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
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.
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