[PK]

To continue answering things:

Emanation obviates the need for an attack roll. It's one of its built-in benefits, implied heavily in the Basic Set but then made more explicit elsewhere. Aura is similar in this way, just longer-lasting.

Otherwise, whether attacking the ground you're standing on with an area effect requires an attack roll is up to the GM. There are about a million different variations on how this could go down, so use common sense. If the attack could feasibly miss, then ask for an attack roll -- though speaking as GM I'd give both the usual +4 for attacking an area and a +2 circumstance bonus for "shooting straight down" (a.k.a. the Inverted Takedown bonus for those of you who play Arkham: Batman games).