Based on black carbon aerosol(BC) observations in Lanzhou,Dunhuang and Tazhong stations,as well as PM10 and aerosol optical depth(AOD) observations from the related stations during March to May 2006 in the northwestern China,the black carbon aerosol distribution is studied.The results show that the BC mass concentration is the highest in Lanzhou compared with Dunhuang and Tazhong by a mean value of 2.22 μg/mm³,1.89 μg/mm³ and 2.07 μg/mm³ respectively.This value is smaller than that in Beijing,Shanghai and Zhujiang Delta,but higher than that in Waliguan atmospheric background station.The diurnal variation of BC mass concentration has an obvious peak and valley characteristics with lower value at 12:00-14:00 BST(Beijing Standard Time) and higher value at 08:00-20:00 BST,which mainly depends on the diurnal variation of BC sources and its turbulent exchange in the surface layer,and the diurnal variation of atmospheric stability.The correlation coefficient between BC and PM₁₀ mass concentration is very small in the northwestern China with the highest correlation coefficient(only 0.24) of hourly observations in Tazhong,indicating that BC mainly comes from human activity and the main composition of PM₁₀ is sand dust aerosol.Once the sandstorm passes the observational station,the BC mass concentration will increase more than 50% for the absorbing characteristic of sand dust aerosol,and BC carried by the abundant sand dust only occupy a small proportion.