They won't disappear until they make contact with the ground. That's just how env_shooter works. A possiple solution, albeit tricky, would be to put an invisible func_wall close to where they spawn, let them fall and hit that, then use the func_push to lift them after they've bounced one. Its possible, give it a shot.
Another possible solution would simply be to use one sprite on a laser, and have the sprite look like a rising, fading stream of smoke. This is a more optimized approach, and the more I think of it, the more I think this is your golden solution. I mean, those particles will always be in the distance, right?