Pollution

• Pollutants in the atmosphere

  • Rapid movement due to winds leads to dispersal over a large area
  • Interaction with electromagnetic radiation such as UV or visible light from the Sun can cause chemical reactions to occur, leading to the production of secondary pollutants
• Smoke


  Atmospheric particulates produced by the incomplete combustion of carbon-based materials
• Smoke particle sizes


  • PM10: particulate matter less than 10 microns in diameter
  • PM5: particulate matter less than 5 microns in diameter
  • PM1: particulate matter less than 1 micron in diameter
• Particle size


  • Smaller particles remain in the atmosphere for longer so they are more likely to be inhaled
• Toxic chemicals found in smoke

  • Fluorides
  • Aluminium
  • Lead
  • Acids
  • Organic compounds such as phenol
• Smoke


  Can act synergistically with other atmospheric pollutants such as sulfur dioxide
• Main sources of smoke

  • Combustion of coal, diesel, general combustible wastes especially in urban areas
  • Combustion of crop waste, wood fuel, grasslands, forests in rural areas
• Smoke particles


  • Small and can remain suspended in the atmosphere for long periods, especially in the stratosphere where there is no rain to wash it out
  • Smoke in the stratosphere can also deplete the ozone layer
• Smog


  Smoke + Fog
• Fog formation


  • Occurs when moist air is cooled until it reaches its dew point, causing water vapour to condense as airborne droplets of water
  • Temperature inversions make the formation of fog more likely
• Smoke in smog


  • More easily inhaled
  • Smog has a very high albedo so the temperature inversion can last for long periods of time allowing pollutant levels to rise to a high level
• The London smog


  1952
• The position of London in a valley has caused atmospheric pollution problems since the 1200s, which became worse as the city grew
• About 12,000 deaths were caused by the 1952 London smog, mainly people who were old, very young or had existing respiratory health problems
• Smoke pollution control measures

  • Legislation: Clean Air Act (1956)
  • Domestic sources: increased use of fuels that do not produce smoke, for example, natural gas, electricity
  • Transport sources: Diesel Particulate Filters (DPF) fitted in the exhaust pipes of diesel engines trap up to 80% of smoke particles
• Electrostatic precipitators


  1. Effluent gases are passed through a chamber with many electrically charged wires or plates

  2. The smoke particles within the gases are attracted to the charged wires and plates and collect together

  3. As the particles accumulate they fall to the floor as fly ash
• Cyclone separators


  1. Effluent gases are forced to rotate in a cylindrical chamber, which throws the suspended particles to the outside surfaces of the chamber where they fall and collect

  2. The clean exhaust air is removed from the middle
• Scrubber


  Uses a fine water spray to wash out suspended solid particulate matter and dissolve soluble gaseous pollutants
• Coal treatment


  Heating coal allows the tar that causes smoke production to be drained off, producing smokeless coal
• Bag filters


  Remove smoke particles from effluent gases by trapping them on a fabric filter
• More efficient combustion technology

  • Increases the amount of oxygen supplied and efficiently mixes it, allowing more complete burning of smoke particles
• Diesel engines produce most smoke when the driver accelerates too quickly, so too much fuel enters the engine for the amount of oxygen available to burn it
• Turbochargers enable more air and therefore more oxygen to be delivered to the combustion chamber, increasing the efficiency of the combustion
• Photochemical smog


  Urban pollution during temperature inversions, often in valleys, involving a wide variety of reactions between primary and secondary pollutants in the presence of sunlight, including UV light
• Sources of primary pollutants in photochemical smog

  • Oxides of nitrogen (NOx) released in the exhaust gases from petrol and diesel engines
  • Unburnt hydrocarbons from a variety of sources, especially unburnt fuel in engine exhaust, fuel evaporation, and spillages
• Some cities are particularly prone to photochemical smogs because of their topography, heavy vehicle use, and climate
• Control of photochemical smogs

  1. NOx and hydrocarbons can both be controlled using catalysts such as platinum and palladium in exhaust pipe catalytic converters

  2. NOx are chemically reduced in catalytic converters to re-form the oxygen and nitrogen gases that originally reacted together

  3. Unburnt hydrocarbons can be controlled using catalytic converters, vapour collection and condensation, or adsorption onto activated carbon
• If the primary pollutants are controlled, the secondary pollutants will not be formed because the reactants will not be present
• Acid rain


  Rain that is more acidic than pH 5.6 due to the dissolution of pollutant gases such as sulfur dioxide and oxides of nitrogen
• Pollutants involved in acid rain

  • Sulfur dioxide
  • Oxides of nitrogen
  • Hydrogen chloride
  • Ozone
• Sources of the gases involved in acid rain

  • Combustion or oxidation of materials containing sulfur, especially burning coal and smelting sulfide ores
  • High temperature combustion, especially in power stations and petrol and diesel internal combustion engines
  • Ozone is a secondary pollutant produced by photochemical reactions
• Pollutants involved in acid rain

  • Sulfur dioxide
  • Oxides of nitrogen
  • Hydrogen chloride
  • Sulfur trioxide
• Sulfur dioxide dissolves in water

  Produces sulfurous acid
• Sulfur dioxide is oxidised in the atmosphere


  Produces sulfur trioxide
• Sulfur trioxide is dissolved

  Produces sulfuric acid
• Oxides of nitrogen dissolve

  Produce nitrous and nitric acids
• Hydrogen chloride dissolves to form hydrochloric acid
• Ozone is involved in the oxidation of sulfur dioxide to sulfur trioxide
• Ozone also harms plants directly
• Sources of the gases involved in acid rain

  • Combustion or oxidation of materials containing sulfur, especially burning coal and smelting sulfide ores
  • High temperature combustion, especially in power stations and petrol and diesel internal combustion engines
  • A secondary pollutant produced by photochemical reactions involving oxides of nitrogen
  • The combustion of coal and the incineration of wastes containing chlorine, for example PVC plastic waste
  • Oxidation of sulfur dioxide by ozone