NEET Chemistry Notes Environmental Chemistry – Air Pollution
It occurs when the concentration of a normal component of the air or a new chemical substance added or formed in air, build up to undesirable proportions causing harm to humans, animals, vegetation and materials. The chemical substances and particles causing pollution are called air pollutants.
Structure of Atmosphere
The lowest region, called the troposphere extends up to the height of —10 km from sea level. Above troposphere, between 10 and 50 km above sea level, lies the stratosphere. Troposphere contains about 80% of the total mass of air and water vapours while stratosphere contains nitrogen, oxygen and ozone.
Mesosphere extends 50-85 km from earth surface. N2 and 02 are present in low concentration in this region.
Thermosphere extends between 85-500 km from earth surface and in it temperature rises to 1200°C.
The outermost part of atmosphere is exosphere and unbounded area beyond exosphere is known as inter-stellar space.
Mesosphere and thermosphere are collectively known as ionosphere.
Sources of Air Pollution
Sources of air pollution are:
- Burning of fossil fuels such as coal, wood and oil.
- Exhaust gases emitted by internal combustion engines of vehicles.
- Chemical industries and their released products.
Consequences of Air Pollution
Greenhouse Effect and Global Warming
The phenomenon in which atmosphere of earth traps the heat coming from the sun and prevents it from escaping into the outer space is called green house effect. Certain gases, called green house gases (carbon dioxide, methane, ozone, chlorofluoro carbon compounds (CFCs) and water vapour] in the atmosphere absorb the heat given by earth and radiate back it to the surface of the earth.
Thus, warming of the earth led to the warming of air due to greenhouse gases, which is called global warming.
Consequences of Greenhouse Effect (or Global Warming)
The greenhouse gases are useful in keeping the earth warm with an average temperature of about 15° to 20°C. There may be less rainfall in this temperature zone and more rainfall in the dried areas of the world. Increase in the concentration of C02 in the atmosphere leads to increase in the temperature of the earth’s surface. Resulting in melting of glaciers and polar ice caps and hence, level of sea water may rise.
The pH of normal rain water is 5.6 due to the dissolution of C02 from atmosphere.
When the pH of rain water drops below 5ppm, it is called acid rain (by Robert Augus.) Oxides of N and S are responsible for making rain water acidic. Much of the NOx and SOx entering in the atmosphere are converted into HN03 and H2S04 respectively. The detailed photochemical reactions occurring in the atmosphere are given as:
HN03 is removed as a precipitate or as particulate nitrates after reaction with bases (like NH3, particulate lime etc).
The presence of hydrocarbons and NOx step up the oxidation rate of the reaction. Soot particles are also known to be strongly involved in catalysing the oxidation of S02.
Acid rain causes extensive damage to building and sculptural materials of marble, limestone, slate, mortar etc.