Introduction
The stratosphere, which is the region between 15 and 50 km above the Earth's surface, is characterized by the presence of ozone (O3) that protects it from the ultraviolet rays emitted by the Sun. The changes that occur in the ozone stratospheric zone are closely linked to the issues of global warming on Earth (see in-depth analysis on the greenhouse effect). Ozone is continuously formed as a result of the absorption of ultraviolet radiation at short wavelengths and, at the same time, it is continuously destroyed by various chemical reactions that convert it back to molecular oxygen. The balance between ozone formation and its destruction provides the ozone concentration in the atmosphere. The first alarms about the destruction of the ozone protective layer date back to the early 70s, but it was only in 1985, with the discovery of the "hole" over Antarctica, that the problem began to manifest itself in all its severity.
The calculation model
The main culprits in the breaking of ozone molecules are CFCs and HCFCs (already mentioned as greenhouse gases) produced by human activities. These compounds are very stable, so much so that they can reach the stratosphere unchanged: here their molecules are "broken" by ultraviolet rays, releasing chlorine that immediately attacks ozone molecules. The destruction of the ozone layer poses risks to human health associated with exposure to UV rays such as skin cancer or immune system problems.
As for the greenhouse effect, a standardization system has been developed to quantify, with a single unit of measurement, the contributions of various gases to this effect. This method is based on the "Ozone Depletion Potential" (ODP), implemented through conversion factors that transform the quantities of the responsible gases into kg of CFC-11 equivalents (category indicator). The ozone depletion potential of a substance is very similar to the GWP and is defined as the ratio between the number of ozone molecule breakage reactions in an equilibrium state following the emission into the atmosphere of a given substance in one year (kg/year) and the corresponding number of breakage reactions caused, under the same conditions, by an equal quantity of CFC-11 (WMO, 1989).
