Air Pollution

Biosphere can be defined as zone where all three Lithosphere, Hydrosphere and Atmosphere meets, and is capable of supporting the life.

Presence of one or more air contaminants in such concentration and for such duration that it starts affecting the life in the biosphere is called Air Pollution.

Air Pollutants are broadly classified into Primary & Secondary Pollutants:

• Primary pollutants have directly identifiable origin whereas when one or more primary air pollutants react under favourable conditions, it leads to the formation of secondary pollutants.
• Primary Pollutants – Oxides of Sulphur, Oxides of Carbon, Oxides of Nitrogen, Lead, Hydrocarbons etc.
• Secondary Pollutants – Sulphuric Acid, Ozone, Formaldehyde, PAN

Note: Out of all Nitrogen compounds, Only NO and NO2 is considered as pollutants.

Gas Law

• PV = nRT.
• P in N/m2, T in Kelvin, R(Universal Gas Constant) = 8.314 kJ/mole.
• The temperature of the air parcel decreases with elevation.
• All gases at STP (Standard Temperature & Pressure) i.e. 0ᵒC, 1 atm occupy 22.4 litres/ mole.
• The concentration of suspended particles and gaseous pollutants are expressed in µg/m3 at atmospheric temperature and pressure.

• The L/mol at STP is 22.4.
• The value at any other temperature and pressure can be found by –                               (P1V1)/T1 = (P2V2)/T2

Suspended Particulate Material (SPM)

Thousands of different solid or liquid particles suspended in  air which includes soil particles, soot (black powder after burning of organic matter), lead, asbestos, sea salt, fog, mist and sulphuric acid droplets.

SPM is dangerous due to following reasons:

• May contain toxic materials or material which has carcinogenic effects.
• Extremely small particles can become lodged in lungs leading to bronchitis, asthma etc.

Note: Pneumoconiosis – lung disease caused due to inhalation of silica & coal dust (usually to the people working in coal mines)

Nitrogen and Sulphur Oxides

•Nitrogen Oxides

   –At high temperature, Gas produced by the chemical interactions between atmospheric            nitrogen and oxygen.

   –Problems

• 
  

  Greenhouse gases
• 
  

  Cause difficulty in breathing

•Sulphur Oxides

   –Gases produced by the chemical interactions between sulphur and oxygen

   –Causes acid rain

   –Originated from refineries, chemical plants, burning of fossil fuels etc.

Carbon Oxides and Hydrocarbons

•Carbon Oxides

   –Carbon monoxide(CO) and carbon dioxide(CO2)

   –Greenhouse gases

   –Chief source is automobile exhausts

   –CO is colourless, odourless, toxic gas and combines with haemoglobin to form                            Carboxyhaemoglobin which reduces the oxygen carrying capacity of the blood.

•Hydrocarbons

  –Diverse group of organic compounds that contain only hydrogen and carbon (ex: CH4 ,           C6H6 etc. )

  –Some hydrocarbons are related to photochemical smog and greenhouse gases.

Ozone

•Tropospheric Ozone

  –Man Made pollutant

  –Present in lower atmosphere

  –Secondary air pollutant

  –Component of photochemical smog

•Stratospheric Ozone

  –consider as Essential component which screens out UV radiation in the upper atmosphere

  –Man-made pollutants (ex: CFCs) can destroy it.

Photochemical Smog

•Smog is a mixture of Smoke (respiratory problems) and Fog (reduces visibility).

•Conditions for formation of Photochemical Smog – 

  –Hydrocarbons

  –Nitrogen Oxides

  –Stagnant air (for chemicals to interact)

  –Abundant Sunlight (happens during day or afternoon)

•It consists of – PAN, Ozone, CO, CO2, Formaldehyde.

CONTROLLING AIR POLLUTION

1.Electrostatic Precipitator–Smoke Stack with Electrostatic Precipitator

–Efficiency of Electrostatic Precipitator

   Properties of ESPs

•    The gas is passed through a highly ionized zone and get electrically charged & are separated out from the gas with the help of electrostatic forces in powerful electric field
•    Widely used in thermal power plants, mining, iron  and steel plants etc.
•     Quite sensitive to varying dust loading & flow rates
•    Wet or dry particle can be collected
•    99% or more efficiency can be obtained
•    Operated at high temperature
•   Maintenance is nominal & power required is more as compared to other devices

  2.Gravitational Settling Chamber 

   – Simple to design and maintain

   –Low head loss

   –Requires large space, Collection efficiency is low

   –Only large sized particles are separated

 3.Centrifugal collector

  –Simple to design and economical

  –Requires less floor area

  –Used for industries like cement, petroleum, asphalt etc.

  –Efficiency is low for smaller particle and quite sensitive. 

  –Can handle large volumes of gases at high temperature temperature.

 4.Fabric/Bag Filter

  –Fabric Filters or Baghouses use filtration to separate dust particulates from dusty gases.

  –They are highly efficient types of Dust Collecting Filters available in market and can achieve a collection efficiency of more than 99% for very fine particulates. They are cost efficient also.

  –Dust-loaded gases enter the baghouse and pass through fabric bags that act as filters. The   bags is in either a tube or envelope shape and made of woven or felted cotton, synthetic, or glass-fiber material.

Also Read: Dispersal of Air Pollutants and Air Plume Behaviour