I wonder if the question [ppmV versus μmol/mol for atmospheric CO2][1] might do better in Chemistry or Physics SE. The science behind the answer probably will need the correction to the ideal gas law as expressed in [van der Waals equation][2] or [van der Waals Equation of State][3] as applied to mixtures of non-ideal gasses.

But the simplest answer could be just a conversion factor close to 1.0 which would be close enough for me for now.

Any thoughts?

**edit:** I've just won a "Tumbleweed" with this question! :) I've also added a `+50` bounty on the original question. If still no luck, I'll either ask for migration, or just delete and repost.

[![enter image description here][4]][4]


  [1]: https://earthscience.stackexchange.com/q/10157/6031
  [2]: https://en.wikipedia.org/wiki/Van_der_Waals_equation
  [3]: http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/waal.html#c1
  [4]: https://i.sstatic.net/1KKtq.png