Global distributions of CO₂ volume mixing ratio in the middle and upper atmosphere from MIPAS high resolution spectra

Global distributions of the CO2 vmr (volume mixing ratio) in the mesosphere and lower thermosphere (from 70 km up to 142 km) have been derived from high resolution mid-IR spectra. This is the first time that the CO2 vmr has been retrieved in the 120–140 km range. The CO2 vmrs have been retrieved using MIPAS daytime limb emission spectra from the 4.3 µm region in its upper atmosphere (UA) mode (data version v5r_CO2_622). The dataset spans from January 2005 until March 2012. The retrieval of CO2 has been performed jointly with the line of sight (LOS) by using a non-local thermodynamic equilibrium (non-LTE) retrieval scheme. The non-LTE model incorporates the accurate vibrational-vibrational and vibrational-translational collisional rates recently derived from the MIPAS spectra. It also takes advantage of simultaneous MIPAS measurements of other atmospheric parameters, as the kinetic temperature (up to ~100 km) from the CO2 15 µm region, the thermospheric temperature from the NO 5.3 µm emission, and the O3 measurements (up to ~100 km). The latter is very important for the calculations of the non-LTE populations because it strongly constrains the O(1D) concentration below ~100 km. The estimated precision of the retrieved CO2 vmr profiles varies with altitude ranging from ~1 % below 90 km, to 5% around 120 km and larger than 10 % above 130 km. There are some latitudinal and seasonal variations of the precision, which are mainly driven by the solar illumination conditions. The retrieved CO2 profiles have a vertical resolution of about 5–7 km below 120 km and between 10 and 20 km at 120–142 km. We have shown that the inclusion of the LOS as joint fit parameter improves the retrieval of CO2, allowing a clear discrimination between the information of CO2 concentration and the LOS and also leading to significantly smaller systematic errors. The retrieved CO2 has a much better accuracy than previous limb emission measurements, because of the highly accurate rate coefficients recently derived from MIPAS, and the simultaneous MIPAS measurements of other key atmospheric parameters needed for the non-LTE modeling like the kinetic temperature and the O3 concentration. The major systematic error source is the uncertainty of the pressure/temperature profiles, inducing errors of up to 15 % above 100 km, and of ~5% around 80 km at mid-latitude conditions. The errors due to uncertainties in the O(1D) and O(3P) profiles are within 3–4 % in the 100–120 km region, and those due to uncertainties in the gain calibration and in the near-IR solar flux are within ~2 % at all altitudes. The retrieved CO2 shows the major features expected and predicted by general circulation models. In particular, its abrupt decline above 80–90 km and the seasonal change of the latitudinal distribution, with higher CO2 abundances in polar summer from 70 km up to ~95 km and lower CO2 vmr in the polar winter. Above ~95 km, CO2 is more abundant in the polar winter than at mid-latitudes and polar summer regions, caused by the reversal of the mean circulation in that altitude region. Also, the solstice seasonal distribution, with a significant pole-to-pole CO2 gradient, lasts about 2.5 months in each hemisphere, while the seasonal transition occurs quickly.