This is an introduction to MODTRAN spectra and their interpretation
This is an emission spectrum of the US Standard Atmosphere containing 380 ppmv of CO2, about the present day concentration. It is generated by using the MODTRAN programme and the red curve is the simulated spectrum. Note that the features of the spectrum are very similar to the experimentally derived spectrum from data collected by the Nimbus-4 satellite in 1974 as shown on the previous page. The units of radiance are watts per square metre per wavenumber. The area under the red curve represents the total radiant intensity leaving the atmosphere at an altitude of 70 km.
The influence of the greenhouse gases are indicated. The region up to 580 cm-1 is dominated by emissions from water molecules as they lose rotational energy by emitting appropriate photons. Although they are not obvious, in the same region there are small emissions from nitrous oxide [even though it is a linear molecule, it possesses a permanent dipole moment and thus is permitted by the quantum rules to absorb and emit quanta [photons] which cause or result in the molecule increasing or decreasing its rotational energy]. The photons directed outwards to space are the ones calculated by the MODTRAN programme.
Molecules losing energy by the emission of photons do so in all directions, the photons directed downwards to the Earth's surface either are absorbed by the surface or never reach the surface if they are absorbed by the greenhouse gases or by clouds.
Between 580-750 cm-1 belongs mainly to CO2. Because the molecule does not possess a permanent dipole moment it is forbidden to lose rotational energy by emitting photons. It is permitted to lose vibrational energy emissively and does so in a very complex manner, the details of the spectrum are described in a subsequent page. Essentially the molecule loses vibrational energy coupled with the loss or gain of some rotational energy. Although it is not obvious from this spectrum, there are some rotational transitions of water molecules also occurring in this region and are obscured by the strong emissions from CO2.
The infrared window is clear for all to see between 750-1250 cm-1 with an intervention from the ozone molecule which emits around 1040 cm-1.
The final piece of the spectrum is crowded by emissions from the vibrational loss of energy by water, methane and nitrous oxide, their spectra overlapping considerably.