I was reviewing through the lecture slides and came across the following question: Why do we not consider absorption of IR by N2 and O2, which are way more abundant in the atmosphere, then CO2? Is the answer because N2 and O2 are symmetrical, unlike CO2, and therefore they have a negligible dipole moment; and, with IR absorption such as in IR spectroscopy, a dipole moment must be present in order for IR to be absorbed? Thank you. (Almost. We need to consider change of the dipole moment, not the dipole moment itself. - GS).