they are stored in the ufos folder - those ufo script files that starts with a ptl_* (but be aware, that you can't use every particle as a static map particle - some are only for weapons) - the best would be to open an existing map and have a look at the particle definitions in the mapeditor for this map. Just to get a better feeling for them. They are normal entities so just select them and hit n to get the entity properties