Vertical structure of the atmosphere

According to the homogeneity of atmospheric composition, two layers can be defined in the atmosphere. The lower layer, up to an altitude of about 80 km above sea level is the homosphere, where due to the continuous turbulent mixing the composition of the atmosphere is relatively constant for chemical species which have long mean residence times. This region is closed by a thin transition layer, called turbopause. Above the turbopause, in the heterosphere, the molecular diffusion dominates and the chemical composition of the atmosphere becomes stratified and varies according to the molecular mass of chemical species (Figure 1.7). The lower heterosphere are dominated by nitrogen and oxygen molecules and the lighter gases being concentrated in the higher layers. Up to 1,000 km the oxygen atoms and above this height the helium and hydrogen are the dominant species.
In the upper part of the atmosphere – from about 60 km to 2000 km above the Earth's surface – ionic species or free radicals (O⁺, O₂⁺, NO⁺, N₂⁺, free electrons) can also be found, and high number of ionized particles affect the propagation of radio waves. This region of the atmosphere is called ionosphere. There are three important layers in the lower part of the ionosphere (at altitudes between about 60 km and 600 km), where the absorption of solar extreme ultraviolet radiation and x-rays ionize the neutral atmosphere. These are the D (60–90 km), E (90–150 km) and F regions (150–500 km) with F1 and F2 sub-layers. The ion density of each layer depends on the solar activity and time of day (Figure 1.8).

Based on the variation of temperature with height, the atmosphere can be divided to different layers (Figure 1.9).
Troposphere:
The lowest major atmospheric layer is the troposphere, extending from the Earth's surface to the tropopause (Figure 1.9). The thickness of the troposphere varies with latitude: it is about 7 km in polar region, generally 11–12 km in the mid-latitudes and even 18 km over the Equator. The height of the tropopause is also depends on season, weather condition and time of day.
Troposphere contains about 80% of total mass of the atmosphere, nearly all water vapour and dust particles can be found here. Almost all weather phenomena and cloud formation take place in this layer. The troposphere is heated from below by the Earth’s surface. Incoming solar radiation first warms the surface, which radiates heat into the atmosphere. The warmer air in the near surface layer generates turbulent vertical motions, which transfer water vapour and other tracers to higher altitudes.
Temperature decreases with increasing height in the troposphere to away from the warming surface. The changing rate of temperature with height is called “lapse rate”^[11]. Tropospheric air temperature is generally proportional with distance from surface and lapse rate is fairly uniform, it is about 6,5 °C / 1000 m, but this rate is affected by water vapour content. Temperature is generally lower than –50 °C at the top of the troposphere (in mid-latitude, temperature is –56.5 °C at 11 km based on ICAO standard atmosphere^[12]).
However, in the lower troposphere, the atmospheric stratification can differ from normal, and temperature can increase with height in the function of time of day and weather condition. This situation is called inversion^[13], which generally occurs at night. When temperature remains the same with height, the stratification is isothermal. The atmospheric stratification and thereby the stability conditions play important role in dispersion of tracers.
The troposphere can be divided into two main parts. The lower part is the planetary boundary layer (PBL) or atmospheric boundary layer, extending upward from the surface to a height that ranges from about 100 to 3000 m in the function of season, weather condition and time of day. Above this layer, the free troposphere can be found.
Stratosphere:
At the tropopause, the decrease of temperature halts and to about 50 km above ground level, an inversion layer can be found, when temperature increases with height. This layer is the stratosphere. Temperature increase in the stratosphere (Figure 1.9) is due to the relatively high concentration of ozone. Ozone strongly absorbs uv radiation^[14] from the Sun in the bands between 210 and 290 nm (more information about ozone see Chapter 8). This absorption by the ozone is the primary cause of temperature increase in the stratosphere. Without ozone layer, a further decrease of temperature with increasing height would be observable in the stratosphere (Figure 1.10).
Stratosphere holds about 19% of total mass of the atmosphere, and it contains only a very small amount of water vapour. Due to the vertical stratification, stratosphere is a stable layer and the mixing is weak. Particles that reach the stratosphere from the troposphere (e.g. from a large volcanic eruption) can stay a long time (many years) in the stratosphere without removing from it. Polar stratospheric clouds^[15] (PSCs) can be observed in winter polar stratosphere between 15 and 25 km height. They form at only very low temperature (below −78 °C). Different types of PSCs contain water, and different particles (e.g. nitric acids) or only water ice (see more information about polar stratospheric clouds in Chapter 8).
The stratosphere is bounded above by the stratopause at about 50 km height, where the average temperature is generally just below 0 °C.


Mesosphere:
Over the stratopause, the next layer is the mesosphere from about 50 km to 85–100 km above the Earth’s surface (Figure 1.9). Air density is tow low to absorb solar radiation, thus the mesosphere is warmed from below by the stratosphere and hence the temperature decreases with increasing height. However the atmosphere is still thick enough to slow down meteoroids enter to the atmosphere. The upper boundary of mesosphere is the mesopause, which is the coldest region of Earth’s atmosphere, where the temperature is around –100 °C.
Within the mesosphere, noctilucent clouds^[16] can be appeared, when Sun is below the horizon and the lower layers of the atmosphere are in the Earth's shadow. These thin clouds are composed from tiny ice crystals, but their emergences, properties and relationships with global climate change are still not fully understood.
Upper atmospheric electrical discharges (like red sprites or blue jets) over tropospheric thunderstorms also occur in the mesosphere.
However, in the absence of frequent direct measurements (only by occasionally sounding rockets), mesosphere is a less known layer of the atmosphere.
Thermosphere:
In the thermosphere, over the mesopause, temperature rise continually with increasing height due to the direct absorption of high energy solar radiation by atmospheric gases. Temperatures are highly dependent on solar activity, and can rise well beyond to 1000 °C. However, this value is not comparable to those of the lower part of the atmosphere, as the air density is extremely low in this layer.
Considering the composition, this layer is a part of the heterosphere, where the atmospheric compounds stratified by their molecular mass. Major layers of the ionosphere (see above) are situated in the thermosphere. Auroras, form by collisions of energetic charged particles with atoms, occur also in the thermosphere.
Over about 500–1000 km above the Earth’s surface (depending on solar activity), the collisions between atmospheric constituent become negligible. This layer is often called as exosphere, which gradually merge into interplanetary space.