Physicochemical characteristics of black carbon aerosol and its radiative impact in a polluted urban area of China

Black carbon (BC) aerosol plays an important role in the Earth's radiative balance. An intensive measurement campaign was conducted at Xi'an, China, from December 2012 to January 2013 to investigate the sources and physicochemical characteristics of refractory BC (rBC) and its direct radiative forcing at the surface. The overall average rBC concentration for the campaign was 8.07.1 mu gm(-3). Source apportionment based on positive matrix factorization showed that traffic was the dominant rBC source (46.0%), followed by coal burning (33.9%) and biomass burning (20.1%). The rBC mass size distributions were monomodal and lognormal with larger mass median diameters for coal burning source (215nm) compared with the traffic source (189nm). Coal burning rBC was more strongly associated with sulfate than traffic rBC, suggesting a higher cloud condensation nuclei activity. The slope of a robust linear regression between rBC and carbon monoxide (CO) for all samples was 5.9 mu gm(-3)ppm(-1), and the slope for the coal burning source (4.5 mu gm(-3)ppm(-1)) was larger than that for the traffic source (2.7 mu gm(-3)ppm(-1)). The net rBC emission during winter of 2009 was estimated to be 4.5Gg based on the relationship between rBC and CO. A Tropospheric Ultraviolet and Visible radiation model showed that the average daytime value for the clear-sky direct radiative forcing due to rBC from 23 December 2012 to 31 January 2013 was -47.728.9Wm(-2), which amounted to an average of 45.7% of the total surface atmospheric aerosol forcing.