18.4

• The Earth's atmosphere is changing
• Scientific measurements have documented shifts in chemical composition throughout the lower atmosphere, as well as substantial alterations in the chemical composition in the upper atmosphere
• Composition of Earth's atmosphere


  • It continuously changes
  • Changes are caused by natural factors and human activities
  • Atmospheric changes result in global climate change
• Human activities that cause harmful effects on the atmosphere


  • Photochemical smog
  • Thermal inversion
  • Acid deposition
  • Ozone depletion
• Photochemical smog


  It is the result of the reactions between the smoke emissions and sunlight. It is common in dry and sunny areas where peak is at midday of summer.
• Photochemical smog is a mixture of pollutants that are formed when nitrogen oxides and volatile organic compounds react to sunlight, creating a brown haze above cities
• Photochemical smog tends to occur more often in summer because that is when we have the most sunlight
• Sources of photochemical smog


  • Automobiles
  • Oil refineries
  • Industries
  • Natural sources
• The main chemicals involved in photochemical smog are nitrogen oxides and hydrocarbons forming ozone
• Photochemical smog is produced when sunlight reacts with nitrogen oxides and at least one volatile organic compound (VOC) in the atmosphere
• Nitrogen oxides come from car exhaust, coal power
• Effects of photochemical smog


  • Eye irritation/poor visibility
  • Respiratory problems
  • Decrease in crop yields
  • Materials become brittle
• Volcanic smog (vog)


  Visible haze created when sulfur dioxide gas and other pollutants emitted from volcanic activity interact chemically with atmospheric moisture, oxygen, dust and sunlight
• Smog


  A special type of air pollution, a combination of harmful pollutants introduced into the atmosphere by both natural and human induced processes
• Thermal inversion


  It happens when a layer of warm air descends and form an inversion layer. The inversion layer and the cold air from the bodies of water trap smog which increases air pollution. A reversal of the normal behaviour of temperature in the troposphere, in which a layer of cool air at the surface is overlain by a layer of warmer air.
• Thermal inversion is typical in cities with many tall buildings because the buildings prevent air from circulating. The warmer air rises and acts as a lid trapping the colder air close to the ground. Pollution, including that from road traffic is also trapped, so the air layer closest to the ground becomes more and more polluted.
• Acid Deposition


  It refers to the acid precipitation into Earth's surface, commonly known as acid rain.
• Harmful gases combine with the oxygen and water in the air. Water in the air comes down as rain, sleet, hail or snow, it carries it with gases. Acid rain is very damaging to all life forms.
• Acid rain is rain that has become acidic due to pollutants in the air. When fossil fuels like coal, oil, or gas are burned, they release gases called sulfur dioxide (SO2) and nitrogen oxides (NOx) into the atmosphere. These gases can react with water vapor, oxygen, and other chemicals in the air to form sulfuric acid (H2SO4) and nitric acid (HNO3), which then mix with rainwater, snow, fog, or other forms of precipitation.
• Forms of acid deposition


  • Wet depositions (acid rain)
  • Dry depositions (acidic particles and gases without moisture)
• Rusting and corrosion are effects of dry deposition of acid
• Ozone depletion


  It is the destruction of the ozone layer, one of the major consequences of human activities in the atmosphere. It is the gradual thinning of Earth's ozone layer in the upper atmosphere caused by the release of chemical compounds containing gaseous chlorine or bromine from industry and other human activities.
• Effects of ozone depletion


  • Harmful UV rays may penetrate Earth's surface
  • Prolonged exposure to UV rays may cause skin cancer, impaired immune system, and cataracts
• In 1985, the British Antarctic Survey Team confirmed that there is a hole in the ozone layer over the South Pole or the Antarctic. It is as wide as the US and as deep as the Mount Everest.
• Chlorofluorocarbons (CFCs)


  Major contributor to ozone depletion. They were developed during the 1930s as a coolant for air conditioners and refrigerators. They are a stable substance and can last up to around 70 to over 100 years in the atmosphere.
• CFCs were thought to be safe until they were discovered that they can destroy the ozone layer. They were banned in 1996 by the US and Montreal Protocol.
• CFC's were phased out via the Montreal Protocol due to their part in ozone depletion. An international ban on ozone-depleting chemicals preserved the ozone layer and prevented a significant increase in global warming.