Particulates - as Pollutants

Small solid particles and liquid droplets suspended in the air are collectively are called as particulates. Some important characterstics of these particulates are as under :-

* Size. Their size varies from 5nm to 500000 nm.
* Mass. Their mass varies from 60ug to 2000ug per cubic meter in urban and industrial areas.
* Concentration. Their number varies from few hundred per cubic cm in clear air to polluted air.
* Surface Area. They have large surface Area due to which they have good sorption for organic as well as inorganic matter.
* Chemical nature. Different types of particulates have a different chemical nature.

Sources of Particulates :-

* Natural Sources :- These include volcanic eruptions , grinding , blowing of dust and soil by the wind , spraying of salts by seas and oceans etc.

* Man-made sources :-

(A) Soot :- These are the smallest particulates. They are the most common particulates introduced in the air due to incomplete combustion of fossil fuel such as coal wood , fuel oil , natural gas etc.

(B) Fly Ash :- These are ash particles coming alongwith furnace flue gases and are the biggest particulates. They are produce due to high ash fossil fuels as in the thermal power plants or in smelting and mining processes.

(C) Inorganic particulates :- A few common inorganic particulates are :-
* Metallic particles :- These are discharging into the air during finishing of metals.
* Metal oxides :- They are released into the atmosphere during the combustion of fuels containing metals compounds.
* Lead halides :- Tetraethyl lead is added to gasoline to act as an anti - knocking agent.
* Asbestos dust :- Asbestos is a fibrous silicate mineral which is introduced in the atmosphere by the industries manufacturering asbestos sheets, ropes etc.
* Sulphuric acid and nitric acid :- These particulates in the form of droplets produce in the atmosphere when oxides of sulphur or nitrogen come in contact with the moisture.

(D) Organic Particulates :- Organic Particulates include paraffins, olefins and aromatic compounds , particularly polycyclic aromatic hydrocarbons. These originate from the combustion of static fuels or automobiles fuels or from petroleum refineries. The polycyclic hydrocarbon are easily absorbed on the soot particles and prove to be great health hazards.

Harmful effects of particulates pollutants :-

* Effect on human :- Different type of lung disease are causes by different type of particulates. e. g asbestos cause asbestosis , dust containing free silica cause silicosis etc.
Lead particulates are highly poisonous and have a serious effect on a children's brain and cause cancer.
* Effacts on Plants :- Particulates deposit on the leaves of the plants thereby blocking the stomata and retarding and transmission of mineral from the soil.

* Effects on material :- Particulates damage a large number of material e.g suit , dust , fumes , clothes , building , soil , etc. They also accelerates corrosion of metals.

* Effect on Visibility :- The light scattered by particulates result in poor visibility of the object in front of us as it happened at night when we are driving a car or scooter especially on foggy nights. They have a very harmful effects on eye retina and harmful effects on Visibility of a human beings which is a very dangerous for any one of human beings.

Effect on Climate :- Particulates act as a nuclei for cloud formation and hence effect the climate. Further , they scattered and reflected back the heat of the sunlight and controlling the warming up of the earth surface during to increase in carbon dioxide I.e. they counteract the green house effect.

Control of Particulates Pollution :- To reduced the pollution in the air , the removal of Particulates is very essential. This is done by using the following techniques which are based on the settling or washing away of the particulates.

* Gravity settling chamber :- It consist of a horizontal rectangular tank. It is so large that the effluented gases which are allowed to enter into the slow down.

* Cyclone Collector :- In this techniques, the gas is allowed to flow at tight circular spiral. Due to the centrifugal force , the particulates move towards the wall and start settling down.

* Wet Scrubbers :- In this techniques, spray towers and chamber are used into which a suitable liquid is introduced in the form of a fine spray which washes away the particulates.

* Electrostatic precipitation :- This is the most effective and efficient method for the removal of Particulates. About 99% of the particulates can be removed by this method.