The statement that Earth's atmosphere contains a specific total amount of oxygen gas is a matter of everyday physical reality and common knowledge; no specific citation is required to establish that a finite planet has a finite atmospheric mass and composition.
The claim states that the Earth's atmosphere contains a specific total amount of oxygen gas. This is a trivially true, observable property of a finite planetary system containing a finite atmosphere with a measurable percentage of oxygen. Per instructions, common knowledge claims do not require citations, and papers are not attached as receipts for the obvious.