Classification of Air Pollutants
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The air pollutants can classify on the following basis:
Rating Based on Origin
According to the origin, air pollutants can organise into Primary air pollutants and Secondary air pollutants.
Primary air pollutants
Primary air pollutants are emitted directly from the sources and found in the atmospheres in the form in which they emanated.
For Example SO2, NO2, HC, ash, smoke, dust, mist etc.
Secondary air pollutants
The secondary air pollutant is formed in the atmosphere by chemical interactions between primary contaminants and atmospheric constituents.
For Example Ozone, Sulphur trioxide, PAN, aldehydes, ketones etc.
Classification based on the state of matter
According to the state in which air pollutants found in the atmosphere, they classified as Gaseous air pollutants
Particulate air pollutants Gaseous air pollutants:
Gaseous air pollutants are those air pollutants which are located in the gaseous state at standard temperature and pressure in the atmosphere.
For Example SO2, NO2, HC, CO, CO2etc.
Particulate Air pollutants
These include suspended droplets, solid particles or their mixtures in the atmosphere, commonly referred as particulates.
For Example aerosols, dust, smoke, fumes, mist, fog, fly ash, soot, and natural particulates such as pollen grains, protozoa, fungal spores and volcanic soil.
Air pollutants – Sources and Effects
Carbon monoxide (CO)
It is a colourless, odourless, tasteless gas. Moreover, It does not affect the average concentration (0.1ppm) but at higher conc. It severely affects the human metabolism.
Natural sources such as Volcanic eruption, natural gas emissions, electrical discharge during cloud forming, marsh gas production etc. contribute a small amount of CO in the atmosphere.
Transportation sources contribute about 64% of CO in air.
Forest fire and agricultural burning contribute about 17 % off in the air.
Industrial processes such as electric furnace and blast furnaces in iron and steel industries, Petroleum refining, Paper industry, Gas manufacture, Coal mining etc.
It reduces the oxygen-carrying capacity of the blood by selectively combining with haemoglobin forming carboxyhemoglobin. This causes giddiness, laziness, and exhaustion.
It reduces vision and causes cardiovascular disorders.
CO is a very dangerous asphyxiant, and its high levels are fatal to human life.
It may cause coma, respiratory failure, and even death.
Carbon Dioxide (CO2)
The content of carbon dioxide in the air has increased by approximately 15% during the last century in spite of the fact that photosynthesising green plants balance the CO2 – O2 ratio to a large extent.
- Fossil fuel burning
- Agricultural practices (e.g. Deforestation)
- Respiration process
It is the primary greenhouse gas responsible for the rise in an average temperature of the atmosphere. CO2 is less dangerous than CO and cause nausea and headache.
Oxides of Nitrogen (NOx)
NOx group include six different oxides of nitrogen (NO, NO2, N2O, N2O3, N2O4, N2O5). Nitric oxide and nitrogen oxide are the critical pollutants. NO is colourless, odourless gas but is NO2 reddish brown and have suffocating odour.
You can see the Nitrogen cycle for further details.
- Fuel combustion in automobiles and industries.
- Forest fire
- Bacterial decomposition of organic matter
- Nitric oxide combines with haemoglobin and reduces the oxygen-carrying capacity of blood.
- NO2 is more toxic than NO and may affect lungs and cause bronchitis.
- NO2 is reacting with atmospheric moisture to form nitric acid causes acid rain and affects vegetables and metals.
Oxides of sulfur (SOx)
SOx include sulfur dioxide and sulfur trioxide. SO2 is the colourless gas having the pungent and suffocating odour.
- Most of the SOx pollution (67%) due to volcanic activities and other natural sources.
- Burning of solid and fossil fuels
- Industries like paper mfg. plants, refineries, sulphuric acid plant
- Open burning of refuse and municipal incinerator.
It causes cardiac diseases, asthma, bronchitis, eye irritation, throat troubles etc.
Long-term exposures to high levels of sulfur dioxide gas cause respiratory illness and heart diseases.
Oxides of sulfur attacks building materials especially marbles and limestone. (eg. Taj Mahal at Agra)
SO2 reacts with moisture in the atmosphere to form sulphuric acid which causes acid rain affects vegetables and metals.
Oxides of sulfur may affect clothes, leather, paper, and plants.
The gaseous and volatile hydrocarbons are mainly responsible for air pollution. Familiar HC includes methane, ethane, acetylene etc.
- Natural fires
- Incomplete combustion from car engines
- Industrial sources (refineries)
- Forest fires
- Agricultural burning
- Coal waste fires
Some aromatic HC may cause cancer.
Unburned HC with oxides of nitrogen in the presence of sunlight from Photochemical oxidants (like ozone, PAN) which can have adverse effects on humans and plants.
The primary photochemical oxidant is ozone. Ozone is produced in the upper atmosphere by the solar reaction; small concentrations of this gas diffuse downwards and become the primary concern in the air pollution.
In the presence of sunlight, the oxides of nitrogen react with the unburned HC released by the exhausts of automobiles and following a series of complex reactions produce secondary pollutants like PAN, Ozone, aldehydes, and Ketones etc.
Unburnt Hydrocarbon + NOx Abundant sunlight Photochemical smog
- Automobile exhausts
Photochemical oxidants irritate eye, nose, and throat, headache etc.
Ozone damage chromosomes.
Ozone and PAN cause damage to plants by interfering with plant cell metabolism, especially in leafy vegetables. Premature fall and yellowing of leaves are due to this pollutant.
Photochemical oxidants also affect the materials like rubber, plants, textile fibres etc.
Particulate Air Pollutants
These are small, solid particles and liquid droplets present in the atmosphere in reasonably large numbers and sometimes pose serious air pollution problems. The size of particulate ranges from 0.02 µ to 500 µ.
Aerosols: These include all airborne suspensions of solid or liquid particles smaller than 1 mm.
Dust: It consisted of small solid particles (size 1 to 200 µm) and generated by material crushing, grinding or blasting.
Smoke: Moreover, It consists of fine solid particles (size 0.1 to 1 µm) resulting from the incomplete combustion of organic particles like coal, wood, tobacco or other chemical processes.
Fumes: These are fine solid particles (size 0.1 to 1 µm) formed by the condensation of vapours or solids materials.
Mist: Moreover, It consists of liquid droplets formed by the condensation of vapours in the atmosphere or released from an industrial operation.
Fog: It the mist is made up of water droplets whose concentration is high and dense enough to obscure vision then the fog is called fog.
Fly ash: This consists of finely divided non-combustible particles present in the gases arising from fuel combustion.
Soot: Incomplete combustion of carbon-containing materials release carbon particles.
Natural particulates: Natural particulates are pollen grains, spores bacteria viruses, protozoa, fungal spores and volcanic dust.
- Volcanic eruption
- Dust storms
- Spraying of salts by oceans
- Fly ash from the combustion of fossil fuels
- Smoke from vehicles
- Agriculture burning
Health effects – Particulates less than 10µm can enter deep into the lungs and may also get into bloodstreams. It can cause problems like irritation, aggravated asthma, chronic bronchitis and other lung diseases.
- Particulates accelerate corrosion of metals, damages buildings, paints etc.
- Aerosols (Fluorocarbons, NOx, SOx) released from emissions from the jet, and supersonic planes deplete the ozone layer in the higher atmosphere.
- Moreover, A dust coating on leaves affects photosynthesis and reduces plant growth.
- Fly ash reduces pH balance and potability of water. Metal dust containing heavy metals and cotton dust may also cause the respiratory problem.