Stratospheric Chemistry and Ozone Hole

a) Stratosphere

In this chapter particular properties of the stratosphere (air layer above the tropopause) will be explained. Because the temperature does not decrease anymore above the tropopause in 10-18 km altitude (boundary layer between the troposphere and the stratosphere) an intensive mixing of these two air layers is widely prevented. Therefore many gases emitted by plants and human activity never reach the stratosphere and the chemistry in this region differs substantially from the chemistry in air layers near surface.

A short overview on the chemistry of the stratosphere will be given. Reactions on ice crystals as well as the chemistry of chlorine- and oxidised nitrogen compounds play an important role in this region of the atmosphere.

Due to the higher residence time of compounds in the stratosphere direct emissions have a stronger impact in this region than in the troposphere. This direct input can occur e.g. via volcanic eruptions, however it is occurring by far more regularly by high flying air traffic. In order not to disturb the equilibrium in this region an intensive use of the airspace above 10-12km by air planes has been avoided so far, however, many planes fly at the border.

b) Ozone hole

In the stratosphere also lies the ozone layer. Its function is explained here. Its destruction especially due to the input of chlorofluorocarbons (CFC) into the atmosphere was probably the first massive environmental problem at which the global impact of human activities became clear. The processes leading to ozone depletion are outlined and it is explained why the problem is more pronounced over the Antarctic than over the Arctic and the ozone hole has a temporal periodicity.

On the one hand it is made clear that (in contrast to the widespread opinion) global warming and the ozone hole are caused by various human activities. On the other hand it is explained how these two processes interact (reduced light absorption by the thinner ozone layer, cooling of the stratosphere due to global warming). An outlook into the future shows the decrease of the CFCs in the air layers, but it is also mentioned that a recovery of the ozone layer in the next 50 years can not be guaranteed, one reason for this being the stratospheric cooling.