Greenhouse Gases
- When radiation from the Sun hits the Earth, it is radiated back from the Earth's surface as long-wave radiation
- A greenhouse gas is a gas that absorbs this re-radiated radiation, trapping it in the Earth's atmosphere so that it is not lost to space
- Greenhouse gases in the atmosphere have a similar effect to the glass in a greenhouse, hence the term greenhouse gas
- There are many greenhouse gases, and those that contribute most to the greenhouse effect are:
- Carbon dioxide (CO2)
- Water vapour (H2O)
- These have the most significant impact on the greenhouse effect
- There are other greenhouse gases which have a lesser effect, such as:
- Ozone (O2 and O3)
- Methane
- Nitrous oxides
Greenhouse Gas Absorption Spectrum: Ozone absorbs nearly 100% ultraviolet rays, carbon dioxide absorbs radiation with wavelengths between 1.5 - 30 µm and water vapour from 0.8 - 35 µm. Most of the ultraviolet, visible, infrared and microwave radiation is absorbed by the atmosphere. The dark parts show radiation that is absorbed by each type of greenhouse gas. It is the ozone that restricts most of the outgoing infrared radiation from leaving the Earth's atmosphere.
Molecular Mechanisms
- The greenhouse effect occurs due to the particular molecular structure of greenhouse gases
- High-frequency UV light is energetic and able to break bonds within molecules
- Infrared light, on the other hand, causes atoms to vibrate
- The greenhouse gases have a natural frequency that falls in the infrared region
- This means when they absorb infrared light, they begin to resonate, causing the molecules to heat up
- They absorb the infrared radiation and subsequently emit it back towards the Earth’s surface
Solar radiation is primarily short-wave, while the radiation that is re-emitted by earth is long-wave radiation
Examiner Tip
You may have heard of a separate environmental concern, described as the 'hole in the ozone layer'; this is not something that you need to know about. Ozone is an atmospheric gas that absorbs harmful UV radiation before it reaches earth, but any concerns about ozone depletion have nothing to do with the greenhouse effect. The problem of ozone depletion is one that has improved significantly due to measures taken to reduce certain types of emissions; humans can get it right sometimes!
You do not need to know the specific sources of each type of greenhouse gas – all you need to know is that each greenhouse gas has both natural and man-made origins