Particulates

Particulates

Particulates refer to finely divided solid or liquid particles suspended in air. The particulates, usually present in atmosphere are soot, water particles, metal oxides & metal salts, fly ash, asbestos powder, dust, solid hydrocarbons etc. Liquid aerosols include sulphuric acid & nitric acid mists.

Some common particulates & their sources

(i) Soot

The most common particulate injected into the atmosphere through human activity is soot which is produced by incomplete combustion of carbonaceous fossils fuels such as coal, fuel oil, natural gas, wood etc in insufficient supply of oxygen.

(ii) Metal particles

These are released by various metal finishing operation. The micro particles of toxic metal & SO2 gas present in the polluted atmosphere get absorbed on the particles rendering them highly toxic.

(iii) Metal oxides

They are generated by combustion of fuels containing metallic compounds.

(iv) Lead salts

Their source is lead tetraethyl (Pb(C2H5)₄) which is added to gasoline to improve its antiknock property. In order to avoid deposition of PbO suitable amounts of C2H4Cl2 & C2H4Br2 are added to gasoline along with Pb(C₂H₅)₄.

(v) Fly ash

It originates from the combustion of high ash fossil. It contains partially burnt particles of the fuels.

(vi) Asbestos dust

It originates from industrial units manufacturing asbestos sheets, gaskets ropes etc. Asbestos flowing & asbestos insulations also contribute towards asbestos dust in the atmosphere.

(vii) Solid Hydrocarbons

These are emitted from petroleum refineries & comprise of paraffins, olefins & aromatics.

(viii) Dust Particulates

Dust particulates originate from natural, domestic, industrial or agricultural sources. These are thrown into atmosphere by volcanic eruptions, blowing of dust by wind, mining operations etc.

(ix) Acid mist

Sulphuric acid mist is produced when SO₃ present in the atmosphere comes in contact with moisture. Similarly nitric acid mist is produced when oxides of nitrogen, viz, NO & NO2, undergo the series of reactions in the atmosphere.

Harmful effects of particulates

(i) Effect on human beings

The particulates seriously affect the human respiratory system & cause several respiratory illnesses. The particles with small size are more harmful in this context. The particulates in fact, become the carriers of the toxic substances from the atmosphere to the human & cause big health hazards.

(ii) Effect on visibility

Particulates in the atmosphere cause scattering & absorption of sunlight & reduce the visibility.

(iii) Effect on Materials

The adverse effect of particulates on materials include corrosion of metals (when the atmosphere is humid), erosion & soiling of building, sculptures & painted surfaces & soiling of clothes & draperies.

CONTROL OF AIR POLLUTION

Air pollution can be checked either at source or at the dispersal stage of various pollutants. Although the Prob. of controlling air pollution at source is not easy, yet it can be reduced to some extent by taking steps such as,

Use of appropriate raw materials, selection of proper sites for industrial units & use of modern techniques for efficient reduction of emission & effluents.

Different methods for the control of various air pollutants are as follows:

1. Control of CO

Some of the measures which can be adopted for control of CO pollution are:

(a) Use of anti pollution exhaust catalyst.

(b) Modification & improvement in internal combustion engine.

(c) Use of other fuels such as CNG, LPG & LHG.

2. Control of CO₂

CO₂ level can be maintained by reducing its release from industrial units & controlling vehicular exhaust etc. Hydrogen is being looked upon as pollution less future fuel.

3. Control of NOx Pollution

(i) Power plants & industrial units emit large amounts of NOx. Such emission can be reduced by almost 90% with the help of two step combustion.

(a) In the first step, the fuel is burnt at high temperature in a limited supply of air.

(b) In the second step, the unburnt fuel is burnt at lower temperature in excess of air.

(ii) Acidic scrubbing of flue gases: The large amounts of nitrogen oxides emitted form power plants & industrial units can also be removed by scrubbing the flue gases with sulphuric acid.

(iii) NOx in vehicular exhaust is converted into N2 & trace amount of NH3 with the help of
anti-pollution exhaust catalyst.

4. Control of SO₂ pollution

The process of removal of SO2 is known as flue as flue gas desulphurisation (FGD). Sulphuridioxide from flue gases is commonly removed by using chemical scrubbers consisting of a slurry of CaCO3 where SO2 is absorbed forming CaSO₃.

Smog & its ill effects

Smog is a mixture of smoke & fog. The principal source of classical smog is the combustion of industrial household fuels. Because of the presence of SO2 & carbon (soot particles), classical smog has a reducing character.

Photo Chemical Smog

This is another type of smog which is distinctly different from the classical smog.

The principal constituents of the photo chemical smog are ozone & oxides of nitrogen.

It causes irritation in eyes due to the presence of ozone. This type of smog is formed through photo chemical reactions involving solar radiations, hence is named as photo chemical smog.

Harmful effects of smog

(i) Smog causes respiratory Prob.s

(ii) It reduces visibility

(iii) It damages plants & other material like electronic & electrical equipment.