As technologically advanced as Númenor was in Pharazôn's time, I don't see them having any sort of WMD capability. After all, some of the King's Men escaped the Downfall, becoming the enemies of Gondor. If that sort of technology had been available, surely they'd have used it against Gondor at some point?
Speaking of WMDs in Middle-earth, Sauron apparently used
bioterror weapons in the Third Age. Good thing he didn't pass
that along to the Easterlings or Southrons.