Solar and stellar occultation observations are usually interpreted in an assumption of a spherically symmetric atmosphere, which implies, for aerosol studies, a combination of attenuating layers, variable with altitude and uniform in latitude and longitude. We will call this extinction model ‘standard’ in this Section. Panel a of Figure 9 illustrates retrieved from the ingress occultation #14501 typical extinction profiles in the 2D geometry (we show only a half of the atmosphere)

with three CO2 cloud layers A, B and C located at different altitudes. However, the atmosphere is not necessarily symmetric and many models may be constructed to fit the experimental data.
One of the possible alternative extinction distributions is shown in panel c of Figure 10. It was constructed to satisfy the following conditions:
1) Reproduce the observed aerosol transmission profiles Tra at 3708 cm-1; 2) Have spherically uniform aerosol layers everywhere except for altitudes of the A cloud; 3) Don’t have the local maxima in extinction profiles that correspond to clouds B and C of standard model.

[Figure: Figure 10. Illustration of an alternative aerosol extinction model for the ingress occultation #14501. Panel a: A standard for solar occultation spherically symmetric model. Three CO2 cloud layers A, B and C (shown as orange frames) are located at different altitudes and are uniform in latitude and longitude. Panel b: vertical profiles of aerosol extinction for standard model (black), alternative model (red) and layers II (blue) and III (green) of the alternative model. Panel c: Alternative model. Three CO2 cloud layers I, II, and III (shown as orange frames) are located at the same altitudes, but different latitudes/longitudes.]

As a result, the alternative model has three CO2 cloud layers, marked I, II, and III, that belong to the same spherical shell (are at the same altitudes), but are located at different latitudes/longitudes. The aerosol extinction of layers A and I are equal (panel b of Figure 10). Inside the layers II and III, extinction is vertically not uniform and is equal to extinction of layers B and C at higher altitudes and 2-3 times smaller at lower altitudes. The nadir optical depths of layers II and III equal 2.1·10-3 and 1.3·10-3, and are only marginally higher than values of layers B and C (Table 2).
Demonstrated alternative model could potentially resolve the controversy between the detected CO2 clouds at lower altitudes and extremely low values of saturation that was discussed previously. Indeed, small horizontal variations of temperature (5-10 K) is enough to result in drop in saturation from ~5 to ≲1. However, it is only speculative and needs a priori data to justify the selection of the aerosol extinction model. Solar occultation measurements alone do not contain enough information to do so: the number of transmission measurements in a single profile is ℕ, while the number of sublayers is ~ ℕ2. Moreover, in five out of eleven occultations S was ≪ 1 for all CO2 cloud layers (e.g. #5401 in Figure 9), which requires another explanation.

Also, when constructing the alternative model, we worked only with a single extinction at 3708 cm-1 but not its spectral dependence. Considering the factors mentioned above, we conclude that while a departure from spherical symmetry is possible in some sessions, the primary conclusions and questions raised in the previous Section remain valid.