tf2/tf2_src/utils/particle_test/particles.cfg

sheets
{
  sheet1
  {
    spark
    {
      coords "0 0 10 10"
    }

  }
}

particles
{
  spark
  {
    sheet sheet1
    sprite spark
  }
}

psystems
{
  fireball
  {
    max_particles 1000000
    material effects/firesprite
    ptype spark
    initializers
    {
      emit_from_point
      {
        init_particles 100
        spawn_radius 10
        particle_lifetime "100 200"           // range    
	  }
    }
    operators
    {
      basic_movement
      {
         gravity "0 -10 0"
         drag 0.001
      }
    } 
  }
}