Green’s Dictionary of Slang

(US campus) the stink created by someone having recently broken wind.

1989		P. Munro Sl. U. 24: Mark farted and left the room, leaving in his wake an atomic atmosphere for all to enjoy.
1997		K. Kainulainen ‘University Euphemisms in Calif. Today’ 🌐 Some of the euphemistic expressions I had heard before seemed to me as they were invented by a bunch of guys my age fooling around, [...] ‘Atomic atmosphere’ used to describe the malodorous air caused by a recent fart is an example of this.