The atmosphere is divided into five distinct layers characterized by the slope of its temperature profile:
(i) Troposphere (ii) Stratosphere (iii) Mesosphere (iv) Ionosphere Thermosphere and (v) Magnetosphere/Exosphere
This region extends from the earth (about 8-10 km at the polar latitude, 12 km at the moderate latitude and 18 km at the equator).
The troposphere accounts for about 80 per cent of the atmospheric mass. The vertical temperature gradient of the troposphere is 5° per km in the lower and 7° per km in the upper regions of the troposphere.
The upper region of the troposphere is separated by the lower region of the stratosphere in a narrow range called the Tropopause.
The uppermost region of the troposphere is almost transparent to the sun’s rays. The sun’s rays are mostly absorbed by the earth’s surface due to various materials present in it.
Some of the absorbed solar energy radiates as heat waves from the lower region of the troposphere to the middle and finally to the upper troposphere.
Thus, there occurs a gradual decrement in temperature with height. Again, the non-uniform heating of the ground surface of the earth, due to uneven distribution of materials, ascending and descending air currents so produced result in turbulence and mixing of air mass.
This mixing helps air to maintain a good quality since it rapidly disperses pollutants. The temperature in the troposphere region varies from ground temperature to – 56°C with altitude.
The average pressure on the earth’s surface is 1,014 millibars, close to 1 atmosphere (atm). With increased height the air pressure decreases. Natural happenings like rains, storms, thunder showers, etc., are found in this layer only. Because of climatic changes, this layer has tremendous effect on the eco-system.
All living beings live in the troposphere. Although, they are surrounded by air they cannot see it, but only feel it when it blows or when they breathe.
Air has no colour, odour or definite shape, the composition (per cent by volume) of clean air being: nitrogen 77.2 per Cent, oxygen 20.6 per cent, carbon dioxide 0.04 per cent, water vapour varies from place to place, inert gases 0.94 per cent and other traces approximately 0.03 per cent.
It is a colourless, tasteless and odourless gas. It is a diatomic gas. The structure is (: N & N :). At room temperature it is inert and only at higher temperature it shows some chemical reactivity.
It dilutes the effect of oxygen and controls combustions. It is very essential for plant and animal life as it forms amino acids and proteins. Oxygen (02): It is a diatomic gas.
It is a colourless, tasteless and odourless gas. The structure is (:0 = 0:). Pure oxygen is very necessary for most forms of life. Oxygen helps in combustion, oxidation and respiration of all living beings.
Plants in all ecosystems release about 8 miles of oxygen per year per square meter of the earth’s surface during photosynthesis.
This amount of oxygen is utilized by plants and heterotrophic organisms in respiration, so that there is a balance between the amount of oxygen production and utilization.
Carbon dioxide (C02):
It is a colourless gas with faint pungent smell with a resonance hybrid of three structures.
It is very essential for photosynthetic activity. The biochemical formula of this process is given as
In this process CO and H20 is consumed and carbohydrate (CH20) is produced and 02 is released. C02 has a unique property of absorption of infrared radiation of the solar spectrum keeping the earth warm. C02 has a linear structure (O = C = O), thus the net dipole moment is zero. However, in the infrared region here, it is vibration, rather than the rotation which gives rise to a dipole change.
Inert gases (Helium [He], Neon [Ne], Argon [Ar], Krypton [Kr], Xenon [Xe] and Radon [Rn]): They are zero or 18 group elements in the periodic table.
They undergo very few chemical reactions under drastic conditions and are therefore known as noble gases. Neon gas is very effective in formation of chlorophyll. These gases are generally required to create inert atmosphere.
The layer above the tropopause is called the stratosphere. Its thickness is about 50 km above the earth surface and it consists of a rich layer of ozone. The short wavelength ultraviolet energy (190nm – 3 80nm) is absorbed by ozone and oxygen, causing the air to be heated and simultaneously this layer acts as a shield to protect life on earth from the harmful effect of this radiation. The temperature in this region continues to increase with height and at 50 km, attains a maximum of-2°C.
Stratosphere is therefore, a stable layer of very dry air with composition of ozone, oxygen and atomic oxygen.
The atomic oxygen which is produced has less residence period as it combines with molecular oxygen to form ozone and hence, it has less contribution to heating effect on the stratosphere unlike its role in thermosphere/ionosphere.
Pollutants that find their way into the stratosphere may remain there for many years before eventually drifting back to the troposphere and are then finally removed by settling or precipitation.
The upper region of the stratosphere is separated from the lower region of the mesosphere by a narrow range called the stratopause. Ozone: It is a pale blue gas and has a strong odour. When inhaled, it causes headaches and large quantities of it prove fatal. Ozone molecules are considered to be the resonance hybrid of two structures-
The 0-0-0 bond angle in ozone is 116.8° and 0-0 bonds are of equal length (127.8 pm).
Oxygen in the upper atmosphere absorbs ultraviolet radiation and changes to atomic oxygen. Atomic oxygen thus produced, is combined with molecular oxygen to form ozone
Ultraviolet rays harmful to animals (including man) are absorbed by oxygen to form ozone and the ozone thus formed prevents the remaining ultraviolet rays to reach the earth’s surface.
This is the region above the stratosphere and extends up to a height of 85 km above the earth’s surface. Concentration of ozone is very low in this region and decreases rapidly with increase in height.
Thus, there is a decrease in absorption of solar radiation and temperature falls to – 92°C. The dominant chemical species found in this region are 02 and NO.
It has the highest clouds which often are visible in clear weather. Meteors and upward convection air currents cause these clouds. This layer is also responsible for reflecting all sound waves moving upwards from the earth. So beyond this layer, a valley of silence prevails.
The upper region of the mesosphere is separated from the lower region of the thermosphere by a narrow range called the menopause. Nitric oxide (NO): It is a colourless gas. Nitric oxide is paramagnetic, having an odd electron. It is a resonance hybrid of two structures.
The region immediately above the menopause, where the temperature rises very rapidly with increasing altitude is known as the thermosphere. It extends up to 500 km above the earth surface. The most dominant chemical species in this region are 02, NO and atomic oxygen (O).
They absorb uv-radiation, x-rays and y-rays and undergo ionization. The maximum temperature in this region is about 1200°C. The heating of the thermosphere is mainly due to the absorption of solar energy by atomic oxygen (O).
Within the thermosphere, is a relatively dense band of charged particles (O, N0+,0+ and electrons) called the ionosphere.
The ionosphere is high electrical conductor as it contains charged particles produced from NO as well as from hydrogen, oxygen, helium, ozone, etc.
Appreciable levels of positive ions, such as 02, 0+, NO+ exist in this region with considerable residence times. The charged species mainly form due to the absorption of UV radiation. During the nights when UV radiations are not present, the positive ions combine with the free electrons and form neutral species.
Before the induction of satellites, the ionosphere was important for worldwide communication due to its ability to reflect short radio waves back to earth.
(v) Magnetosphere/ Exosphere:
Very little is known about this layer. It exists above the ionosphere and extends approximately up to 2000 km above the earth. This layer is almost airless and empty. It probably contains hydrogen gas in ionized state. It has very high temperature and merges into airless dark black interplanetary space.