Photochemical Modeling of the Venus Middle Atmosphere

Details Photochemical Modeling of the Venus Middle Atmosphere Photochemical Modeling of the Venus Middle Atmosphere

Jul 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997dps....29.3504m&link_type=abstract

American Astronomical Society, DPS meeting #29, #35.04; Bulletin of the American Astronomical Society, Vol. 29, p.1043

Astronomy and Astrophysics

Astronomy

Scientific paper

The primary photochemical cycle of the Venus middle atmosphere is the photolysis of CO_2 to form CO and oxygen atoms on the dayside, and the re-formation of CO_2 from CO and oxygen via catalytic cycles. Previous modeling used ClO_x [Krasnopolsky & Parshev 1983, Yung & DeMore 1982], SO_x [Winick & Stewart 1980], and HO_x [Sze & McElroy 1975] radicals to catalyze the re-formation of CO_2. These models qualitatively explained the stability of Venus' CO_2 atmosphere, but, despite the powerful catalytic cycles introduced, none could quantitatively explain either the low column abundance of molecular oxygen (two-sigma upper limit of ~ 0.3 - 1.0 x 10(18) molecules cm(-2) depending on the assumed altitude for optical depth unity, Trauger & Lunine 1983) or the intense nightside airglow in the O_2 ((1) Delta ) band. We have developed a revised one-dimensional, steady-state model based on the latest kinetic and photoabsorption data and observations of the abundances of HCl, SO_2, SO, and H_2O. The vertical eddy diffusion profile and the abundance of SO_2 at the lower boundary (58-km altitude) were adjusted to simultaneously reproduce (within the stated error bars and temporal/spatial variability) the retrieved SO profile [Na et al. 1994], the retrieved SO_2 abundance and scale height at the cloud tops [Na et al. 1994], and the retrieved CO profile [Clancy & Muhleman 1991]. Using only gas-phase chemistry, the predicted column abundance of molecular oxygen (above 58-km altitude) has been reduced to ~ 3 x 10(18) molecules cm(-2) by adjusting the rates for selected reactions within their one-sigma uncertainties. Although still larger than the observed upper limit, this column abundance is a factor of 10 smaller than would have been predicted using the Yung & DeMore 1982 model with the currently accepted abundance for HCl (0.4 ppm, Connes et al. 1967, Pollack et al. 1993). Our preliminary results affirm the importance of the ClO_x catalytic cycles in the chemistry of the Venus middle atmosphere but imply some additional catalytic process, such as heterogeneous chemistry or BrO_x chemistry, may also be important.

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