(covers information from several alternate timelines)
A dispersal node was a device designed to distribute a substance, either gas or energy, over a predetermined area. Dispersal nodes were components of starship environmental systems, responsible for the distribution of gaseous substances into the internal atmosphere of the ship.
In 2372, while attempting to sabotage the USS Voyager, Captain Braxton utilized a dispersal node to generate an energy form capable of blocking transporter signals in an effort to elude capture by the crew of the USS Relativity. The timeline in which these events occurred was later negated when Captain Janeway traveled back in time and apprehended Braxton before he had a chance to initiate his act of sabotage. (VOY: "Relativity")
In 2373, while attempting to distribute an antigen with which to defend Voyager against infestation by a macrovirus, Captain Janeway and The Doctor utilized the ship's environmental controls, setting the dispersal node to one part per thousand. (VOY: "Macrocosm")