Environmental Engg.

Lesson-28 Introduction to air pollution

Introduction to air pollution

As we all know, air is the most essential for our living.  A person cannot survive for five minutes without air.  The important life controlling element, viz. air, sometimes becomes an enemy when it gets polluted.  Air pollution is nothing but a system where presence of any substance (solid, liquid or gas) in the atmosphere in such a concentration that may or may tend to cause injuries to human, crops or property and to the atmosphere itself. The substances which cause air pollution are called as air pollutants. 

The polluted air affects not only living beings but also non living things and has a deleterious effect on mankind. Air pollution is a serious problem in many countries of the world.  Centre for Science and Environment (CSE) has observed that air pollution is the fifth leading cause of death in India after high blood pressure, indoor air pollution, tobacco smoking and poor nutrition, with about 620,000 premature deaths occurring from air pollution-related diseases. Like China, India faces an unprecedented public health crisis due to air pollution.  Half of the urban population breathes air laced with particulate pollution that has exceeded the safety standards. As much as one third of urban population is exposed to critical level of particulate pollution. Smaller cities are among the most polluted in the country.

The green think tank released its own assessment and the global study's India specific data during February 2013 warning that the number of premature deaths due to air pollution had increased six fold over the last 10 years.

Air quality data generated by the Central Pollution Control Board (CPCB) for 2007 under the National Air Quality Monitoring Programme (NAMP) presents deadly facts about air pollution levels in Indian cities. CSE has analysed the official data to assess the state of air quality and trend in Indian cities. The most widely monitored pollutants in India are particulate matter (PM), nitrogen dioxide (NO₂), sulphur dioxide (SO₂), and on a limited scale carbon monoxide.  Some of the worst forms of air pollutions are found in Indian cities. The Central Pollution Control Board (CPCB) considers air to be ‘clean’ if the levels are below 50 per cent of the prescribed standards for pollutants. During 2007 only 2 per cent cities have low air pollution on the basis of PM₁₀. In about 80 per cent of cities (of a total of 127 cities/towns monitored under the NAMP) at least one criteria pollutant exceeded the annual average ambient air quality standards. This has serious public health implications. There are very few cities, which can be termed clean keeping PM₁₀ levels (respirable particulates) as criteria however over the years SO₂ levels have fallen sharply in many cities but the NO₂ levels are increasing in many cities.

Issues of concern in air pollution

Global warming

In past few decades, there has been a large amount of hue and cry regarding the issue of earth getting hotter and hotter year after year.   This concept of global warming is predominantly because of major changes in the human lifestyle.   Generally, green house gases viz., Carbon dioxide, methane, etc. in the lower atmosphere act as a shield in trapping some of the heat, as it radiates back to the atmosphere from the Earth.  Such gases because of their heat and warmth have made survival for life on our mother Earth.   But when the quantities of these green house gases increases, excessive heat is generated and makes living almost impossible. 

The reasons for increase in quantity of green house gases :

• Continuous and excessive burning of fossil fuels increases carbon dioxide level

• Excessive deforestation leads to increase in carbon dioxide level

• Large scale decomposition of organic matter in swamps, rice fields, live stock yards has lead to increase in methane level

• Level of Chloro Flouro Carbons (CFCs) increasing due to change in human life style

Acid rain

Rainfall through unpolluted environment is slightly acidic with its pH value ranging just less than 7.  However, when the environment is polluted with primary pollutants like SO_x and NO_x gases, the resultant precipitation tends become more acidic.  This acidity if because of the formation of secondary pollutants like sulphuric acid and nitric acid due to the reaction of water vapour with SO_x and NO_x gases.  It has been specified that when the pH value of rainfall is less than 5.6 or below, then the rain is specifically termed as acidic.

Photochemical smog

Photochemical smog is a unique type of air pollution which is caused by reactions between sunlight and pollutants like hydrocarbons and nitrogen dioxide. Although photochemical smog is often invisible, it can be extremely harmful, leading to irritations of the respiratory tract and eyes. In regions of the world with high concentrations of photochemical smog, elevated rates of death and respiratory illnesses have been observed.

Ozone depletion

Ozone depletion is the most dreaded aspect of air pollution, having wide spread implications, extending over the entire atmosphere.  This problem is caused by the reduction of naturally available ozone layer in the atmosphere.   Scientists are also becoming aware of the possible connection between ozone depletion and climate change due to global warming.  The increased concentrations of green house gases leading to warmer climate at the Earth’s surface, infact causes cooling effect at altitudes where the ozone layer is found.  This cooling of the stratosphere enhances the chemical reaction that destroys the ozone, leading to further ozone depletion.

Chlorofluoro carbons (CFCs) are the chief agents of ozone destruction.  They are largely used and subsequently released in modern world, in refrigeration, air-conditioning, fire extinguishers.  These inert CFCs do not disintegrate in the lower atmosphere and do rise several kilometers up into the stratosphere, where they release chlorine atoms in the presence of sunlight.  Each chlorine atom from CFCs then reacts with an ozone molecule forming Chlorine monoxide.  The Chlorine monoxide thus formed, reacts with another oxygen atom to form a new oxygen molecule and a chlorine atom.  The chlorine atoms, thus replenished can go on to break apart thousands and thousands of more ozone molecules, leading to large scale ozone deficits.  It has been estimated that each atom of chlorine can destroy up to 1 lakh ozone molecules at a faster  rate than the gas is replenished naturally.

Types of air pollution

Personal air exposure

It refers to exposure to dust, fumes and gases to which an individual exposes himself when he indulge himself in smoking 

Occupational air exposure

It represents the type of exposure of individuals to potentially harmful concentration of aerosols, vapors, and gases in their working environment. 

Community air exposure

This is most serious, complex, consists of varieties of assortment of pollution sources, meteorological factors, and wide variety of adverse social, economical, and health effects.

• Natural Sources –Volcano, forest fire, dust storms, oceans, plants and trees

• Anthropogenic Sources - created by human beings

Classification of sources of Air pollution

Air pollutants may be classified by sources as stationary or mobile

-Stationary sources

• Point sources (Industrial processing, power plants, fuels combustion etc.)

• Area sources (Residential heating coal gas oil, on site incineration, open burning etc.)

- Mobile sources

• Line sources (Highway vehicles, railroad locomotives, channel vessels etc.)