## 3 Results  
The Concordia lidar observatory has been active since 2014. Lidar profiles have been recorded during the months of major PSC occurrences, namely from June through September. Usually, two to four profiles are recorded every day, with the exception of prohibitive meteorological conditions, such as storms, precipitation, or strongly absorbing tropospheric clouds. In 2019, both lasers broke down and no data acquisitions were possible. Here, we show the PSC observations performed in 2021 as an example of the seasonal variations in PSC occurrences.  
The formation of PSCs is mainly dictated by temperature, since the different kinds of PSCs exist in a limited temperature range (see, e.g., refs. [21; 22; 23]). Above a certain threshold temperature, they simply evaporate and cease to exist as solid or liquid particles. On the other hand, the formation processes can follow different pathways depending on temperature history induced by either large synoptic fields or mesoscale features. The upper temperature threshold for PSCs is the formation temperature for NAT (T\({}_{NAT}\)), which is around 195 K depending on the water vapor and nitric acid mixing ratios, local  
Figure 7: The figure shows the criteria using the backscatter ratio \(R\) and the perpendicular backscatter coefficient \(\beta_{perp}\) to classify the different PSC types.  
pressure, and temperatures [24]. T\({}_{NAT}\) is the highest PSC formation temperature that is thermodynamically possible, but usually NAT forms at about 2.5-3 K below T\({}_{NAT}\)[20]. Water ice PSCs have the lowest formation temperature (about 188 K), the so-called frost temperature T\({}_{ice}\) (see, e.g., ref. [25] or [26] for different methods to calculate the frost point).  
The nucleation and growth of the different PSCs have been extensively discussed elsewhere [27]. The abundance of water in the troposphere and stratosphere, with respect to nitric acid, sulfuric acid, and other condensable species, produces ice PSCs when the local temperature is low enough. Homogeneous ice nucleation from SSA (supercooled sulfuric acid) or from STSs occurs typically at T\({}_{ice}\)-4 K [28; 29; 30], while heterogeneous nucleation on NAT particles already occurs below T\({}_{ice}\) but above T\({}_{ice}\)-4 K [31; 32].  
For the formation of NAT particles, several pathways have been proposed [33]. The heterogeneous nucleation on ice particles requires that the temperature be low enough to produce ice PSCs, implying that other formation processes are required for T \(>\) T\({}_{ice}\).  
The heterogeneous nucleation on background aerosols containing meteoritic material [34] or other condensation nuclei might occur readily below T\({}_{NAT}\). However, the possibility of homogeneous nucleation on STSs was rejected by [30].  
Liquid STS droplets, being ternary solutions with a variable stoichiometry, do not form below a definite threshold temperature but rather grow by a continuous uptake of nitric acid by sulfuric acid aerosols, which becomes massive below 195 K [35]. The composition of the STS droplets changes with temperature; while temperature decreases, the HNO\({}_{3}\) concentration increases while the H\({}_{2}\)SO\({}_{4}\) concentration decreases.  
The formation of PSCs causes a depletion of water and nitric acid in the gas phase, which leads to dehydration and denitrification of the stratosphere.