Using nParticle collision events, you can combine particle systems, emit new particles into particle systems in the advanced frames of a simulation, and accurately kill particle systems when they collide.
A collision event involves two or more nParticle system: a original particle system, whose particles execute the event when they collide with other objects, and a target particle system, which is the nParticle system that emits particles when the event is executed.
You can create and edit collision events using the Particle Collision Event Editor.