the Kitt Peak National Radio Astronomy Observatory (NRAO) on April 10–12, 1992. The spectral bandwidth and resolution of the observed collisional line shapes are appropriate to retrieving profile abundances for O 3 , H 2 O, and HO 2 in the upper stratosphere and mesosphere (45‐ to 70‐km altitude) with 20–30% uncertainties. The derived profile for O 3 exhibits 20–30% larger mesospheric ozone abundances than indicated by the April 30°N average profiles from Solar Mesosphere Explorer (SME) and Stratosphere Aerosol Gas Experiment (SAGE) II and 60–80% larger than a photochemical model employing current photochemical rate constants and an H 2 O profile constrained by the microwave H 2 18 O observations. Surprisingly, the microwave measurement of HO 2 yields mesospheric HO 2 abundances roughly twice those predicted by the standard model photochemistry. As HO x (OH, HO 2 , H) is the primary agent for catalytic destruction of ozone in the mesosphere, the model underpredictions of both HO 2 and O 3 suggest a change in the model photochemistry that is distinct from errors in O x production and HO x loss rates. Increases in O x production and/or increases in HO x loss rates have been considered as the likely sources for the persistent data‐model disagreement in upper stratospheric/mesospheric ozone abundances. Based on the microwave measurements of O 3 and HO 2 , we propose changes to model rates for partitioning between OH and HO 2 .Specifically, a large (60–80%) decrease in the rate coefficient for the reaction O + HO 2 → OH + O 2 is shown to be uniquely capable of increasing model abundances for O 3 and HO 2 , in accordance with the microwave observations as well as the outstanding underprediction of upper stratospheric/mesospheric ozone by photochemical modeling.