Notes: Mon 16 Sept

The Atmosphere, continued

From last time: remember that CO is carbon monoxide, the same lethal gas you generate from car exhaust, whereas CO2 is carbon dioxide, which is a natural gas but which may help to increase the temperature of the atmosphere. Carbon dioxide and aerosols have the opposite effects on temperature; aerosols may actually cool down the atmosphere by blocking out incoming radiation.

Modern atmospheric composition: as discussed last time, it is mostly nitrogen, oxygen, and argon. "Modern" means within about the last 600 million years of Earth's 4.6 billion-year-old history. Compared to the primordial atmosphere, this composition is completely different. What happened? You must consider:

  • were the original gases inert?
  • what are their chemical and physical properties on Earth's surface?
  • what are the masses of the gases?

    With these questions in mind, some of the factors in the evolution of the atmosphere are:

  • the earlier gases, called volatiles, were much lighter and reactive. They dissipated and escaped into space.
  • volcanic outgassing added different gases to the atmosphere over time.
  • radioactive decay added or increased amounts of certain elements (for instance the decay of a potassium isotope creates more argon).
  • biological and chemical activity at Earth's surface (like chemical synthesis, which came before photosynthesis).
  • photodissociation, or the breakdown of water by UV radiation. The process leaves behind hydrogen gas, which goes into space, and oxygen, which stays on Earth.

    The present atmosphere is primarily the result of volcanic outgassing, though. Some evidence: the composition of gases from volcanoes today is similar to the composition of atmospheric gases. The argon:nitrogen ratio in volcanoes and the atmosphere is the same. However, volcanoes put out much more carbon dioxide and H2O compared to what we see in the atmosphere. Where does all this extra CO2 and H2O go? H2O is taken up by the oceans. CO2 is changed to CaCO3 in the oceans and is used up in photosynthesis as well.

    Other sources:

    Structure of the atmosphere
    Classifying the atmosphere based on temperature characteristics is the most popular method. Note that the book places the "top" of the atmosphere way too high up.

    80-480 km: thermosphere (top is "thermopause")

  • temperature increases with increasing height
  • average kinetic energy of particles here is high, but there are very few particles, so it isn't sensible heat.

    50-80 km: mesosphere (top is "mesopause")

  • temperature decreases with increasing height
  • has few gas molecules

    18-50 km: stratosphere (top is the "stratopause")

  • temperature increases with increasing height
  • this layer is very stable and resistant to mixing

    0-18 km: troposphere (top is the "tropopause")

  • temperature decreases with increasing height
  • how fast temperature changes with height is called the lapse rate. For the troposphere it is about -6.4 degrees C/km.
  • all weather systems (which includes clouds) are found here.

    Why is weather found in the troposphere and not the stratosphere?
    Well, if we have some air that is warm at the bottom and cool at the top, the cool air is going to want to sink and the hot air is going to want to rise. The result is turbulent mixing, or "convection." This is the situation in the troposphere. If we look at the stratosphere, we see temperature is colder on the bottom and hotter on the top. This is stable because cold air naturally wants to be at the bottom, and so the stratosphere will not want to mix itself. This stable situation is called an inversion.

    Why does the atmosphere have 2 inversions?
    The first inversion in the stratosphere is due to ozone. Ozone absorbs UV radiation, which causes it to heat up. (More UV is soaked up at the top, so the top becomes hotter.)
    The second inversion in the thermosphere is also caused by absorption, but of even shorter wavelength radiation (higher energy). The wavelength of energy absorbed depends also on the size of the molecule doing the absorbing.

    The top part of the atmosphere is also called the magnetosphere because it is the region where charged particles in the solar wind interact with Earth's magnetic field (remember the Northern & Southern lights?). This illustrates that using temperature to classify the atmospheric structure is just one way to do it. Other methods are based on the purpose of the classification, such as satellite or radio technology.

    Back to GS 201 homepage