4.2: Two mechanisms for atmospheric absorption of sunlight
The solar radiation that enters Earth's atmosphere undergoes some degree of absorption, so only part of it reaches the ground.
The figure shows the AM0 spectrum as the black line  and the terrestrial spectrum without the blue sky (red line) ,
and the transmittance of those atmospheric gases that are the main responsible for absorption .
There are two main mechanisms that cause absorption in gas molecules:
- The photon excites an electron to a higher atomic/molecular shell (shell transitions). In atoms and simple molecules, the atomic shells
have very distinct energies, so shell transitions cause mostly rather narrow absorption lines.
- The photon causes parts of a molecule to rotate and/or to vibrate differently (rotational-vibrational transitions). The more complicated
a molecule is, the more ways are there to rotate and vibrate it with hardly distinct energies, and this is the cause for broad absorption bands.
Looking at the graph, what do you think: Which molecules absorb due to shell transitions, which due to rotational-vibrational transitions?
→ Hover here for a hint.
Note that the AM0 spectrum has absorption lines too, especially above 400 nm. They exist because the sun is surrounded by gases
(called photosphere) where absorption takes place, too. These lines are visible in the terrestrial spectrum, primarily in spectral
ranges where there is no absorption in the earth atmosphere.
Figure: Sources of absorption in the AM1-5d spectrum.