Abstract:Fog is one of the most severe weather events in Beijing in winter, and restrains the diffusion of air pollutants. During heavy fog days from 30 November 2002 to 4 December 2002, air pollutants were greatly retained by the wind shadow, low convergent zone, and conditional stability. The air was polluted heavily. The variation characteristics of air pollutants and the evolvement and characteristics of the related synoptic processes are analyzed.