This study mainly investigated the causes of high black carbon (BC) episodes and effects of meteorological conditions on air quality in winter at Xian.Continuous BC mass concentration was measured from September 2003 to August 2007 at the site of Institute of Earth Environment,an urban site at Xian.Averaged BC concentrations were higher in winter and autumn than those in summer and spring.High BC concentration often appeared in winter and the magnitude was higher than those measured at other urban sites.Annual averaged BC concentration showed a linear decline that indicated the reduction of emissions and proportion of coal burning in the total energy consumption in winter.Relationships between BC concentration and meteorological conditions showed that an inversion layer,descending motion in the low troposphere and weak surface wind jointly contributed to high BC concentration in winter at Xian.Significant negative relationships between BC concentration and boundary layer height,wind speed implied that meteorological conditions directly affected seasonal variation of BC concentration.Additionally,less precipitation was also a key factor that led to high BC concentration in winter at Xian due to BC accumulation in the atmosphere.