Monday, March 13, 2017

Sources, classification and effects of air pollutants

Air pollution
Air pollution is defined as the presence of any undesirable substance in the environment in an excessive concentration for an excess duration that causes damage to the living and non-living components of the environment.

The undesirable substance is termed as "air pollutant". The source of air pollution is any activity that causes pollutants to be emitted in the air. Natural sources of air pollution are called "biogenic" sources while pollutants generated due to human activities are called "anthropogenic" sources.

Air pollutants can be classified in the following ways:

  • Based on the source of pollutants, air pollution is classified as:
    • Natural air pollution. Examples are
      • Forest fires
      • Volcanic eruptions
      • Biological decay
      • Disintegration of rocks
    • Artificial air pollution. Examples are
      • Vehicular pollution
      • Cooking (domestic and industrial) 
      • Industries
  • Based on movement of air polluting source, air pollution is classified as
    • Stationary source
      • Stationary sources are inturn classified as
        • Point sources. Examples are
          • Industrial processes
          • Power plants
          • Volcano
        • Area sources. Examples are
          • Domestic heating of coal and gas
          • Onsite incineration
    • Mobile source
      • Mobile sources are inturn classified as 
        • Line sources. Examples are
          • Highway vehicles
          • Railways
        • Area sources. Examples are
          • Railway station
          • Bus stand
          • Airports
The term used to describe a variety of vehicles, engines and equipment that generate air pollution and can be moved from place to place is Mobile sources
Mobile sources are in-turn classified as 
  • On-road sources and
  • Non-road sources
On road sources are used on road for transportation of passengers or freight. Examples are:
  • LDVs (Light Duty Vehicle or passenger car)
  • HDVs (Heavy Duty Vehicle used for transportation of freight) and
  • Motorcycles
Non-road sources include gasoline and diesel powered vehicles, engines and equipment used for construction, agriculture, transportation, recreation and several other purposes.
Mobile sources contribute to air pollution by way of combustion and evaporation of fuel.
Petrol and diesel are mixtures of hydrocarbons. Complete combustion of fuel gives carbondioxide and water while incomplete combustion (which occurs mostly in every engine) yields unburnt hydrocarbons, oxides of nitrogen, carbon monoxide, carbon dioxide and water.

Air pollution is also classified based on state of pollutant as
  • Gaseous pollutant. Examples are
    • Carbon monoxide (CO)
    • Oxides of Nitrogen (NOx)
    • Oxides of sulphur (SOx), etc
  • Particulate pollutant. Examples are
    •  Fumes
    • Mist
    • Fog
    • Smoke
    • Dust, etc
Gaseous pollutants mix with air and normally do not settle. Particulate pollutants consist of finely divided solid or liquid particles and often exist in colloidal state as aerosols.

 Based on Origin, air pollutants are classified as primary or secondary pollutants.
  • Examples of primary air pollutants are
    • Carbon monoxide (CO)
    • Carbon dioxide (CO2)
    • Sulphur dioxide (SO2)
    • Suspended Particulate Matter (SPM) and
    • Halogen compounds
  • Examples of secondary air pollutants are
    • Ozone (O3)
    • Peroxy Acetyl Nitrate (PAN)
    • Smog
    • Formaldehyde, etc
Primary pollutants are emitted from identifiable sources while secondary pollutants are produced by reaction between two or more primary air pollutants or between primary air pollutants and constituents of the atmosphere with or without sunlight as a catalyst.

Effects of air pollutants:
  • Effects of air pollutants on human beings
    • Carbondioxide is the primary pollutant responsible for global warming
    • Carbonmonoxide causes asphyxiation (breathlessness) and in high doses forms carboxyhaemoglobin that results in unconsciousness, coma and death.
    • Other pollutants irritate the respiratory tract, cause allergic reactions, lead to eye-watering, poor visibility
    •  Sulphur dioxide (SO2) is found in association with particulate matter and is mainly released into the air by combustion of sulphur containing fuels. It causes significant respiratory damage.
    • Oxides of Nitrogen (NOx) originate from both natural and anthropogenic sources. It is produced by burning fuel in automobiles and power plants. It affects the respiratory system of human beings. It causes pneumonia and bronchitis.
    • Hydrocarbons (HC) are found in fossil fuels. They cause eye irritation, coughing and drowsiness. Hydrocarbons with high molecular weight cause mutagenic or carcinogenic problems
    • Presence of excess lead causes behaviour disorders in children and adults. In high concentrations it causes coma, severe and irreversible brain damage. Lead poisoning weakens the central nervous system resulting in convulsions, uncontrolled mental disturbances, coma and eventually death.
    • Cadmium is present in urban atmosphere and cigarette smoke. It causes cardiovascular diseases, hypertension, kidney and liver failure.
    • Mercury present in the gaseous form causes neurological damage, birth defects, damage to the cerebellum and cortex.
  • Effects of air pollutants on materials
    • Air pollutants like NO2, CO, SO2, acids and aerosols cause discoloration and deterioration of limestone and building materials.
    • SO2 in the form of acid (dry deposition) or gas reduces the tensile strength of textiles
    • Ozone (O3) and other oxidants are responsible for cracking and loss of strength of rubber
    • CO, O3, SO2, H2S and SPM cause discolouration, surface erosion and soiling of paints
    • SO2, acids and gases cause embrittlement and discolouration of paper
    • NO2, SO2, acids and gases are responsible for corrosion, tarnishing and loss of strength in metals
    • SO2 and acid gases are responsible for disintegration and loss of strength of leather.
  • Effects of air pollutants on vegetation
    •  SO2 causes chlorosis (disappearance of chlorophyll and yellowing of leaves)
    • NO2 causes premature fall of leaves and suppressed growth of plants resulting in reduced yield
    • Ozone causes necrosis (dead areas on leaves), leaf damage and reduced yield
    • PeroxyAcetylNitrate(PAN) causes premature fall of leaves, discoloration and epinasty (downward curvature of leaves)
  • Effects of air pollutants on animals
    • Animals ingest fluorides in air through plants. This affects their bones and a reduced production of milk
    • Lead particulates in air lead to lead poisoning in animals
    • Extreme air pollution leads to paralysis of animals.
  • Effects of air pollutants on physical properties of atmosphere
    • Air pollution leads to 
      • Decrease in visibility
      • Reduction of solar radiation
      • Affects weather conditions and
      • Affects the atmospheric composition

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