This figure shows number density (red squares) for each brightness of the galaxies derived from observation this time. (alma-telescope.jp)
R.J. calculated the spatial number density of ISOs. (nature.com)
I don't know how absorption probabilities go with pressure off the top of my head, but for sure they decrease with decreasing pressure; focusing on band structure while excluding the huge drop in number density with altitude is obfuscatory. (realclimate.org)