I just bought Particular so I'm still learning the ropes.
I have a project that calls for an irregularly shaped object to dissolve. I have everything set up with the precomped wiping mask as well as the layer below it that linear wipes along with the Particular layer referencing the first mask wiping layer so it works fine but no matter what graphic I want to dissolve, the emission of the particles is never really quite exactly from the edges of the graphic...many of them are offset about 25 pixels or so and this kind of ruins the coolness/illusion.
The first thing that comes in mind, the camera/emitter positions are offset to the layer you are trying to dissolve. Make sure, the emitter layer and the object layer both are 3D and at the exact pos-rot-scale. Also, go to the Particle and check if Opacity-Over-Life and Size-Over-Life curves are "flat-100%" at left side, so you can see where exactly those particles are emitted from.