  The Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter
(TGO) monitors the Martian atmosphere through different spectral intervals in
the infrared light. We present a retrieval algorithm tailored to the analysis
of spectra acquired in nadir geometry by TIRVIM, the thermal infrared channel
of ACS. Our algorithm simultaneously retrieves vertical profile of atmospheric
temperature up to 50 km, surface temperature, and integrated optical depth of
dust and water ice clouds. The specificity of the TIRVIM dataset lies in its
capacity to resolve the diurnal cycle over a 54 sol period. However, it is
uncertain to what extent can the desired atmospheric quantities be accurately
estimated at different times of day. Here we first present an Observing System
Simulation Experiment (OSSE). We produce synthetic observations at various
latitudes, seasons and local times and run our retrieval algorithm on these
synthetic data, to evaluate its robustness. Different sources of biases are
documented, in particular regarding aerosol retrievals. Atmospheric temperature
retrievals are found robust even when dust and/or water ice cloud opacities are
not well estimated in our OSSE. We then apply our algorithm to TIRVIM
observations in April-May, 2018 and perform a cross-validation of retrieved
atmospheric temperature and dust integrated opacity by comparisons with
thousands of co-located Mars Climate Sounder (MCS) retrievals. Most differences
between TIRVIM and MCS atmospheric temperatures can be attributed to
differences in vertical sensitivity. Daytime dust opacities agree well with
each other, while biases are found in nighttime dust opacity retrieved from
TIRVIM at this season.
