Boden wrote:

Norminn wrote:

....
CO is a bit heavier than air. If the atrium was only open at the top
the result was to be expected.


CO is slightly lighter than air (insignificantly so).
CO is 28 g/mol.
Air is (roughly) an 80/20 mixute of N2 (28 g/mol) and O2 (32 g/mol).