To get the best smoke effect in a HL Singleplayer map I'd say get Spirit and use an env_particle entity and use the blacksmoke preset.
Otherwise create a few env_sprites going upwards in a line, using the scale property make them bigger the higher they are.
The get the name of the sprite (.spr) file that you want to use het sprite_explorer
here and search the pak0.pak file in the Valve folder.