Hi,
I don’t have Particular on this machine but I don’t think that an Obscuration layer would produce the same effect then the wall feature in Particle Playground. If I recall right it was only meant to intersect your particles with a 3D layer.
Depending on your animation and the look you are after you may want to simply apply CC Sphere to your particle layer and switch its “Render” to “Outside”. Play with the lighting.
Or simply precompose your particle layer including all attributes, then apply a feathered mask to your precomp. But as I said it all depends on what you are after.
Maybe someone more familiar with Particular will answer your question here or at the Trapcode forum.
Salut.
Serge