CO absorbs the ground's thermal radiation mainly at wavelengths between 13 and 17 micron. At this wavelength range, it is almost solely responsible for the attenuation of radiation from the ground. The amount of ground radiation that is transmitted through the atmosphere in each wavelength is related to the optical depth of the atmosphere at this wavelength, OD, by:
The optical depth itself is given by Beer–Lambert law:
where &sigma; is the absorption cross section of a single CO molecule, and n(y) is the 
number density of these molecules at altitude y. Due to the high dependence of the cross section in wavelength, the OD changes from around 0.1 at 13 microns to ~10 at 14 microns and even higher beyond 100 at 15 microns, then dropping off to ~10 at 16 microns, ~1 at 17 microns and below 0.1 at 18 microns. Note that the OD depends on the total number of molecules per unit area in the atmosphere, and therefore rises linearly with its CO content.
Looked upon from outer space into the atmosphere at a specific wavelength, one would see to different degrees different layers of the atmosphere, but on average one would see down to an altitude such that the part of the atmosphere from this altitude and up has an optical depth of ~1. Earth will therefore radiate at this wavelength approximately according to the temperature of that altitude. The effect of increasing CO atmospheric content means that the optical depth increases, so that the altitude seen from outer space increases; as long as it increases within the troposphere, the radiation temperature drops and the radiation decreases. When it reaches the tropopause, any further increase in CO levels will have no noticeable effect, since the temperature no longer depends there on the altitude.
At wavelengths of 14 to 16 microns, even the tropopause, having ~0.12 of the amount of CO of the whole atmosphere, has OD>1. Therefore, at these wavelengths Earth radiates mainly in the tropopause temperature, and addition of CO does not change this. At wavelengths smaller than 13 microns or larger than 18 microns, the atmospheric absorption is negligible, and addition of CO hardly changes this. Therefore, the effect of CO increase on radiation is relevant in wavelengths 13–14 and 16–18 microns, and addition on CO mainly contributes to the opacity of the troposphere, changing the altitude that is effectively seen from outer space within the troposphere.