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F. P. Mills (Caltech), D. Crisp (JPL), M. A. Allen (Caltech, JPL), Y. L. Yung (Caltech)
The primary photochemical cycle of the Venus middle atmosphere is photolysis of CO2 to form CO and O on the day side and the re-formation of CO2 via catalytic cycles. Previous models (Krasnopolsky & Parshev [1983]; Yung & DeMore [1982]) qualitatively explained the stability of the atmosphere but could not quantitatively explain the low O2 column abundance (< 0.3 \times 1018 molec cm-2 [Trauger & Lunine 1983]) or the intense night side O2 (a1 \Delta) airglow [Crisp {\it{et al.\ }}1996]. Our one-dimensional, steady-state model (based on the latest laboratory data and observations) has been able to reproduce (within measurement uncertainty and temporal/spatial variability) the SO profile [Na {\it{et al.}} 1994], the SO2 abundance and scale height at the cloud top [Na {\it{et al.}} 1994], the CO profile [Clancy & Muhleman 1991], and the ``global average'' O2 (a1 \Delta) airglow [Crisp {\it{et al.\ }}1996] using only gas-phase chemistry by adjusting selected reaction rates within their one-sigma uncertainties. If the stability of ClCO is increased by its assessed uncertainty, then radiative transfer model calculations indicate the predicted ``global average'' O2 column abundance from the photochemical model is consistent with the observed 2 \sigma upper limit. Calculations indicate horizontal transport is important in understanding the distribution of oxygen in the Venus atmosphere and suggest simultaneous, spatially-resolved retrievals of CO mixing ratios and temperatures are needed.