I reproduced the glow that is around the sparks shader for you. It is not exact but if you play around with it, you can get different areas of the sphere to not be there. Obviously I only let this go a few samples to show you what it does, it will clean up a lot better as it goes along. I don't have the emitter in the same place here either. But this will give you a better Idea of what I did to get the effect. Think of it as oil on water, in an atmosphere....