Spoiler:
They take two gas tanks: one as scrubber tank and one as pump reservoir; they therefore also have three modes: pump in/out, and scrub contaminants.
This allows atmos techs to become walking firebombs,, then, after making their mess, lets them return the air to breathable status in a more efficient fashion.
Bonus: if you want to nerf the pneumatic cannon make it require air from one of these instead of an ordinary tank and don't even think about hooking up a flamethrower.