From Valve Developer Community
Particle Editing Tool
Note: Valve has broken the editors for both TF2 and HL2:EP2. Currently, the particle editor can only be launched through Portal.
- To run the particle editor, run the game with -tools on the commandline and select 'Particle Editor' from the tool menu on the far right.
- To load a map so you can test your particle, look to the console window at the bottom of the screen and type "map <levelname>"
- A helpful command to restart all particle systems in a level - useful when tweaking them in the editor : bind "h" = "ent_fire info_particle_system stop;wait 120;ent_fire info_particle_system start"