i'm struggling to understand the pic (which looks nice, btw) and what, within the pic, you want to control or cause a trigger, etc.
since you have the emitter start location and are adding, theoretically, the direction, etc, you should be able to do some calculations to determine when a given particle reaches a particular location. within reason, that is, and depending on what you might be doing to the particles once they've been spit out.
otherwise, doing those kinds of checks, calculations, etc, would probably undermine the point of the particle system in the first place (which strips away a lot of that to increase performance), as i understand it, leaving you to write your own (which shouldn't be too difficult).
meanwhile, if you have some basic code you can share (because i honestly don't know what, in the pic, i should be focusing on), and exactly what
you want to happen when
(IE, the effect
you're trying to achieve), i'm sure a solution can be found. or, a short vid and what to watch for, might help, too.