Rocket-borne ultraviolet spectrometer measurement of OH resonance fluorescence with a diffusive transport model for mesospheric photochemistry

Abstract

An upper limit on the column density of the hydroxyl radical in the earth's upper atmosphere was determined with a rocket-borne scanning spectrometer using resonance fluorescence techniques in the near ultraviolet. The (0–0) band of the A2Σ-X2Π transition at 3064 A was studied between 60 and 108 km under evening twilight conditions from Wallops Island, Virginia, on June 27, 1969. The solar zenith angle was 92°. Particular attention is given to the problem of determining the Rayleigh scattered background to allow an accurate determination of the column emission rate from the OH molecule. An emission rate factor calculation for OH is presented that considers the rotational structure of the molecule and the effect of absorption by OH in the solar atmosphere. By using this emission rate factor, the upper limit on the column density of OH is found to be 6×1012 cm−2 at 65 km, 3×1012 cm−2 at 75 km, and 7×1011 cm−2 at 85 km. The dissociation rate for water vapor was calculated by using a high-resolution analysis of the O2 absorption cross section. A significant amount of H2O dissociation was found to occur throughout the mesosphere down to the stratopause. A mesospheric oxygen-hydrogen model that includes diffusive transport of water vapor and the recalculated H2O dissociation rate is considered in order to interpret the experimental results. It is demonstrated that the upper limit on the hydroxyl column density requires the eddy diffusion coefficient to be less than 4×106 cm2/sec in the lower mesosphere. A model for the concentration of H, OH, HO2, and H2O in the mesosphere is presented that uses a water vapor mixing ratio of 5×10−6 at 50 km and an eddy diffusion coefficient of 1×10−6 cm2/sec.