Unit 06

COMPOSITION AND STRUCTURE OF THE ATMOSPHERE

Unit Overview

This unit discusses the atmosphere, which is one of our planet's major spheres. The main sections are:

  • Contents of the atmosphere
  • The layered structure of the atmosphere
  • Research Frontiers

The atmosphere consists of the gases (both constant and variable), droplets, and particles surrounding the Earth's surface. Three constant gases - nitrogen, oxygen, and argon - account for 78.1%, 21%, and 0.9%, respectively, of the atmosphere's volume. Although variable gases comprise a much smaller percentage of the atmosphere's volume, they are very important with respect to the "greenhouse effect," precipitation, and the absorption of ultraviolet radiation. Water vapour and carbon dioxide are the two most important "greenhouse" gases; water vapour is also a precursor to clouds and precipitation.

Another "greenhouse" gas - ozone - also protects living organisms from ultraviolet radiation but is a major atmospheric pollutant. Particles in the atmosphere can also modify the planetary energy balance (e.g., generally lead to a cooling of the Earth's surface); they are also important atmospheric pollutants.

The atmosphere can be divided into the following generalized layers: troposphere, stratosphere, mesosphere, and thermosphere. Humans and other organisms live in the troposphere, while the state of the other layers impacts the welfare of humans, and of the planet as a whole.

Unit Objectives

  • To describe the constituents of the atmosphere and their relative concentrations
  • To survey the four layers of the atmosphere together with their major properties
  • To discuss the problem of ozone depletion and its consequences


Glossary of Key Terms

Aerosols Tiny solid or liquid particles suspended in the atmosphere.
Carbon dioxide cycle Dominated by exchanges occurring between the air and the sea; carbon dioxide is directly absorbed by the ocean from the atmosphere and is released during the photosynthesis of billions of small organisms known as plankton.
Chlorofluorocarbons (CFCs) Ozone-depleting chemicals widely used as coolants, propellents, cleaning solvents, and as components in plastic foam prior to 1990.
Climate The long- term conditions (over at least 30 years) of aggregate weather over a region, summarized by averages and measures of variability; a synthesis of the succession of weather events we have learned to expect at any given location.
Constant gases Atmospheric gases always found in the same proportions; two of them constitute over 99 percent of the air, nitrogen (78 percent) and oxygen (21 percent).
Hydrologic cycle The complex system of exchange involving water in its various forms as it continually circulates among the atmosphere, lithosphere, hydrosphere, biosphere, and cryosphere.
Impurities Solid particles floating in the atmosphere whose quantities vary in time and space; among other things, they play an active role in the formation of raindrops.
Lapse rate The rate of decline in temperature as altitude increases; the average lapse rate of temperature with height in the troposphere is 0.65°C/100 m (3.5°F/1000 ft).
Mesopause The upper boundary of the mesosphere, lying approximately 80 km (50 mi) above the surface.
Mesosphere The third layer of the atmosphere, lying above the troposphere and stratosphere; here temperatures again decline with increasing elevation as they do in the troposphere.
Nitrogen cycle Cycle in which nitrogen-fixing bacteria convert atmospheric nitrogen into the organic compounds in plants. This organic material is consumed by animals (including humans) when the plants are eaten. When animals die, organic proteins are transformed by other bacteria and microorganisms to ammonia, urea, nitrates, and finally back into gaseous nitrogen.
Oxygen cycle Oxygen is put back onto the atmosphere as a by-product of photosynthesis, and is lost when it is inhaled by animals or chemically combined with other materials during oxidation.
Ozone layer Also known as the ozonosphere, the ozone-rich layer of the atmosphere that extends between 15 and 50 km (9 and 31 mi) above the surface; the highest concentrations of ozone are usually found at the level between 20 and 25 km (12 and 15 mi).
Stratopause The upper boundary of the stratosphere, lying approximately 52 Ian (32mi) above the surface.
Stratosphere The atmospheric layer lying above the troposphere; here temperatures are either constant or start increasing with altitude.
Temperature inversion Condition in which temperature increases with altitude rather than decreases - a positive lapse rate; it inverts what we, on the surface, believe to be the "normal" behavior of temperature change with increasing height.
Thermosphere The fourth layer of the atmosphere, lying respectively above the troposphere, stratosphere, and mesosphere; in this layer, temperatures increase as altitude increases.
Tropopause The upper boundary of the troposphere along which temperatures stop decreasing with height.
Troposphere The bottom layer of the atmosphere in which temperature usually decreases with altitude.
Variable gases Atmospheric gases present in differing quantities at different times and places; three are essential to human well-being: carbon dioxide, water vapour, and ozone.
Water vapour The invisible gaseous form of water; the most widely distributed variable gas of the atmosphere.
Weather The immediate and short-term conditions of the atmosphere that impinge on daily human activities.


Unit Outline

  • Contents of the atmosphere
    • Homosphere (surface to 50-63 mi above)
    • Heterosphere (above the homosphere)
    • Homospheric gases
      • constant gases
        • nitrogen (78% of air)
        • oxygen (21 % of air)
        • respiration
        • fossil fuels
        • inert gases
      • variable gases
        • carbon dioxide
        • water vapour
        • ozone
      • variable gases in minute quantities
        • hydrogen
        • helium
        • sulfur dioxide
        • oxides of nitrogen
        • ammoma
        • methane
        • carbon monoxide
    • Impurities (aerosols)
      • smoke
      • dust
      • bacteria
      • plant spores
      • salt crystals
    • Atmospheric cycles
      • atmosphere in dynamic equilibrium
      • hydrologic cycle
      • oxygen cycle
      • nitrogen cycle
      • carbon dioxide cycle
  • The layered temperature structure of the atmosphere
    • Troposphere (bottom of atmosphere)
    • Tropopause (at approximately 12 km above surface)
    • Stratosphere (12 km to 50 km)
    • Stratopause (at 50 km above surface)
    • Mesosphere (50 km to 80 km)
    • Mesopause (at 80 km above surface)
    • Thermosphere (top of atmosphere - above 80 km)
  • Research frontiers
  • Research continues into the layers of the effective atmosphere


Review Questions

  1. What are chlorofluorocarbons and how do they affect the atmosphere?
  2. List the constant gases and variable gases of Earth's atmosphere.
  3. Define and contrast the terms weather and climate.