STRATOSPHERE: CHARACTERISTICS, FUNCTIONS, TEMPERATURE - ENVIRONMENT - 2024

The stratosphere 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 middle latitudes of the planet. Its upper limit is 50 km of altitude above the Earth's surface.

The Earth's atmosphere is the gaseous envelope that surrounds the planet. According to the chemical composition and the variation in temperature, it is divided into 5 layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere.

Figure 1. Stratosphere seen from space. Source: NOSA Galician Space Agency

The troposphere extends from the Earth's surface up to 10 km high. The next layer, the stratosphere, ranges 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.

Stratosphere characteristics

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 equatorial line.

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 of altitude. The upper limit is 50 km above the Earth's surface.

Structure

The stratosphere has its own layered structure, which are defined by temperature: cold layers are at the bottom, and hot layers are at the top.

Also, the stratosphere has a layer where there is a high concentration of ozone, called the ozone layer or ozonosphere, which is between 30 to 60 km above the earth's surface.

Chemical composition

The most important chemical compound in the stratosphere is ozone. 85 to 90% of the total ozone present in the Earth's atmosphere is found in the stratosphere.

Ozone is formed in the stratosphere through a photochemical reaction (chemical reaction where light intervenes) that oxygen undergoes. Much of the gases in the stratosphere enter from the troposphere.

The stratosphere contains ozone (O ₃), nitrogen (N ₂), oxygen (O ₂), nitrogen oxides, nitric acid (HNO ₃), sulfuric acid (H ₂ SO ₄), silicates and halogenated compounds, such as chlorofluorocarbons. Some of these substances come from volcanic eruptions. The concentration of water vapor (H ₂ O in the gaseous state) in the stratosphere is very low.

In the stratosphere, the vertical gas mixing is very slow and practically nil, due to the absence of turbulence. For this reason, chemicals and other materials that enter this layer remain in it for a long time.

Temperature

The temperature in the stratosphere exhibits an inverse behavior to that of 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 (O ₃) intervenes. There are considerable amounts of ozone in the stratosphere, which absorbs high-energy ultraviolet radiation from the Sun.

The stratosphere is a stable layer, with no turbulence for gases to mix. The air is cold and dense in the lower part and in the upper part it is warm and light.

Ozone formation

In the stratosphere, molecular oxygen (O ₂) is dissociated by the effect of ultraviolet (UV) radiation from the Sun:

O ₂ + UV LIGHT → O + O

Oxygen (O) atoms are highly reactive and react with oxygen (O ₂) molecules to form ozone (O ₃):

O + O _{2 →} O ₃ + 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.

Features

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 decomposes into atomic oxygen (O) and molecular oxygen (O ₂), as shown by the following chemical reaction:

O ₃ + UV LIGHT → O + O ₂

In the stratosphere, the processes of formation and destruction of ozone 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 CFCl ₃ (Freon 11), CF ₂ Cl ₂ (Freon 12), C ₂ F ₃ Cl ₃ (Freon 113) and C ₂ F ₄ Cl ₂ (Freon 114). These compounds are easily compressible, relatively unreactive, and non-flammable.

They began to be used as refrigerants in air conditioners and refrigerators, replacing ammonia (NH ₃) and liquid sulfur dioxide (SO ₂) (highly toxic).

Subsequently, CFCs have been used in large quantities in the manufacture of disposable plastic articles, as propellants for commercial products in the form of aerosols in cans, and as cleaning solvents for electronic device cards.

The widespread and large use of CFCs has created a serious environmental problem, since those used in industries and refrigerant uses are discharged into the atmosphere.

In the atmosphere, these compounds slowly diffuse into the stratosphere; in this layer they suffer decomposition due to the effect of UV radiation:

CFCl _{3 →} CFCl ₂ + Cl

CF ₂ Cl _{2 →} CF ₂ Cl + Cl

Chlorine atoms react very easily with ozone and destroy it:

Cl + O ₃ → ClO + O ₂

A single chlorine atom can destroy more than 100,000 ozone molecules.

Nitrogen oxides

Nitrogen oxides NO and NO ₂ react to destroy ozone. The presence of these nitrogen oxides in the stratosphere is due to the gases emitted by the engines of supersonic aircraft, 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 cut in 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 on the use of CFCs

In 1978 many countries banned the use of CFCs as propellants in 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 failed to comply with 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.

Why don't planes fly in the stratosphere?

During the flight of an airplane, 4 basic forces act: lift, airplane weight, drag, and thrust.

Lift is a force that supports the plane and pushes it upward; the higher the density of the air, the greater the lift. Weight, on the other hand, is the force with which the Earth's gravity pulls the plane towards the center of the Earth.

Resistance is a force that slows or prevents the aircraft from moving forward. This resistance force acts in the opposite direction to the plane's path.

Thrust is the force that moves the plane forward. As we can see, the thrust and lift favor flight; the weight and the resistance act to disadvantage the flight of the airplane.

Aircraft that

Commercial and civil aircraft at short distances fly approximately 10,000 meters above sea level, that is, at the upper limit of the troposphere.

All aircraft require cabin pressurization, which consists of pumping compressed air into the aircraft cabin.

Why is cabin pressurization required?

As the aircraft ascends to higher altitudes, the external atmospheric pressure decreases and the oxygen content also decreases.

If pressurized air was not supplied to the cabin, passengers would suffer from hypoxia (or mountain sickness), with symptoms such as fatigue, dizziness, headache and loss of consciousness due to lack of oxygen.

If a failure in the supply of compressed air to the cabin or a decompression occurs, an emergency would arise where the airplane must descend immediately, and all its occupants should wear the oxygen masks.

Flights in the stratosphere, supersonic planes

At altitudes greater than 10,000 meters, in the stratosphere, the density of the gaseous layer is lower, and therefore the lift force that favors flight is also lower.

On the other hand, at these high altitudes the content of oxygen (O ₂) in the air is lower, and this is required both for the combustion of the diesel fuel that makes the aircraft engine work, and for an effective pressurization in the cabin.

At altitudes greater than 10,000 meters above the earth's surface, the plane has to go at very high speeds, called supersonic, reaching over 1,225 km / hour at sea level.

Figure 2. Concorde supersonic commercial aircraft. Source: Eduard Marmet

Disadvantages of supersonic aircraft developed to date

Supersonic flights produce so-called sonic booms, which are very loud noises similar to thunder. These noises negatively impact animals and humans.

Additionally, these supersonic aircraft need to use more fuel, and therefore produce more air pollutants than aircraft that fly at lower altitudes.

Supersonic aircraft require much more powerful engines and expensive special materials to manufacture. Commercial flights were so costly economically that their implementation has not been profitable.

References

1. SM, Hegglin, MI, Fujiwara, M., Dragani, R., Harada, Y et all. (2017). Assessment of upper tropospheric and stratospheric water vapor and ozone in reanalyses as part of S-RIP. Atmospheric Chemistry and Physics. 17: 12743-12778. doi: 10.5194 / acp-17-12743-2017
2. Hoshi, K., Ukita, J., Honda, M. Nakamura, T., Yamazaki, K. et all. (2019). Weak Stratospheric Polar Vortex Events Modulated by the Arctic Sea ‐ Ice Loss. Journal of Geophysical Research: Atmospheres. 124 (2): 858-869. doi: 10.1029 / 2018JD029222
3. Iqbal, W., Hannachi, A., Hirooka, T., Chafik, L., Harada, Y. et all. (2019). Troposphere-Stratosphere Dynamical Coupling in Regard to the North Atlantic Eddy-Driven Jet Variability. Japan Science and Technology Agency. doi: 10.2151 / jmsj.2019-037
4. Kidston, J., Scaife, AA, Hardiman, SC, Mitchell, DM, Butchart, N. et all. (2015). Stratospheric influence on tropospheric jet streams, storm tracks and surface weather. Nature 8: 433-440.
5. Stohl, A., Bonasoni P., Cristofanelli, P., Collins, W., Feichter J. et all. (2003). Stratosphere ‐ troposphere exchange: A review, and what we have learned from STACCATO. Journal of Geophysical Research: Atmospheres. 108 (D12). doi: 10.1029 / 2002jD002490
6. Rowland FS (2009) Stratospheric Ozone Depletion. In: Zerefos C., Contopoulos G., Skalkeas G. (eds) Twenty Years of Ozone Decline. Springer. doi: 10.1007 / 978-90-481-2469-5_5