Particulates

Particulate refers to 'finely divided solid or liquid particles suspended in air'. The size of particulates varies from about 0.0002 to about 500

(1 = 10^-4 cm). In the polluted air, the number of particulate particles is as high as 10¹¹ per m³. In urban areas the mass level of particulates varies from 60 g to 2 x 10³ g per cubic meter.

The particulates usually present in atmosphere are soot, metal particles, metal oxides, metal salts, fly ash, asbestos powder, dust, solid hydrocarbons, etc. Liquid aerosols include sulphuric acid and nitric acid mists.

Particulate matter with a diameter greater than 1 micrometer scatter incoming light away from the Earth's surface and affects the climate. This adversely affects plant growth by reducing photosynthesis. It also reduces the precipitation of water vapor.

The first regulation of air pollution resulted from particulates when King of England, Edward I banned smoky coal burning in London in 1272.

Common Particulates and their Sources

Soot

It is produced by incomplete combustion of carbonaceous fossil fuels such as coal, fuel oil, natural gas, wood, etc., due to insufficient supply of oxygen. Soot is the most common particulate injected into the atmosphere through human activity.

Metal Particles

The micro-particles of toxic metals and SO₂ gas present in the polluted atmosphere get adsorbed on metal particles rendering them highly toxic. These are released by various metal-finishing operations.

Metal Oxides

The combustion of fuels containing metallic compounds generates metal oxides. For instance, the particulate Fe₃O₄ is produced by the combustion of coal containing iron pyrites (FeS₂).

Lead Salts

The most prevalent source of lead salts in both air and water bodies is tetraethyl lead [Pb(C₂H₅)₄] which is added to gasoline to improve its antiknock property. Tetraethyl lead is oxidized to PbO, which tends to deposit on spark plugs and valves. In order to avoid deposition of PbO, suitable amounts of C₂H₄Cl₂ and C₂H₄Br₂ are added to gasoline along with Pb(C₂H₅)₄. These halides convert PbO into PbCl₂ and PbBr₂ both of which are volatile and get eliminated through the exhaust.

Fly Ash

This pollutant is released into the atmosphere from the combustion of high-ash fossil fuels, such as those used in thermal power stations, smelters and mining operations. It contains burnt particles of the fuels.

Asbestos Dust

Industries manufacturing asbestos sheets, asbestos flooring and asbestos insulations create fine asbestos dust in the atmosphere. Mining and processing operations and units manufacturing gaskets, ropes, etc. also contribute towards this pollutant, which cause serious respiratory problems.

Dust Particulates

Dust particulates originate from natural, domestic, industrial or agricultural sources. These are thrown into the atmosphere by blowing of dust by wind, mining operations etc. Rapid deforestation leading to exposed soil cover and erosion has increased this type of pollution. Natural volcanic eruptions and the spraying of salts and other solid particles by the oceans also cause dust particulates.

Depending upon sources, dust particulates may contain number of harmful pollutants such as Pb, Cd, Hg and hydrocarbons. These particles do not remain dispersed in air but gradually settle down on the surface because of gravitational forces.

Solid Hydrocarbons

A hydrocarbon is the name given to the mixture of gases caused by the incomplete combustion of fossil fuels. These comprise of paraffins, olefines and aromatics that are emitted from petroleum refineries. Some of these substances are suspected carcinogens, such as the polynuclear aromatic hydrocarbons of benzo (-) pyrene, chrysene and dibenzanthracenes. These compounds are readily adsorbed on the surface of soot particles present in the atmosphere and thus become toxic producing smog, posing serious health hazards. They also retard plant growth.

Acid Mist

Vapours of any acid introduced into the atmosphere causes acid mist. 'Sulphuric acid mist' is produced when SO₃ present in the atmosphere, comes in contact with moisture. Sulfur trioxide (SO₃) is produced by complete combustion and forms sulfuric acid by the mechanism below:

SO_x, NO_x mixed with water is a component of nitric acid mist. NO_x also reacts in the air with ozone and hydrocarbons and is part of the brown haze known as mist or smog. It is produced when oxides of nitrogen, viz., NO and NO₂, undergo series of reactions in the atmosphere.

For example:

Acid rain is harmful to humans and can harm plant and animal populations on land and in water. It also causes material damage.

Harmful Effects of Particulates

Effect on Human Beings

Particulates seriously affect the human respiratory system and cause people to cough, get sore throats and obscure vision. The particles with small size are more harmful as they reach the lungs easily and get accumulated there, causing several respiratory illnesses. Particulates become the carriers of toxic substances from the atmosphere to the humans and cause health hazards. Grime is also deposited on personal belongings through particulates.

Due to their larger surface areas the smaller particles provide effective sites for the adsorption of carcinogenic substances such as polynuclear aromatic hydrocarbons, asbestos etc. They cause various allergic reactions, bronchitis, bronchial asthma, emphysema and lung cancer when particulates combine with them.

Effect on Visibility

Particulates in the atmosphere cause scattering and absorption of sunlight and reduce the visibility. They also produce blurring effect on vision. Further, reduced visibility caused in conditions of haze and smog by such pollution is especially dangerous for aircraft and motor vehicle drivers.

Effects on Materials

The adverse effects of particulates on materials include damages to electronic equipment through chemical or mechanical action on electrical contacts, erosion and soiling of buildings, sculptures and painted surfaces. Corrosion of metals (when the atmosphere is humid) also takes place along with soiling of clothes and draperies.

Effect on Climate

Particulate matter affects local climate since particulates with a diameter greater than 1 micrometer scatter incoming light away from the Earth's surface. This affects weather and plant production.

Air Pollutants and their maximum permissible exposure for safe health

Problem

2. State three remedies (with examples) for combating air pollution from automobiles?

Solution

(i) Create laws/legislation to force the reduction of air pollution e.g. strict automobile emission standards. In the National Capital Region of Delhi, the pollution laws state that all vehicles being sold have to confirm to Euro-II emission standards, vehicles have to use unleaded fuel and vehicles older than 15 years that have old combustion processes are not allowed on the road.

(ii) Improve technology to reduce emissions e.g. Using anti-pollution exhaust catalyst in vehicles, such as finely divided platinum supported on ceria or alumina, to reduce carbon monoxide, nitrogen oxides and hydrocarbons.

(iii) Convert on to alternate fuels such as CNG, LPG etc. In the national capital region of Delhi all auto rickshaws, Taxis and public transport buses have to convert to CNG/LPG, by order of law enforced by the supreme court of India.