create a graphic that contains a circle with radial fill. the fill can be whatever color you want and have it fade to alpha=0 at its boundard.
now click the spray brush tool and in the properties panel, change the symbol used by the tool to your graphic. spray. now refine your graphic until you see the results you want. there are also some spray brush options in the properties panel that may help you achieve the look you want. but you have more flexibility editing the shape(s) in your graphic.
you can even animate that graphic if you want to get exotic.