Hemispherical distribution of CO above the Venus’ clouds by ground-based 2.3 μm spectroscopy

The abundance of carbon monoxide in the Venus’ dayside atmosphere above the clouds was measured by ground-based 2.3 μm spectroscopy for 4 days. The hemispherical distributions found show no significant latitudinal or longitudinal structure. The disc-averaged mixing ratio of 58 ± 17 ppm found at a representative height of 62–67 km is consistent with previous measurements. Such a flat distribution of CO abundance above the clouds seems to be controlled by an efficient horizontal eddy diffusion with a time scale of 30 days or shorter although the CO distribution below the clouds seems to be controlled by the meridional circulation. The pole-ward wind speed of the meridional circulation above the clouds is estimated to be 0.2 m s−1 or less based on the difference between the CO mixing ratios above and below the clouds.