Composition of the Atmosphere

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Composition of the Atmosphere: Introduction

Earth’s atmosphere consists of different types of materials such as solids, liquids, and gases. These elements in the atmosphere have their definitive share in the composition of the atmosphere and play an important role in controlling human activities on planet Earth. A description of the various components of the atmosphere is given below:

Composition of the Atmosphere: Dry Air

The atmosphere is made of different types of gases. Nitrogen, Oxygen and CO₂ are important gases which are found in their uniform or stable proportion within the lower layer of the atmosphere close to the earth. Four gases-nitrogen, oxygen, argon, and carbon dioxide account for more than 99 per cent of dry air.

Nitrogen alone constitutes nearly four-fifths of the air by volume and oxygen one-fifth. The principal remaining stable gases are neon, helium, methane, krypton, hydrogen, nitrous oxide, and xenon. Various other less stable gases, including ozone and radon, also occur in the air.

Important gases of dry air are discussed below.

Composition of the Atmosphere: Nitrogen

Nitrogen is a very important constituent of the atmosphere and occupies 78% of the atmosphere by volume in dry air. This gas works as a diluent and neutralizes the combustion phenomena in the atmosphere by diluting oxygen. Nitrogen indirectly helps in various types of oxidations.

Atmospheric nitrogen is used in nitrogenous compounds found in various types of organisms. Besides, it is also used by plants and trees to make protein which is very useful for their growth.

Table 1.1 Share of Gases in the Atmosphere (Composition of the Atmosphere)

Composition of the Atmosphere: Oxygen

O₂ is the second important constituent of the atmosphere. Among all the gases, oxygen is an essential gas for human life. We all inhale oxygen from the atmosphere through breathing. Survival of living beings on the earth’s surface is not possible in the absence of oxygen.

It easily combines with other chemical elements to form a variety of compounds. It also helps in the burning of substances. In the combustion process, O₂ is also used. Oxygen shares about 21 per cent of the volume of dry air. Therefore, the presence of oxygen in the atmosphere is very useful.

Composition of the Atmosphere: Carbon dioxide

The third important gas in the atmosphere is carbon dioxide. The share of Carbon dioxide (CO₂) in the composition of the atmosphere is very insignificant. It plays an important role on the earth as well as in the atmosphere. The advantage of CO₂, are as follows-

• It absorbs long-wave radiation and reflects short-wave radiation. Therefore, it is transparent for visible sunlight and opaque for terrestrial radiation. Consequently, heat is trapped between the atmosphere and the earth’s surface and ultimately temperature on the earth is controlled.
• Carbon dioxide is also consumed by plants in the formation of food through the process of photosynthesis.

However, CO₂ is added to the atmosphere by burning fossil fuels such as coal, oils etc. The automobile industry and deforestation are other important contributors to the addition of CO₂ in the atmosphere. The amount of CO₂ in the atmosphere is increasing at a tremendous rate.

In the hundred years from 1890 to 1990, it increased more than 10 times. Increasing its proportion in the atmosphere causes global warming through the greenhouse effect. Remember that about half of the increased amount of this gas is absorbed by the oceans or used by vegetation, the other half remains present in the atmosphere.

Composition of the Atmosphere

Composition of the Atmosphere: Ozone

The zone of the atmosphere between 19 to 35 km above the earth’s surface is known as the ozonosphere because of the presence of ozone gas. Ozone (O₃) is a very minor constituent of dry air in the atmosphere. It is a powerful oxidizing agent, but it occurs in such minute quantities (occupies only 0.00006 per cent of the volume of dry air) and at such great altitudes (lies at an elevation between 19 to 35 km above the earth’s surface) that its role in this respect is very limited.

Ozone absorbs a part of the ultraviolet sun rays and thereby limits the quantity that reaches the earth’s surface to the amount essential for life. It also protects us from sunburn.

Also, the study exposes that the ozone layer is depleting because of the release of nitrogen oxide from supersonic jet planes and chlorofluorocarbon (CFC) from refrigerator industries. This depletion in the amount of ozone gas has resulted in blood cancer, cataracts etc., diseases in developed countries like USA and Canada.

Composition of the Atmosphere: Water Vapors

Water vapor is another important constituent of the atmosphere. It is a variable element and is close to the earth’s surface. About 90 percent of water vapors lie up to 5 km in height in the atmosphere. Only 1 percent of the total humidity of the atmosphere is found above 10 km.

The proportion of water vapor in the air varies from as little as 0.02 percent of the air in desert environments to over 4 percent under extremely humid conditions. The vapor in the atmosphere is added by evaporation from water bodies like oceans, seas, lakes, rivers, etc., by transpiration from the plants, and precipitation from the atmosphere. The share of vapor, though, is very little to the volume of air, but climatically it plays a very significant role. It affects climate and weather in the following respects.

• All the greenery of the earth and all the living beings have maintained their life only because of water.
• Water vapors are the source of various forms of condensation such as snow, frost, fog, cloud, mist, etc., and precipitation on the earth’s surface. It also determines the type (rain, Hail, sleet) and amount of precipitation.
• It is a regulator of the heat on the earth’s surface. Water vapor is almost transparent for shortwave solar radiation and opaque for longwave terrestrial radiation. Thus, the sun’s rays reach the earth’s surface and terrestrial radiation is not allowed to go back to space. Hence, the earth’s surface and the lower atmosphere are heated up by the greenhouse effect.
• It is also the source of energy that helps in heating the atmosphere to some extent. After the condensation of vapor, latent heat is released into the atmosphere. This latent heat is also steering the storms and thunderstorms in the atmosphere. It also has a great effect on the stability of the atmosphere.
• Water vapors also affects the rate of cooling of the human body. The amount of vapor in the atmosphere is inversely related to the rate of the cooling effect of the body.

The word ‘Atmosphere’ has been derived from the Greek word atmos, which means “vapor”. In this way, if the atmosphere is called a “region of vapor”, then it will not be an exaggeration.

Scientists estimated that the atmosphere receives 16 million tons of water vapor per second from the ground level. If the entire humidity of the atmosphere condenses and falls on the ground in the form of rain, then it will be equal to 2.5 cm of rain on the entire ground.

Table: 1.2 Share of Variable Gases or Particles in the Atmosphere (Composition of the Atmosphere)

Composition of the Atmosphere: Dust Particles

In addition to gas or water vapor, all the solid substances present in the form of particles in the atmosphere are called dust particles. These solid particles may be in the form of flower pollen, fine dust coming out of the wick of the fire, or particles of sand blown by the desert wind. While sitting in our room, we usually do not see dust particles in the air, but when sunray comes through a window, then we see innumerable microscopic dust particles floating in the air. Many particles are not visible even with a microscope.

Like the vapor, dust particles are also variable in both horizontal and vertical distribution. They are found in larger numbers in a desert environment and lower in a humid environment. These particles are added to the atmosphere by volcanic activity, cyclonic storms, breaking of salt, ploughing of land, etc. The presence of dust particles in the atmosphere is very crucial because they affect the weather conditions in the following ways.

• Dust particles scatter the radiation from the sun in the atmosphere. The varied colors i.e., red and orange at sunset and sunrise are only because of the scattering of selective sunlight. The blue color of the sky is the result of the scattering of blue light in the atmosphere. The unusual amount of dust particles especially over town and urban centers reduce the insolation on the earth’s surface due to the scattering of sunlight in the atmosphere.
• They help in the process of condensation of water vapor in the atmosphere. Dust particles like salt particles act as hygroscopic nuclei around which the condensation of water vapor takes place. In the absence of dust particles, there would have been no formed precipitation and snow on the earth’s surface.

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