  We have obtained spatially resolved near-infrared spectroscopy of the Venus
nightside on 15 nights over three observing seasons. We use the depth of the CO
absorption band at 2.3 microns to map the two-dimensional distribution of CO
across both hemispheres. Radiative transfer models are used to relate the
measured CO band depth to the volume mixing ratio of CO. The results confirm
previous investigations in showing a general trend of increased CO abundances
at around 60 degrees latitude north and south as compared with the equatorial
regions. Observations taken over a few nights generally show very similar CO
distributions, but significant changes are apparent over longer periods. In
past studies it has been assumed that the CO latitudinal variation occurs near
35 km altitude, at which K-band sensitivity to CO is greatest. By modeling the
detailed spectrum of the excess CO at high latitudes we show that it occurs at
altitudes around 45 km, much higher than has previously been assumed, and that
there cannot be significant contribution from levels of 36 km or lower. We
suggest that this is most likely due to downwelling of CO-rich gas from the
upper atmosphere at these latitudes, with the CO being removed by around 40 km
through chemical processes such as the reaction with sulfur trioxide.
