Your best bet or his would be to first try mikes tornado tutorial, as that is pretty much the same "volume" idea, and should lead you down the right path. (no pun intended)
Otherwise, I've included a 4.5 file that migh help you
Please check EVERY attribute to learn how I kept the particles from being thrown out of the volume
if you need anything else, lemme now
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com