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

摘要

Global distributions of the CO vmr (volume mixing ratio) in themesosphere and lower thermosphere (from 70 up to  ∼  140 km) have beenderived from high-resolution limb emission daytime MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) spectra inthe 4.3 µm region. This is the first time that the CO vmr hasbeen retrieved in the 120–140 km range. The data set spans from January2005 to March 2012. The retrieval of CO has been performed jointly withthe elevation pointing of the line of sight (LOS) by using a non-localthermodynamic equilibrium (non-LTE) retrieval scheme. The non-LTE modelincorporates the new vibrational–vibrational and vibrational–translationalcollisional rates recently derived from the MIPAS spectra by[Jurado-Navarro et al.(2015)]. It also takes advantage of simultaneous MIPASmeasurements of other atmospheric parameters (retrieved in previous steps),such as the kinetic temperature (derived up to  ∼  100 km from theCO 15 µm region of MIPAS spectra and from 100 up to 170 kmfrom the NO 5.3 µm emission of the same MIPAS spectra) and theO measurements (up to  ∼  100 km). The latter is very important forcalculations of the non-LTE populations because it strongly constrains theO() and O() concentrations below  ∼  100 km. The estimatedprecision of the retrieved CO vmr profiles varies with altitude rangingfrom  ∼  1 % below 90 km to 5 % around 120 km and larger than10 % above 130 km. There are some latitudinal and seasonal variations ofthe precision, which are mainly driven by the solar illumination conditions.The retrieved CO profiles have a vertical resolution of about 5–7 km below120 km and between 10 and 20 km at 120–140 km. We have shown that theinclusion of the LOS as joint fit parameter improves the retrieval ofCO, allowing for a clear discrimination between the information onCO concentration and the LOS and also leading to significantly smallersystematic errors. The retrieved CO has an improved accuracy because ofthe new rate coefficients recently derived from MIPAS and the simultaneousMIPAS measurements of other key atmospheric parameters (retrieved in previoussteps) needed for non-LTE modelling like kinetic temperature and Oconcentration. The major systematic error source is the uncertainty of thepressure/temperature profiles, inducing errors at midlatitude conditions ofup to 15 % above 100 km (20 % for polar summer) and of ∼  5 % around 80 km. The errors due to uncertainties in theO() and O() profiles are within 3–4 % in the 100–120 kmregion, and those due to uncertainties in the gain calibration and in thenear-infrared solar flux are within ∼  2 % at all altitudes. The retrieved CO shows the majorfeatures expected and predicted by general circulation models. In particular,its abrupt decline above 80–90 km and the seasonal change of thelatitudinal distribution, with higher CO abundances in polar summer from70 up to  ∼  95 km and lower CO vmr in the polar winter. Above ∼  95 km, CO is more abundant in the polar winter than at themidlatitudes and polar summer regions, caused by the reversal of the meancirculation in that altitude region. Also, the solstice seasonaldistribution, with a significant pole-to-pole CO gradient, lasts about2.5 months in each hemisphere, while the seasonal transition occurs quickly.