14.3 Pollutants

A pollutant is a waste material that pollutes air, water or soil. Three factors determine the severity of a pollutant, its chemical nature, concentration and persistence. These pollutants are being created and discharged to the environment by human activities. They make the environment (air, water or soil) harmful to life. So the pollutants are those substances which cause pollution. While contaminants are those substances that make something impure.

The harmful substances present in air are called air pollutants. Even a beneficial substance beyond a specific concentration may be harmful. Air pollutants change the weather, badly affect the human health, damage the plants and destroy buildings.

14.3.1 Types of Pollutants

Major air pollutants are classified as primary pollutants and secondary pollutants. Primary pollutants are the waste or exhaust products driven out because of combustion of fossil fuels and organic matter. These are oxides of sulphur (SO₂ and SO₃); oxides of carbon (CO₂ and CO); oxides of nitrogen (specially nitric oxide NO); hydrocarbon (CH₄); ammonia and compounds of fluorine. Secondary pollutants are produced by various reactions of primary pollutants. These are sulphuric acid, carbonic acid, nitric acid, hydrofluoric acid, ozone and peroxy acetyl nitrate (PAN).

14.3.2 Sources of Air Pollutants

As you know $99\%$ of atmosphere consists of $\mathsf{N}_{2}$ and $\mathsf{O}_{2}$ . Although, other gases are minor constituents, they can have major effects on our environment. Because atmosphere determines the environment in which we live. So, these minor constituents are safe upto a concentration limit. But in some areas this limit has been crossed considerably during the last 60 years because of human activities. Different sources of air pollutants are described as:

(i) Oxides of Carbon (CO₂ and CO)

Sources of oxides of carbon are as follows:

Both of these gases are emitted due to volcanic eruption and decomposition of organic matter naturally. However, the major source for the emission of these gases is the combustion of fossil fuels (coal, petroleum and natural gas). Fossil fuels burnt in combustion engine of any type of automobile, kiln of any industry or open air fires emit ${\mathrm{C}\mathrm{O}}_2$$\mathrm{CO}_{2}$ and CO.

Forest fires and burning of wood also emit ${\mathrm{C}\mathrm{O}}_2$$\mathrm{CO}_{2}$ and $\mathrm{C}\mathrm{O}$$\mathrm{CO}$ . Especially, when supply of oxygen is limited, emission of $\mathrm{C}\mathrm{O}$$\mathrm{CO}$ dominates.

Greenhouse Effect and Global Warming

The ${\mathrm{C}\mathrm{O}}_2$$\mathrm{CO}_{2}$ forms a layer around the Earth like an envelope. It allows the heat rays of the Sun to pass through it and reach upto the Earth. These rays are reflected from the Earth surface and go back to upper atmosphere as shown in figure 14.3(a). Normal concentration of ${\mathrm{C}\mathrm{O}}_2$$\mathrm{CO}_{2}$ layer retains enough heat to keep the atmosphere warm. So, normal concentration of ${\mathrm{C}\mathrm{O}}_2$$\mathrm{CO}_{2}$ is necessary and beneficial for keeping the temperature warm. Otherwise, the Earth would have been uninhabitable. The Earth's average temperature would be about $-{20}^{\circ }\mathrm{C}$$- 20^{\circ}\mathrm{C}$ , rather than presently average temperature $15^{\circ}C$ . ${\mathrm{C}\mathrm{O}}_2$$\mathrm{CO}_{2}$ is not an air pollutant. Rather, it is an essential gas for plants as ${\mathrm{O}}_2$$\mathrm{O}_{2}$ is essential for animals. Plants consume ${\mathrm{C}\mathrm{O}}_2$$\mathrm{CO}_{2}$ in photosynthesis process and produce ${\mathrm{O}}_2$$\mathrm{O}_{2}$ .

While animals use ${\mathrm{O}}_2$$\mathrm{O}_{2}$ in respiration and give out ${\mathrm{C}\mathrm{O}}_2$$\mathrm{CO}_{2}$ . In this way, a natural balance exists between these essential gases as represented here.

But this balance is being disturbed by emitting more and more ${\mathrm{C}\mathrm{O}}_2$$\mathrm{CO}_{2}$ in air through different human activities.

Although, $\mathbf{CO}_{2}$ is not a poisonous gas, yet its increasing concentration due to burning of fossil fuels in different human activities is alarming. $\mathbf{CO}_{2}$ in the atmosphere acts like a glass wall of a green house. It allows UV radiations to pass through it but does not allow the IR radiations to pass through it. It traps some of the infrared radiations emitted by the Earth.

Hence, increased concentration of $\mathbf{CO}_{2}$ layer absorbs the infrared radiations emitted by the Earth's surface that prevents heat energy escaping from the atmosphere.

It helps to stop surface from cooling down during night. As the concentration of $\mathbf{CO}_{2}$ in air increases, less heat energy is lost from the surface of the Earth. Therefore, the average temperature of the surface gradually increases. This is called greenhouse effect as shown in figure 14.3(b). This effect is proportional to amount of $\mathbf{CO}_{2}$ in air. Greater is amount of $\mathbf{CO}_{2}$ , more is trapping of heat or warming. Due to increased warming this phenomenon is also called global warming.

Effects of global warming