What Do We Know?
As you have learned, greenhouse gases contribute to the heating of the atmosphere by absorbing IR radiation that is emitted by the Earth and increasing the height at which emission into space can occur, thereby trapping energy within the atmosphere. However, how does the absorption of IR radiation by greenhouse gases actually translate into an increase in atmospheric temperature?
First of all, recall the understanding of temperature in terms of kinetic molecular theory. How did the temperature of a gas sample relate to the movement of gas particles?
Open the Collisional Heating Learning Tool to explore the mechanism by which greenhouse gases increase atmospheric temperature. Move the purple bar over one of the IR absorbance peaks in the absorption spectrum of the carbon dioxide molecule displayed in the center of the screen. Now go to the display tab in the 'Options' menu and turn on the atmosphere. What do you observe?
Absorption of IR photons is vital in the regulation of Earth’s climate. What happens to IR photons when they are absorbed by atmospheric carbon dioxide molecules? Use the Collisional Heating Learning Tool as necessary.
There are two ways that a carbon dioxide molecule can lose the energy it has absorbed:
- It can re-emit a photon in a random direction, or
- It can collide with another molecule, such as oxygen or nitrogen, causing the other molecule to gain kinetic energy (move faster). This will lead to atmospheric heating.
Both these interactions can be seen in the Collisional Heating Learning Tool, although you may have to watch for awhile to observe both possibilities.
In order to see these situations, ensure that the bar on the IR spectrum is located in one of carbon dioxide’s absorption peaks, so that the molecule is able to absorb the IR photons. Watch until a photon is absorbed by the molecule. If you have the 'Atmosphere' option selected, you will observe collisions between the carbon dioxide molecule and the other atmospheric gas molecules.
Also, after absorbing a photon of IR radiation, the carbon dioxide molecule may re-emit a photon of radiation. It may take awhile to see this occur in the Learning Tool, but it is easier to see if the 'Atmosphere' option is not selected. When the molecule re-emits a photon, you will see a red circle expand outwards from the molecule.