All occultation sessions described in the 2.4 Section have been processed. Figure 8 shows a map of all cases of CO2 clouds detections from the TIRVIM and MIR solar occultation using the 2.7 μm band. CO2 ice clouds were detected in 4 out of 910 TIRVIM sessions (0.4 % occurrence) and in 8 out of 864 MIR sessions (0.9 % occurrence). Lower frequency of CO2 clouds detection from the TIRVIM data could be related to its much lower intrinsic SNR (before averaging). As was written earlier, both TIRVIM and MIR channels operated and detected CO2 clouds during the egress occultation #5401. Overall, we have 11 sessions with CO2 clouds detections out of 1663 combined ACS solar occultation observations (0.7 % occurrence). This value of occurrence is close to 1.0 % (4 out of 412) from SPICAV UV (Montmessin et al., 2006) and 0.6 % (2 out of 309) from IUVS (Jiang et al., 2019) observations. Liuzzi et al. (2021) reports 26 detections covering period LS = 340° of MY 34 to LS = 325° of MY 35. Though authors did not detail the total number of analyzed sessions, according to nomad.aeronomie.be site, there were 4575 NOMAD solar occultations in this period. The estimate of NOMAD occurrences is therefore 0.6 % and is close to our result.
Analysis of the diurnal distribution shows a significant asymmetry between the terminators: only three detections of the CO2 clouds were made at the evening one while eight happened at the morning one. One of the possible explanations is the local time variation of temperature induced by the thermal tides. Fedorova et al. (2023) studied the differences in temperature between the morning and the evening terminators at different altitudes, latitudes and seasons from the ACS NIR solar occultation data. Despite the complicated latitude-altitude dependence of the temperature difference, that was also observed by other instruments (e.g. Kleinböhl et al., 2013; Fan et al., 2022; Guerlet et al., 2022), temperature was generally higher at the evening terminator in the low and middle latitudes at altitudes 40-80 km, where the most of the detected CO2 clouds is observed.
CO2 clouds were detected from 39 km up to 90 km. While in principle solar occultations can sound altitudes close to the surface, it is impossible with our method to detect CO2 clouds at altitudes ≲30 km due to saturation of CO2 gaseous absorption (see Figure 2). In five out of eleven cases, there were two or three layers of CO2 clouds that were separated by aerosol-free gaps, with a total number of 19 CO2 ice layers. A typical gap width was 5 km, reaching 15 km in a single case. Altitude range of CO2 cloud detections is wider than 60−80 km from the TES observations (Clancy et al., 2007) and 55−75 km range from the CRISM data (Clancy et al., 2019), not as high as 80−105 km from the SPICAM UV measurements (Montmessin et al., 2006) and 80−100 km from the UIVS observations (Jiang et al., 2019), and almost identical to 40−95 km from the NOMAD solar occultations (Liuzzi et al., 2021). In addition, in 2 out of 26 NOMAD cases there were two CO2 ice layers separated by ~5 km gaps.
Values of retrieved effective radius of CO2 particles are in range 0.1−2.2 μm with a median value of 0.6 μm. We must note that 0.1 μm value falls on the edge of the parameter search space (0.1−20 μm). In this case, the quality of fit was reduced: the FWHM of the absorption peak from the model was equal to 2.5 cm-1 (both for reff = 0.01 μm and for reff = 0.1 μm with veff = 0.1) compared to 1.3 cm-1 from the MIR data. This fact indicates that the model poorly represents extinction cross section for very small CO2 ice particles due to a low spectral resolution of refractive index, and that the actual effective radius of the particle could be <0.1 μm. A high-resolution (0.1 cm-1) absorption spectra of solid-state CO2 ice gives 1.1 cm-1