The vertical and horizontal distribution of CO2 densities in the upper mesosphere and lower thermosphere as measured by CRISTA

Abstract

The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment measured the global distribution of CO2 4.3 μm infrared emissions in the mesosphere and lower thermosphere during two Space Shuttle missions in November 1994 and August 1997. The daytime radiances have been inverted to CO2 number densities in the 60–130 km range by using a nonlocal thermodynamic equilibrium model. A detailed sensitivity study of retrieved CO2 number densities was carried out. The O(1D) excitation mechanism and model parameters constitute the most important uncertainties of retrieved CO2, typically 10–20%. The inaccuracy due to uncertainties in other atmospheric parameters is usually less than 10%. The CO2 volume mixing ratio (VMR) deviates from being well mixed between 70 and 80 km, which is significantly lower than indicated by previous rocket‐borne mass spectrometer data and model calculations but is in good agreement with the data obtained by other 4.3 μm emission and absorption experiments. The global distribution of CRISTA‐2 CO2 density shows significant longitudinal and latitudinal structures. The zonal mean CO2 densities are increasing toward polar summer latitudes below 90 km and above 115 km. Between 90 and 115 km, the latitudinal gradient is reversed. At 100 km, the gradient is mostly pronounced, reaching up to 50% difference between low and high latitudes. These variations are compared with results obtained by the Thermosphere/Ionosphere/Mesosphere Electrodynamics General Circulation Model (TIME‐GCM), showing very good agreement for the latitudinal distribution. Below 110 km, this variation is mostly due to the change in total density rather than to the CO2 VMR.