The atmospheric system was a system aboard the USS Enterprise that regulated atmosphere aboard the starship.
A panel labeled "atmospheric system" located near section 2 on deck 9 was being worked on by Barnhart moments before he was killed by the M-113 creature. (TOS: "The Man Trap")
Another panel was in a corridor of the ship, near a set of quarters that, in 2266, were assigned to Charles Evans. Moments before a force field was activated in the door to Evans' quarters, the atmospheric system panel was momentarily tweaked by Harrison, though operating it wasn't one of his usual duties. (TOS: "Charlie X")
In the final draft script of "Charlie X", this control board was described as "the panel which will control the force field."