A barystatic filter was a device which removed pollutants from a planet's atmosphere.
The filter functioned by projecting negatively-charged ions into the stratosphere, which then transferred their charge to atmospheric pollutants. The thus-charged pollutants were then collected by the positively-charged intake vents of the barystatic filter. A consequence of this system was that an ionic current was set up in the planet's atmosphere, which on a large enough scale could interfere with the function of transporters.
A thousand barystatic filters were installed on Tagra IV in an effort to combat their pollution problem. (TNG: "True Q")