O3 is considered polar
1 Formation of stratospheric ozone
The ozone in the stratosphere is of elementary importance for life on earth. It prevents the hard and very high-energy UV-B radiation from the sun (in the wavelength range between 280 and 320 nm), which can destroy biomolecules such as DNA and proteins, from penetrating the atmosphere unhindered. Plants and humans are particularly at risk from UV-B, while many animals are protected by fur and feathers and a nocturnal and hidden way of life. In humans, increased UV-B radiation can cause skin cancer and weaken the immune system. Life in the ocean is also affected, depending on the depth and cloudiness of the water. In the open ocean, which is normally only slightly cloudy, UV-B radiation can penetrate to a depth of 20 m. On the other hand, the penetration depth near the coast is much lower because of the greater amount of material introduced. In the sea, the sensitive phytoplankton, which form the basis of the food chain in aquatic ecosystems, are particularly at risk. The habitat of the phytoplankton is located near the water surface, where there is still sufficient visible light available for photosynthesis.
Due to the absorption of short-wave and long-wave radiation, the stratospheric ozone also has an important influence on the vertical temperature distribution. While in the troposphere the temperature decreases from 15 ° C on the ground to less than -50 ° C at the tropopause, the absorption of radiation by ozone results in a significant warming up to 0 ° C at a height of 50 km. An important consequence is a high dynamic stability of the stratosphere and the limitation of strong vertical air movements mainly to the troposphere.
Ozone is created in the stratosphere by combining an oxygen atom (O) with an oxygen molecule (O2). The oxygen atom emerges in the stratosphere, especially above the tropics, from the destruction of oxygen molecules by ultraviolet radiation (photolysis):
- O2 + hv -> O + O
- O + O2 -> O3 (2x)
In the lower stratosphere, ozone is also created by the photolysis of nitrogen dioxide:
- NO2 + hv -> NO + O
- O + O2 -> O3
The stratospheric ozone is also broken down again into an oxygen atom and an oxygen molecule by ultraviolet radiation:
- O3 + hv -> O2 + O
In addition, catalytic reactions with natural radicals such as nitrogen monoxide (NO), chlorine (Cl), bromine (Br) and others play a role, which in the stratosphere under the action of UV radiation from originally tropospheric trace gases such as nitrous oxide (N2O), methane (CH4) and methyl chloride (CH3Cl) arise. The formation and destruction of ozone are largely balanced over longer periods of time in natural processes. Over periods of a few years, however, the ozone balance can also be disturbed by fluctuations in solar radiation or by volcanic eruptions.
2 Distribution of stratospheric ozone
The vertical distribution of ozone in the atmosphere is very uneven. Only 10% of the ozone is in the troposphere, while 90% is in the stratosphere, i.e. at an altitude of 12 to 50 km. 75% of the total amount of ozone is concentrated in the 15-30 km altitude area alone and forms the so-called ozone layer here. But also horizontally, the ozone distribution between the equator and the poles is very different. Both radiation and dynamic processes are responsible for the horizontal ozone distribution. While the ozone is mainly formed on both sides of the equator due to the higher solar radiation, the majority of the ozone is found at higher latitudes. So on both sides of the equator there are only about 280 DU (Dobsonian unit: 1 DU = 2.7 x 1016 O3-Molecules / cm2), but at 60 ° N and 60 ° S over 400 or 360 DU.
The reason for the geographical distribution of ozone is the stratospheric circulation. The mean meridional air mass transport in the stratosphere is determined by the Brewer-Dobson circulation (white-blue arrows). It consists of an ascending branch on either side of the equator, which is driven by the high-reaching tropical convection, and a descending branch above the poles, which is caused by the cooling of the radiation. The polar cooling process, which is more pronounced in the winter hemisphere than in the summer hemisphere, is the actual motor of the Brewer-Dobsonian circulation. It plays a key role in ensuring that ozone-rich air is transported from the tropical areas of origin towards the pole. In the winter half-year, stratospheric air sinks above the pole into the troposphere and creates a stationary polar vortex. The polar vortex forms a transport barrier (green, vertical bar) for meridional currents at a height of 15-30 km. Turbulent exchange processes also transport air in middle and higher latitudes over the tropopause (red arrows), similarly also meridionally in the stratosphere itself.
Since the formation and destruction of ozone are heavily dependent on solar radiation and the transport of weather regimes that fluctuate from year to year, the stratospheric ozone concentration is also subject to seasonal fluctuations. In the northern hemisphere, for example, from 60 ° N towards the poles, due to the higher transport dynamics, maximum values of over 400 DU in the northern spring, while the ozone falls to 300 DU in late summer.
In addition, the total ozone can fluctuate significantly from year to year. These fluctuations are particularly pronounced north of 50 ° N in the winter months. In winter, the stratospheric ozone content in higher latitudes is largely dependent on the supply from lower latitudes, since the photolytic formation almost or completely comes to a standstill due to decreasing or absent solar radiation. The high winter temperature differences between the pole and the tropics intensify the large-scale atmospheric circulation. As a result, the tropospheric and stratospheric circulations are more closely linked than in summer. Depending on the planetary waves in the middle latitudes, the winter circulation can be more zonal or more clearly meridional. Planetary waves are large-scale wave movements in the atmosphere that are stimulated by mountains, temperature differences (e.g. between land and sea) or low pressure cells in the troposphere. Its expression also depends on the Arctic (AO) or North Atlantic Oscillation (NAO). A strong AO leads to weak planetary waves, as it forces the air currents on a zonal path, a weak AO leads to strong planetary waves. With strong planetary waves, the winter circulation is more meridional, which enables stronger ozone transport in the direction of the pole. With a strongly zonal orientation, only a little ozone-rich air is transported towards the winter pole.
3 Depletion of stratospheric ozone
Since the late 1970s, the balance between ozone formation and ozone depletion has been increasingly disturbed by human influences. The cause is the anthropogenic emission of halogenated hydrocarbons (CFCs), which contain chlorine and bromine. CFC molecules take several years to reach the stratosphere. There they are destroyed by the strong UV radiation of the sun, whereby chlorine (or bromine) is released, which then attacks the ozone molecules. The decrease in ozone is detectable in the entire stratosphere. However, it is most pronounced over the Poles. The particularly strong decrease in the ozone concentration in the southern spring over the South Pole is known as the ozone hole.
4 individual proofs
- ^ Weber, M., S. Dhomse, F. Wittrock, A.s Richter, B.-M. Sinnhuber and J. Burrows (2003): The influence of dynamics on ozone transport and ozone chemistry in high latitudes, ozone bulletin of the German Weather Service 93
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