What are the Sources of air pollutants?

Air pollutants can be either natural or may be the result of various human activities such as domestic, industrial and agricultural.

Natural Sources

Air contains natural contaminants such as:

(i) pollen grains

(ii) fungal spores ,

(iii) salt spray

(iv) bacteria

(v) smoke and dust particles from forest fires

(vi) volcanic eruptions

(vii) wind-blown dust

(viii) naturally occurring carbon monoxide from the breakdown of methane

(ix) hydrogen sulphide and methane from the anaerobic degradation of organic matter.

Anthropogenic (Man-made) Sources

In contrast to the natural sources of air pollution, there are contaminants of anthropogenic origin. The sources of these pollutants are numerous and are classified as follows:

1. Mobile sources

(a) Line sources

For example, highway vehicles, aircraft, railroads, ships.

(b) Area sources

For example, airports, rail yards, harbour.

2. Stationary combustion

(а) Point sources

For example, thermal power plants, and fuel combustion such as oil, gas and coal.

(б) Area sources

For example, residential, institutional and commercial heating

3. Industrial processes

For example, chemical, metallurgical and pulp and paper industries and petroleum refineries

4. Solid waste disposal

For example, onsite or offsite incineration of solid waste indicates the types of pollutants emitted due to various industrial activities. Classification of pollutants

Air pollutants can be classified according to the origin, chemical composition and state of matter.

Origin

According to their origin, pollutants may be considered as primary or secondary. Primary pollutants such as oxides of sulphur (SOx), oxides of nitrogen (NOx) and hydrocarbons (HC) are those directly emitted into the atmosphere and found there in the same form. Secondary pollutants are those which are formed in the atmosphere by the interaction of two or more primary pollutants, by processes such as photochemical reaction, hydrolysis and oxidation.

Examples of secondary air pollutants:

1. Ozone

2. Peroxy acetyl nitrate (PAN)

3. Photochemical smog

4. Acid mist.

Chemical Composition

Pollutants can be further classified according to their chemical composition, as either organic or inorganic. Organic compounds contain carbon and hydrogen, and may also contain elements such as oxygen, nitrogen, phosphorus and sulphur. Hydrocarbons are organic compounds containing only carbon and hydrogen. Inorganic compounds found in the atmosphere include carbon monoxide (CO), carbon dioxide (C0₂), sulphur oxides (SOx), nitrogen oxides (NOx), ozone, hydrogen fluoride and hydrogen chloride.

State of Matter

Pollutants can be classified on the basis of the state of matter as particulate and gaseous. Particulate pollutants include dust, fumes, smokes, fly ash, mist, spray and smog. Under quiescent condition, particulate pollutants will settle out of the atmosphere due to gravitational force.

Gaseous pollutants are formless fluids that completely occupy the space into which they are released. They behave much like air and do not settle out of the atmosphere. Common gaseous pollutants are CO, C0₂, SOx, NOx, HC and oxidants.

Particulate matter

Particulates may be further classified according to their physical, chemical and biological characteristics. Physical properties include size of the particle, mode of formation and settling behaviour. Chemical characteristics include organic or inorganic composition and biological characteristics.

Physical characteristics

Mode of formation

Particles can be classified according to their mode of formation as dust, smoke, fumes, fly ash, mist and spray. The first four are solid particles and the last two are liquid particles.

Dusts are solid particles of natural or industrial origin usually formed by the disintegration process. Dust may also come directly from the processing or handling of materials such as coal, cement or grains. The size, of the particle ranges from a lower limit of 1 mew up to about 200 pm and larger.

Smoke consists of fine solid particles formed due to incomplete combustion of organic matter such as wood, coal, etc. The particles size varies from 0.5 to 1 mew

Fly ash consists of finely divided non-combustible particles contained in flue gases arising from combustion of coal. The particle size varies from 1 to 1000 mew

Fumes are solid particles generated by the condensation of vapours by sublimation, distillation, calcination or other chemical reactions. Examples of fumes are zinc oxide and lead oxide resulting from the condensation and oxidation of metals volatized under high temperature. They range in particle size from 0 03 to 0.3 mew

Mist consists of liquid particles which may arise from the condensation of a vapour. Mist is usually less than 10 mew Spray consists of liquid particles generated by atomization of parent liquids such as pesticides and herbicides. The size of the particles usually ranges between 10 and 1000 mew

Settling characteristics

On the basis of settling characteristics, particles can generally be classified as suspended and settle able. Suspended particulate matter can be designated symbolically as SPM, for particulate matter less than 10 mew only. They remain suspended in the atmosphere for a long period of time.

Chemical characteristics

Atmospheric particulates contain both organic and inorganic components. Some of the more common organic particulates include organic acids, phenols and alcohols. Common inorganic particulates include metals such as lead (Pb), manganese (Mn), zinc (Zn) and radium.

Biological characteristics

The biological particulates in the atmosphere include protozoa, bacteria, fungi, spores, pollens and viruses. Microorganisms generally survive only for a short period in the atmosphere because of lack of nutrients and UV radiation from the sun. However, certain bacteria and fungi form spores and can survive for a long period of time.