Atmospheric Composition, or Vacuuming Airy Cats

Just by way of showing how ‘scruciatingly obsessed with minutiae I am, I have actually spent time this morning computing the precise constituents of an Imperial ‘standard atmosphere’ – i.e., what standards documents whose first versions date back to the eldrae’s first space habitats mean when they say ‘standard atmosphere’, since obviously different planets and different species have their own ideas, here – to three decimal places…

And so now I’m going to show you my work.  Standard atmosphere:

74.864% N₂
21.930% O₂
3.164% Ar
0.042% (i.e., 420 ppm) CO₂

at 13.2 psi / 91.0 kPa.  Sprinkle with enough H₂O vapor to bring relative humidity to 40% for your preferred temperature set-point, and serve.

(This is, as you might expect, substantively identical to Eliéran sea-level atmospheric constituents and pressure, except for various minor trace gases all at least an order of magnitude less present than the CO₂. The substantially higher than, say, Earth’s percentage of argon is due to the higher quantities of radioactive elements in its crust that I may well have mentioned before.)