What are the oxygen or water vapor lines?

Question

I was reading the document Recommendation ITU-R P.676-11 (09/2016); Attenuation by atmospheric gases (P Series, Radiowave propagation) in order to calculate the atmospheric losses, and there were some terms that I did not understand.

Specifically, in Annex 1; Line-by-line calculation of gaseous attenuation on page 4 equations 2a and 2b are presented, where they talk about the "imaginary parts of the frequency-dependent complex refractivities" of the oxygen and water vapor lines.

What are these lines? How many should I take into account when computing the atmospheric losses?

Answer 1

These are absorption lines due to molecules of oxygen (O$_{2}$) and water (H$_{2}$O). They are listed in Tables 1 and 2 of that Annex, including all the parameters needed to calculate their individual effects (for use in Equations 3 through 7).

Which ones you want to use depends, I suppose, on what part of the spectrum you're worried about. If you only want to know attenuation at 900 GHz, then you can probably ignore all the lines with frequencies lower than 800 GHz without any problems. If you want to know the attenuation for a bandpass spanning 300 to 500 GHz, then you want to include all the lines within that range, and maybe some of the H$_{2}$O lines at frequencies slightly larger than 500 GHz as well. (Figure 1 and 2 show where the different lines -- or combinations of nearby lines -- dominate within the 1-1000 Ghz range. The oxygen lines are always present; the additional effects of the H$_{2}$O lines show up in the "Standard" curve.)