  Mesospheric CO2 clouds are one of two types of carbon dioxide clouds known on
Mars. We present observations of mesospheric CO2 clouds made by Atmospheric
Chemistry Suit (ACS) onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO).
We analyzed 1663 solar occultation sessions of Thermal InfraRed (TIRVIM) and
Middle InfraRed (MIR) channels of ACS covering more than two Martian years that
contain spectra of 2.7 {\mu}m carbon dioxide ice absorption band. That allowed
us to unambiguously discriminate carbon dioxide ice aerosols from mineral dust
and water ice aerosols, not relying on the information of atmospheric thermal
conditions. CO2 clouds were detected in eleven solar occultation observations
at altitudes from 39 km to 90 km. In five cases, there were two or three layers
of CO2 clouds that were vertically separated by 5-15 km gaps. Effective radius
of CO2 aerosol particles is in the range of 0.1-2.2 {\mu}m. Spectra produced by
the smallest particles indicate a need for a better resolved CO2 ice refractive
index. Nadir optical depth of CO2 clouds is in the range 5*10^{-4}-4*10^{-2} at
both 2.7 {\mu}m and 0.8 {\mu}m. Asymmetrical diurnal distribution of detections
observed by ACS is potentially due to local time variations of temperature
induced by thermal tides. Two out of five cases of carbon dioxide cloud
detections made by the TIRVIM instrument reveal the simultaneous presence of
CO2 ice and H2O ice aerosols. Temperature profiles measured by the Near
InfraRed (NIR) channel of ACS are used to calculate CO2 saturation ratio S at
locations of carbon dioxide clouds. Supersaturation S > 1 is detected in only 5
out of 19 cases of CO2 cloud layers; extremely low values of S < 0.1 are found
in 9 out of 19 cases.
