por The stratosphere It is one of the layers of the Earth’s atmosphere, located between the troposphere and the mesosphere. The altitude of the lower limit of the stratosphere varies, but can be taken as 10 km for the planet’s mid-latitudes. Its upper limit is 50 km above the Earth’s surface.
The terrestrial atmosphere is the gaseous envelope that surrounds the planet. According to the chemical composition and the temperature variation, it is divided into 5 layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere.
The troposphere extends from the Earth’s surface up to 10 km high. The next layer, the stratosphere, goes from 10 km to 50 km above the earth’s surface.
The mesosphere ranges from 50 km to 80 km in height. The thermosphere from 80 km to 500 km, and finally the exosphere extends from 500 km to 10,000 km in height, being the limit with interplanetary space.
Features of the stratosphere
Location
The stratosphere is located between the troposphere and the mesosphere. The lower limit of this layer varies with latitude or distance from the Earth’s equator.
At the planet’s poles, the stratosphere begins between 6 and 10 km above the earth’s surface. At the equator it begins between 16 and 20 km altitude. The upper limit is 50 km above the Earth’s surface.
Structure
The stratosphere has its own layered structure, which is defined by temperature: cold layers are at the bottom, and warm layers are at the top.
Also, the stratosphere presents a layer where there is a high concentration of ozone, called the ozone layer or ozonosphere, which is between 30 and 60 km above the earth’s surface.
Chemical composition
The most important chemical compound in the stratosphere is ozone. Some 85 to 90% of the total ozone present in the Earth’s atmosphere is found in the stratosphere.
Ozone is formed in the stratosphere by means of a photochemical reaction (chemical reaction involving light) that oxygen undergoes. Much of the gases in the stratosphere enter from the troposphere.
The stratosphere contains ozone (O3), nitrogen (N2), oxygen (O2), nitrogen oxides, nitric acid (HNO3), sulfuric acid (H2SO4), silicates, and halogenated compounds, such as chlorofluorocarbons. Some of these substances come from volcanic eruptions. The concentration of water vapor (H2O in the gaseous state) in the stratosphere is very low.
In the stratosphere, the mixture of gases in the vertical direction is very slow and practically zero, due to the absence of turbulence. For this reason, chemical compounds and other materials that enter this layer remain in it for a long time.
Temperature
The temperature in the stratosphere presents an inverse behavior to that in the troposphere. In this layer the temperature increases with altitude.
This increase in temperature is due to the occurrence of chemical reactions that release heat, where ozone (O3) intervenes. In the stratosphere there are considerable amounts of ozone, which absorbs high-energy ultraviolet radiation from the Sun.
The stratosphere is a stable layer, without turbulence that mixes the gases. The air is cold and dense at the bottom and at the top it is hot and light.
ozone formation
In the stratosphere, molecular oxygen (O2) is dissociated by ultraviolet (UV) radiation from the Sun:
O2 + UV LIGHT → O + O
Oxygen atoms (O) are highly reactive and react with oxygen molecules (O2) to form ozone (O3):
O + O2 → O3 + Heat
In this process heat is released (exothermic reaction). This chemical reaction is the source of heat in the stratosphere and causes its high temperatures in the upper layers.
functions
The stratosphere fulfills a protective function of all forms of life that exist on planet Earth. The ozone layer prevents high-energy ultraviolet (UV) radiation from reaching the Earth’s surface.
Ozone absorbs ultraviolet light and breaks down into atomic oxygen (O) and molecular oxygen (O2), as shown by the following chemical reaction:
O3 + UV LIGHT → O + O2
In the stratosphere, the processes of ozone formation and destruction are in a balance that maintains its constant concentration.
In this way, the ozone layer works as a protective shield from UV radiation, which is the cause of genetic mutations, skin cancer, destruction of crops and plants in general.
Ozone layer destruction
CFC compounds
Since the 1970s, researchers have expressed great concern about the damaging effects of chlorofluorocarbon compounds (CFCs) on the ozone layer.
In 1930 the use of chlorofluorocarbon compounds commercially called freons was introduced. Among these are CFCl3 (freon 11), CF2Cl2 (freon 12), C2F3Cl3 (freon 113) and C2F4Cl2 (freon 114). These compounds are easily compressible, relatively unreactive, and nonflammable.
They began to be used as refrigerants in air conditioners and refrigerators, replacing ammonia (NH3) and liquid sulfur dioxide (SO2) (highly toxic).
Subsequently, CFCs have been used in large quantities in the manufacture of disposable plastic items, as propellants for commercial products in the form of aerosol cans, and as solvents for cleaning electronic device cards.
The widespread use and large quantities of CFCs has caused a serious environmental problem, since those used in industries and refrigerant uses are discharged into the atmosphere.
In the atmosphere these compounds diffuse slowly into the stratosphere; in this layer they suffer decomposition due to the effect of UV radiation:
CFCl3 → CFCl2 + Cl
CF2Cl2 → CF2Cl + Cl
Chlorine atoms react very easily with ozone and destroy it:
Cl + O3 → ClO + O2
A single chlorine atom can destroy more than 100,000 ozone molecules.
Nitrogen oxides
Nitrogen oxides NO and NO2 react destroying ozone. The presence of these nitrogen oxides in the stratosphere is due to gases emitted by supersonic aircraft engines, emissions from human activities on Earth, and volcanic activity.
Thinning and holes in the ozone layer
In the 1980s it was discovered that a hole had formed in the ozone layer above the South Pole area. In this area the amount of ozone had been reduced by half.
It was also discovered that above the North Pole and throughout the stratosphere, the protective ozone layer has thinned, that is, it has reduced its width because the amount of ozone has decreased considerably.
The loss of ozone in the stratosphere has serious consequences for life on the planet, and several countries have accepted that a drastic reduction or complete elimination of the use of CFCs is necessary and urgent.
International agreements on restriction in the use of CFCs
In 1978, many countries banned the use of CFCs as propellants for commercial aerosol products. In 1987, the vast majority of industrialized countries signed the so-called Montreal Protocol, an international agreement where goals were set for the gradual reduction of CFC manufacturing and its total elimination by the year 2000.
Several countries have breached the Montreal Protocol, because this reduction and elimination of CFCs would affect their economy, putting economic interests before the preservation of life on planet Earth.