physics particle group collision event



I've made a physics particle group which works great, and its colliding with the rest of the physics objects in my room.

I would like to make it so when one of my "projectile" physics objects collide with the particle group, the projectile is destroyed.

I tried assigning a name to the particle group on creation, like this:

pPgrp = physics_particle_group_begin(flag, phy_particle_group_flag_solid, 576, 192, 0,0,0,0,c_white,1,1,1);

then in the projectile step event I put in the code

if place_meeting(x,y, argument0) instance_destroy();

but I get the error:
Variable obj_destroyer.pPgrp(100079, -2147483648) not set before reading it.

I guess I can't do that because pPgrp isnt exactly an object?
If the collisions are happening anyway, there must be a way to access those events?