unit 10

• Troposphere


  The atmosphere's inner layer, about 17 km deep at the equator and 8 km at the poles
• Composition of clean, dry air in the troposphere

  • 78% nitrogen (N2)
  • 21% oxygen (O2)
  • <1% argon
  • 0.04% carbon dioxide (CO2)
  • Traces of other gases
• Stratosphere


  The layer above the troposphere, extending about 17-48 km above the Earth's surface
• Stratosphere


  • Its volume of water vapour is ~1000 times less, therefore clouds are rarely found above the troposphere
  • Its volume of ozone is ~1000 times greater
• Stratospheric ozone


  Acts as an ultraviolet filter keeping out ~99% of the Sun's harmful radiation from reaching the Earth's surface
• This allows life to exist on land, protects animals from UV's harmful effects and prevents oxygen in the troposphere from being converted to ozone which is considered a pollutant in the troposphere
• Important atmospheric pollutants


  • Gases such as sulphur dioxide (SO2), oxides of nitrogen and carbon, ozone, chlorofluorocarbons (CFC's)
  • Particulates such as dust
• Urban areas with large concentrations of vehicles and factories normally have higher air pollution levels than rural areas
• Primary air pollutants


  Harmful chemicals that enter directly into the atmosphere
• Secondary air pollutants


  Harmful chemicals that are the resultant products of reactions among primary pollutants or between primary pollutants and gases normally present in the atmosphere
• Sources of air pollution

  • Natural sources (e.g. hydrocarbon emission from trees, sulphur dioxide from volcanoes)
  • Human sources (e.g. motor vehicles, industries)
• Smog


  Air pollution that reduces visibility and is localised in urban areas
• Industrial smog


  Smoke pollution with the principal components being sulphur oxides and particulate matter
• Formation of industrial smog

  1. Sulphur dioxide reacts with oxygen to form sulphur trioxide

  2. Sulphur trioxide reacts with water vapour to form sulphuric acid (H2SO4)

  3. Sulphuric acid reacts with ammonia to form solid particulates of ammonium sulphate

  4. Particles of soot give industrial smog its grey colour
• Photochemical or brown air smog


  A mixture of primary and secondary pollutants formed under the influence of sunlight, consisting of more than 100 secondary air pollutants
• Formation of photochemical smog

  1. Nitrogen and oxygen in the air react to produce nitric oxide

  2. Nitric oxide reacts with oxygen to form nitrogen dioxide (NO2)

  3. Nitrogen dioxide may react with water vapour to form nitric acid (HNO3) or with UV light to form nitric oxide (NO) and oxygen atoms (O)

  4. Oxygen atoms and ozone can react with volatile organic compounds to produce aldehydes and peroxyacetyl nitrates (PANs)
• Photochemical oxidants


  • Nitrogen dioxide, ozone and PANs can react with compounds in the air or in human lungs that are not normally oxidised, irritating the respiratory tract
• Photochemical smog may also cause eye watering and may be toxic to plants because it can block one of the pathways in photosynthesis
• When the sun increases surface temperatures

  The air near the ground is warmed, causing a low pressure area near the ground followed by movement of the surrounding air into this low pressure area, preventing toxic pollutants from increasing to dangerous levels near to the ground
• Thermal inversion


  Where air near the ground is colder than the air above it, causing pollutants to be trapped near the ground
• Thermal inversion only occurs for a few hours
• Topography


  Influences the likelihood of thermal inversion, with cities located near valleys, near the coast or on the leeward side of mountains being prone to this problem
• Indoor air pollution


  Air in enclosed spaces may have higher levels of air pollutants than air outside, resulting in illnesses resembling common ailments
• Common indoor contaminants


  • Cigarette smoke
  • Carbon monoxide
  • Nitrogen oxide
  • Pesticides
  • Cleaning solvents
  • Ozone (from photocopiers)
  • Asbestos
  • Microorganisms
  • Dust mites
  • Pollen
  • Other organisms
• Sick building syndrome


  The presence of air pollution inside office buildings that can cause eye irritations, nausea, headaches, respiratory infections and fatigue
• Acid deposition


  A type of air pollution involving secondary pollutants such as sulphuric acid and nitric acid in precipitation (sometimes called wet precipitation)
• Formation of acid deposition

  1. Sulphur oxides and nitrogen oxides react with water to produce dilute solutions of sulphuric acid, nitric acid and nitrous acid

  2. Sulphuric and nitric acid particles can settle out of the air (dry deposition)
• Acid precipitation, including acid rain, sleet, snow and fog, poses a serious threat to the environment
• Natural, unpolluted rain has a pH of about 5.6 due to the presence of dissolved carbon dioxide, but in some areas rain water often has a pH of 4-4.5 and sometimes lower
• Harmful effects of acid deposition

  • Ecological effects when pH falls below 5.1 for terrestrial systems and below 5.5 for aquatic systems
  • Contribution to human respiratory diseases such as bronchitis and asthma
  • Damage to buildings, metal statues and car finishes
  • Damage to forests and vegetation, contributing to forest decline
• How acid deposition harms plants

  1. Acid deposition alters the chemistry of soils, affecting the development of plant roots and their uptake of dissolved minerals and water

  2. Essential plant minerals such as calcium and magnesium wash readily out of acidic soils becoming available for absorption in toxic amounts
• Effects of acid deposition on aquatic life

  • Can kill fish and interfere with the uptake of calcium by crustaceans, preventing them from manufacturing their exoskeletons
  • Can affect birds, causing them to lay eggs with thin, fragile shells that break or dry out before the chicks hatch
• The basic way to control acid deposition is to reduce emissions of sulphur and nitrogen oxides
• Regions affected by acid rain

  • Large portions of acid producing chemicals generated in one country may be exported to others by prevailing winds
  • 16,000 lakes in Norway and Sweden contain no fish
  • 14,000 acidified lakes in Canada are almost fish-less and 150,000 more are in peril
  • 9,000 lakes in USA are threatened
• Liming


  A treatment for acidified lakes and soils
• Controlling air pollutants


  • Smaller more fuel efficient automobiles
  • Develop new technologies to control air pollution
  • Smokestacks fitted with equipment to remove particulate matter
  • Sprinkling water on dry soil that is being moved during road construction
  • Switch to low sulphur fuels such as natural gas
  • Reduce combustion temperatures in automobiles which lessen the formation of nitrogen oxide
• Greenhouse effect


  The natural trapping of heat by gases such as carbon dioxide and methane in the atmosphere, keeping the planet warm and allowing life to exist
• Without this process the average temperature on earth would be 170C
• Human activities have increased the concentrations of many greenhouse gases, enhancing the greenhouse effect and resulting in increased average global temperatures (global warming)
• The Intergovernmental Panel on Climate Change has concluded there's a more than 90 percent probability that human activities over the past 250 years have warmed our planet