Dust is aligned in space to some extent, and this results in selective absorption such that there is a preferred plane of vibration for the light waves. (britannica.com)
One major simplification involves treating all the infrared radiation (IR) as if it consisted of one wave length, although the real situation is of course much more complicated with a wave spectrum, selective absorption bands, and radiation with different frequencies and optical depths (Pierrehumbert 2011). (link.springer.com)
After ruling out the possibility that the helium may be doubly ionized, the authors conclude that the most likely explanation for the discrepancy is selective absorption of helium - ionizing photons by dust. (greenbankobservatory.org)