Figure 4: Backward trajectories of air masses at 2000 m, 1000 m, and 500 m heights by NOAA HYSPLIT model ending at (a) 09:00 LST (00:00 UTC), October 26, 2003 and (b) 21:00 LST (12:00 UTC). Clean air masses from above 2 km height of the north-eastern Mongolia flew toward Gangneung city at each level in (a) and ones from above 3 km height outside Gobi Desert (Nei-Mongo in the northeastern China) reached 1000 m and 2000 m heights of the city, without dust transportation to the city in (b). As weather map or wind fields generated by a meteorological numerical model just show flow patterns of air at a certain height and at a given time of a special day, but particle trajectory using NOAA-HYSPIT model supplies the moving path (main stream) of majority of air particle and its position at every 6 hours for several days, continuously.