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Read this essay to learn about the air pollutants:- 1. Meaning of Air Pollutants and 2. Classification of Air Pollutants.
Essay # Meaning of Air Pollutants:
Particulate matter (both liquid and solid) and some gases, which when present in air in large enough quantities so as to cause deleterious effects on living beings directly or indirectly or bring about undesirable changes in the surroundings are termed as air pollutants.
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The list of air pollutants is a long one. It is becoming longer as newer chemicals are being commercially produced and we are becoming more knowledgeable about their harmful physicochemical properties.
Some of the pollutants are produced by nature and some are produced as a result of domestic, agricultural and industrial activities of man. Such pollutants are referred to as primary pollutants. Some of the primary pollutants when present in the atmosphere react among themselves and thereby produce some other harmful chemicals. These chemicals are termed as secondary pollutants.
Essay # Classification of Air Pollutants:
Air Pollutants may be classified into two groups depending on their physical state, namely, particulate and gaseous. The particulate pollutants are further classified as viable and non-viable. Based on their size both the viable and non-viable particulate pollutants are further sub- classified as detailed hereunder.
1. Particulate Pollutants:
a. Viable Particulate Pollutants:
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The largest sized air-borne viable particulate pollutants are plant fragments, seeds and micro-fauna, which are larger than 100 um in size. The smallest sized are viruses having diameter as small as 0.015 um. Viable particles having intermediate size are bacteria, fungi, algae, protozoa, spores and pollen grains. The larger viable particles are of natural origin.
But the presence of microbes and their density in ambient air depend on the environmental condition created by man. Ambient air in the vicinity of land and water bodies having putrefying organic matter would have a higher density of microbial population
b. Non-Viable Particulate Pollutants:
The non-viable particulate pollutants may be either liquid or solid.
The terminology is normally associated with non-viable particles present in air and there connotations are as follows:
Fog denotes a high concentration of very fine water droplets suspended in air, resulting from condensation of moisture in air due to fall in air temperature at night. Mist and Sprays may result from natural processes as well as due to industrial operations. These are liquid droplets having diameter in the range of 0.1 to 5000 pm dispersed in air.
Gas emitted from gas-liquid contacting devices often carries entrained liquid droplets which are referred to as mist. Spraying devices produce fine liquid droplets, some of which may escape to the atmosphere unless steps are taken to prevent their escape. The term Haze signifies the presence of fine dust, mist and gaseous pollutants in the atmosphere which reduce visibility.
Smoke is produced as a result of incomplete combustion of organics (solid, liquid and gas). It contains fine solid particles (un-burnt) suspended in the flue gas. The particles are rather small in size, ranging from less than 0.01 pm to 1 pm diameter.
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Smog is a mixture of smoke and fog. It is more stable than fog.
Dusts are air-borne fine solid particles. Industrial operations such as size reduction, detonation, pneumatic conveying, screening, loading and unloading of granular and powdery materials and polishing produce particles in the size range of 1 to 1000 pm. Dust particles do not diffuse in air but settle out due to gravity.
Fly ash is entrained ash particles in a flue gas resulting from combustion of powdered solid fuels. Fly ash particles may be finer than 1 um. Fumes are finely divided solid particles that result from condensation of a vapour. Fumes are generally produced during sublimation, distillation and molten metal processing. The particle sizes of fumes range from 0.1 to 1 um.
Aerosols are small particles, either solid or liquid, suspended in a gas. The aerosol particle size ranges from 0.01 to 100 um. They are of two types, namely dispersion aerosol and condensation aerosol. Dispersion aerosol is formed during operations like grinding, solid-liquid atomization and conveying of powders. The particles are coarse and may be individual or slightly aggregated.
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Condensation aerosol is produced when supersaturated vapors condense or when air-borne gases react chemically to form non-viable particles. Individual particles may be less than 1 pm in size. A large number of these particles may combine to form loose aggregates.
2. Gaseous Pollutants:
Gaseous pollutants are classified as primary and secondary pollutants. Primary pollutants are those, which are directly emitted from natural sources or are produced as a result of human activity. Some of the common pollutants of this group are Carbon monoxide (CO), Carbon dioxide (CO2), Sulphur dioxide (SO2), Hydrogen sulphide (H2S), Nitrogen oxides (NO, NO2, N2O), Ammonia (NH3), hydrocarbons and other volatile organic compounds.
Secondary pollutants are those, which are produced as a result of interaction between the primary pollutants under favourable atmospheric conditions. Some of the common pollutants, which belong to this group, are Ozone (O3), Hydrogen peroxide (H2O2), Peroxyacetyl nitrate (PAN), aldehydes (such as formaldehyde) and organic hydro peroxides.
