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Fog field observation data during December 2002 and historic data from 1951 to 2002 in the Shapingba district of Chongqing were analyzed, and major characteristics of fog were described. The occurrence frequency of heavy fogs has a decreasing trend. Heavy fogs in the downtown area are the mixture of natural fog, smoke and dust, and the river valley and urban effects densify fogs. The PBL characteristics associated with heavy fogs are the temperature inversion, humidity increase and temperature decrease in the near-surface layer. The net radiation effect of high-concentration aerosol prevents the daytime mixing layer from developing, leading to the stabilization of PBL. It is the main reason for heavy fogs to last several days in the Chongqing downtown area. Heavy fogs play a role of wet deposition, cleaning the air to a certain degree. It is suggested that the total emission of pollutants should be controlled during heavy fog days to avoid the occurrence of heavy pollution events.