VPE 511: Environment and Environmental Hygiene (2+1)

GLOBAL AIR POLLUTION

Global effect of air pollution

• Troposphere and stratospheric ozone depletion, aerosol scattering and absorption of solar and terrestrial radiation, green-house gas warming, rain and precipitation quality, long-range transport of air-pollutants, heat-islands and urban air quality are known to be main global air pollution problems.
• The environment in which we live is unfortunately the host medium for air pollutants. Man is the sole culprit in polluting the air and he alone should take the responsibility to clean the atmosphere and protect the environment.

Green House Effect (GHE)

• Solar energy is in the form of light radiation has wavelength in the range of 0.2 to 4 µm. It will lose solar energy after striking the earth and will be converted to heat energy of longer wavelengths. Thus the wavelength of this terrestrial reradiation, from earth to atmosphere is more (4-100um).
• After striking earth since energy decrease, the wavelength increases. Carbon dioxide has radiation absorption bands in the range of 12-18 mm wavelengths. Thus if CO₂ is present in the atmosphere, it allows the incoming solar radiation to pass through. This is the origin of the term ‘ Green House Effect’ since the glass in a green house also is transparent to short wavelength solar radiation and absorb the long wavelength radiation, emitted from inside the green-house. CO₂, water vapour, methane N₂O, ozone and CFCs cause similar effects and hence are called green house gases. As these green house gas molecules absorb energy, their temperature increases and they themselves start radiating heat. Only part of it escapes out into the space while the remaining is radiated back to earth further increasing its temperature.
• Green house effect is essential for mankind and life. But man’s activities are accelerating or enhancing the warming process to cause concern.

Sources of Green Houses Gasses

• Main source of GHE is carbon dioxide, contributes about 50% . Presently more than 10,000 million tons of carbon dioxide is released into the atmosphere every year by industries. USA alone produces about 2500 million tones of CO₂ per year.
• The carbon dioxide concentration in the pre-industrial era was 265-290 ppm and today it is about 350 ppm.

Heat Islands

• The heat energy release causes a significant climate change in cities and may result in global climate effects. Because of the thermal capacitance of the streets, buildings and industries for solar input and because of energy dissipation, cities are warmer than their rural surroundings.
• A difference of 6⁰C is common is urbanized and industrialized cities. Hence become heat island.

Acid Rains

• Thermal power plants, industries and other sources release thousand tones of oxides of nitrogen and sulphur into the atmosphere everyday. These gases undergo transformation in the atmosphere from nitrates, sulphates, nitric acid or sulphuric acid droplets. Some of these pollutants can travel 200-300 km in a day. Thus compound emitted in one place may cause effect of concern on another place.
• Rain in the purest source of water ‘Acid Rain’ means any precipitation-rain snow, or dew, which is more acidic than normal. Acid rain in general has the pH less than 5.6. In developed countries acid rains with pH <4.5 are common.

Ozone Holes

• Earth has a protective unmbrella in the form of Ozone Layer of 24 km thickness in the stratosphere about 15 km away from the earth’s surface . The concentration of ozone in stratosphere is about 10 ppm. It is essential for the life to sustain on earth. It absorb the dangerous ultraviolet radiation (UV-β rays with wavelengths from 200-280 nm) from the sun and convert it to heat and chemical energy. It is this activity that is responsible for the rise in temperature. The layer is highest at equator and lowest at the poles.
• In nature , ozone is continuously formed and destroyed through photochemical interaction and equilibrium in ozone concentration is ensured. However this equilibrium is upset due to the discharge of anthropogenic air pollutants such as Chloro Fluoro Carbons (CFSs) into the atmosphere. The CFCs release free radicals of chlorine, fluorine and bromine, which destroy the stratospheric ozone as a result of which the ozone layer is thinned. The patches of thinned ozone layers are known as “Ozone Holes”. By definition the ozone hole represents only a depletion of ozone concentration but not an empty space in the atmosphere.

Indoor Air pollution

• As civilization improved man has constrained his life to indoors and started living more and more indoor within the structures built by him. This led to the sever problem of “ Indoor Pollution”. Man and several thousands of household products made by him pose a serious threat to indoor air quality. These products include cleaners, detergents, paints, air fresheners, etc. Indoor air pollution often leads to severe health hazards like irritation to eyes, nose, throat, headache and respiratory problems. It is proved that indoor pollution increases the tension and decreases the productivity of workers.