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Acid Rain & Ozone Depleting Substances

Acid Rain

  • Acid rain (Ph < 5.6) is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids (H2SO4& HNO3)
  • Acid rain is a popular term referring to the deposition of wet (rain, snow, sleet, fog, cloudwater, and dew) and dry (acidifying particles and gases) acidic components

Acid Rain


 Ozone depleting substances (ODS)

  • Scientific evidence indicates that stratospheric ozone is being destroyed by a group of manufactured chemicals, containing mainly chlorine or bromine, called “ozone-depleting substances” (ODS).
  • ODS are very stable, nontoxic, non-flammable and environmentally safe in the lower atmosphere, which is why they became so popular in the first place
  • However, their very stability allows them to float up, intact, to the stratosphere & once there, they are broken apart by the intense ultraviolet light, releasing chlorine and bromine.
  • Chlorine and bromine demolish ozone at an alarming rate, by stripping an atom from the ozone molecule

 

Main ODS – CFCs, HCFCs, Halons, CCl4, NOx, CH3CCl3 (Methyl chloroform)

Major Sources: Solvent Cleansing products >Refrigeration & Air conditioners > Foam Products

Chlorofluorocarbons (CFCs)

  • The most widely used ODS, accounting for over 80% of total stratospheric ozone depletion.
  • Used as coolants in refrigerators, freezers and air conditioners in buildings and cars manufactured before 1995.
  • Also found in industrial solvents, dry-cleaning agents and hospital sterilants.
  • Also used in foam products — such as soft-foam padding (e.g. cushions and mattresses) and rigid foam (e.g. home insulation)

 

CFCs

 

Halons & Hydrobromofluorocarbon (HBFC)

  • Halons, unlike CFCs, contain bromine, which destroys ozone 100 times more than chlorine
  • Halons are used primarily in fire extinguishers & methyl bromide (Pesticide)

 

Methyl Chloroform

  • Used mainly in industry as cleansing solvent — for vapour degreasing, cold cleaning, adhesives and chemical processing.

 

Carbon Tetrachloride

  • Mainly used as solvent & in fire extinguishers.

 

Hydrofluorocarbons (HCFCs)

  • Major, “transitional” substitutes for CFCs- much less harmful to stratospheric ozone than CFCs
  • But still cause some ozone destruction and are potent greenhouse gases

Impacts of Ozone Depletion

  • Skin cancers, sunburns and premature aging of the skin
  • UV radiation can damage several parts of the eye, including the lens, cornea, retina and conjunctiva
  • Weakening of the human immune system
  • Several of the world’s major crop species are particularly vulnerable to increased UV, resulting in reduced growth, photosynthesis and flowering
  • Plankton are threatened by increased UV radiation

 

Ozone hole mainly in Antarctica not in Arctic pole, because

  • Southern polar region more colder & isolated than north, which favor the formation of polar stratospheric clouds (PSCs) which in turn serve as platforms for catalytic ozone breakdown
  • As a consequence of having less land, the circulation over the Antarctic is more persistent and vortex-like, which favours PSCs
  • Air inside the vortex is prevented from mixing with warmer, ozone-rich air from lower latitudes (This vortex is not a feature of the Arctic)

 

Ozone is measured in Dobson units

  • A Dobson unit of gas is equal to a layer of gas, at the surface of the Earth, with a thickness of one hundredth of a millimeter
  • The ozone in the atmosphere is about 300 Dobsons, which means if you brought all of the ozone in the atmosphere down to a layer of pure ozone at the surface; it would be about three millimeters thick.
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