The below mentioned article provides quick notes on the atmospheric temperature and composition.
Atmospheric Temperature:
Based on the atmospheric temperature profile, the atmosphere is classified into four regions.
The layer next to the earth’s surface is called the troposphere. Its thickness ranges between 8 to 16 km depending upon the season and the latitude of a place.
Its temperature decreases progressively with height and becomes minimum at the ‘tropopause’, where the next layer stratosphere starts.
Under ‘normal’ condition the decrease in temperature in the troposphere is 0.64°C per 100 m increase in altitude. This is termed as the lapse rate. The theoretical lapse rate (considering adiabatic expansion of dry air) is -1°C/100 m. Because of this temperature gradient there is vertical convective movement of air within this layer. Most of the water vapour present in the atmosphere is confined in this layer.
The major constituents of the troposphere being oxygen (O2) and nitrogen (N2), it cannot absorb solar radiation. During the day it receives heat from the earth’s surface by convection. (The earth’s surface absorbs solar radiation during the day as a result its temperature becomes higher than that of the troposphere).
At night as the earth’s surface cools because of radiation to the space, its temperature becomes less than that of the troposphere. Consequently it (the earth’s surface) receives heat from the lower part of the troposphere. As a result the temperature profile in the troposphere changes at night and temperature inversion takes place.
The stratosphere, starting at the tropopause, extends to a height of about 50 km and ends at the stratopause. In this layer the temperature increases with altitude but the rate of increase is slow. The warming of the stratosphere is mainly due to absorption of ultraviolet solar radiation by the ozone molecules of the stratosphere.
As the temperature gradient is not favourable in the stratosphere, convective motion is virtually absent here. Consequently once some gaseous components reach this layer from the troposphere by diffusion they continue to reside there till they are transformed due to chemical reactions.
The layer above the stratosphere is known as the mesosphere, which is about 35 km thick. This layer ends at the mesopause, where the layer known as the thermosphere starts. In the mesosphere the temperature decreases with increase in altitude and becomes less than -100°C at the mesopause. The thermosphere, which starts at the mesopause, extends up to the end of atmosphere. In this layer the temperature goes on increasing with the altitude indefinitely.
Figure 2.1 shows schematically vertical pressure and temperature distribution in the atmosphere.
Atmospheric Chemical Composition:
Chemical composition of the atmosphere is not the same all through. The layer adjacent to the earth’s surface is somewhat homogeneous in composition and hence this layer is termed as the homosphere. It includes the troposphere, the stratosphere and the mesosphere and extends up to an altitude of about 100 km.
A sub-layer relatively rich in ozone exists in the upper part of the stratosphere and the lower part of the mesosphere. The layer is referred to the ozone layer. It shields the earth from the ultraviolet solar radiation. The concentration of ozone in the troposphere is normally low, about 0.02 to 0.08 ppmv, whereas that in the ozone layer may be more than 5 ppmv.
Macro-scale movement of air parcels tends to homogenize the atmosphere, whereas molecular diffusion tends to segregate the atmospheric constituents on the basis of molecular weight. Well above 100 km altitude molecular diffusion becomes dominant as a result the concentration of the lighter (lower, molecular weight) gases increases at higher altitude.
Homosphere:
The composition of ambient air, particularly, in the troposphere, depends on whether it is clean or polluted. The concentration of one of the constituents, namely, water vapour depends on the season, as well as on the location of the place. The major constituents of tropospherical air are nitrogen and oxygen. There are several minor constituents. In Table 2.1.an average composition of the tropospherical dry air is listed.
Of the different constituents of ambient air, oxygen is highly reactive. It is essential for all living beings, as well as for some industrial processes. Even though nitrogen is less reactive, it is an important ingredient of all the living beings. Of the other constituents argon, helium, krypton, neon and xenon are inert.
The other constituents, namely, Carbon dioxide (CO2), Carbon monoxide (CO), Methane (CH4), Nitrogen oxides (NOx), Sulphur oxides (SOx) Ozone (O3) Ammonia (NH3), etc., have some harmful effects on the living beings directly or indirectly at higher concentrations and hence they are designated as pollutants.
Heterosphere:
The layer of atmosphere beyond the homosphere is termed as the heterosphere as its chemical composition is non-uniform. It is made up of several sub-layers. The layer adjacent to the homosphere contains mainly molecular nitrogen. Beyond the molecular nitrogen layer there are layers containing predominantly atomic oxygen, helium and hydrogen respectively. The heterosphere filters out high- energy solar radiation, as a result of which the temperature of the troposphere is conducive to life on earth.
Chemosphere and Ionosphere:
The atmosphere is also classified based on its chemical and physical properties. The layer closest to the earth’s surface is termed as chemosphere. It contains mainly gaseous molecules and some free radicals, such as hydroxyl (OH’), hydro peroxide (HO’2) and some atomic oxygen and hydrogen. The layer beyond is known as the ionosphere as it is made up of a relatively large proportion of ions.
