C. What are Greenhouse Gas Absorption Profiles?
There are three major factors in the amount of infrared radiation that can be absorbed and re-emitted. It is the number of molecules present and the absorption profile. The number of molecules present is determined by the molar concentration and density. The second element is the absorption profile. The third factor is the wavelength or temperature of the infrared radiation. It requires an analysis of each component. If the molar concentration is low, it will directly reduce the amount of absorption. If the absorption profile is low, it will directly reduce the amount of absorption. If the density (pressure) is low, the atmosphere is too thin, e.g. Mars, it will directly reduce the amount of absorption.
The figure below shows the absorption profile of the main Greenhouse Gases. The top panel of the figure shows the amount of radiation that passes directly from the Earth’s Surface to space without significant absorption by any of the Greenhouse Gases. This amounts to 15 to 30% of all up-going thermal radiation that cools the planet and known as the Infrared Window.
The second panel is the total infrared absorption by all gases. The temperature equivalent for the infrared radiation in the Infrared Window is roughly +90°C (190°F) to -66°C (-87° F).
The third panel is the absorption profile of water vapor. This profile dominates the absorption profiles of all other Greenhouse Gases by a large factor up to ten fold. It also has some absorption, albeit small, inside the Infrared Window. It also has a significant absorption via liquid water and ice crystals, which is not shown in the above figure. But, because water vapor has a very large concentration, 20,000 to 40,000 ppmv it does provide significant absorption.
The next profile is for CO2. The figure illustrates that CO2 absorbs most infrared radiation with a temperature equivalent of -66°C (-87° F) to -102°C (-150° F). This means that CO2 works best in the upper colder atmospheres. But these upper cold atmospheres have major problems with reduced density.
The only Greenhouse Gas with an absorption peak in this Infrared Window is Ozone as shown in panel 4 of the above Rhode’s figure. And as shown in Item 2, Ozone is only remotely present at 0.025 ppmv as compared to water vapor at 30,000 ppmv in first place and carbon dioxide at 407 ppmv in second place.
Methane is shown in panel 5 and indicates that it has an absorption peak on the left hand side of the Infrared Window. It is also dominated by water vapor at this temperature. And as shown in Item 2, methane is only remotely present at 1.9 ppmv as compared to water vapor around 20-40,000 ppmv in first place and carbon dioxide at 407 ppmv in second place.
The last panel 6 shows the absorption profile for the nitrous oxides. The peaks for Nitrous Oxide is at the same wave lengths as methane and one at a very high temperature (370°C or 700°F) which is only produced in a few areas on the planet. As shown in Item 2, nitrous oxides are only remotely present at 0.32 ppmv.