A continuous air and precipitation sampling for carbonaceous particles was conducted in a field observatory beside Nam Co,Central Tibetan Plateau during July of 2006 through January of 2007.Organic carbon(OC)was the dominant composition of the carbonaceous particles both in the atmosphere(1660ng/m3)and precipitation(476ng/g)in this area,while the average elemental carbon(BC)concentrations in the atmosphere and precipitation were only 82ng/m3 and 8ng/g,respectively.Very high OC/BC ratio suggested local secondary organic carbon could be a dominant contribution to OC over the Nam Co region,while BC could be mainly originated from Southern Asia,as indicated by trajectory analysis and aerosol optical depth.Comparison between the BC concentrations measured in Lhasa,those at"Nepal Climate Observatory at Pyramid(NCO-P)"site on the southern slope of the Himalayas,and Nam Co suggested BC in the Nam Co region reflected a background with weak anthropogenic disturbances and the emissions from Lhasa might have little impact on the atmospheric environment here,while the pollutants from the Indo-Gangetic Basin of Southern Asia could be transported to the Nam Co region by both the summer monsoon and the westerly.