Specific absorption cross-section and elemental carbon content of urban aerosols

To investigate the light absorption efficiency of aerosols, samples of fine particulate matter (PM2.5) were collected during a field experiment in Taipei, Taiwan. Mass concentration and the elemental carbon (EC) content of the PM2.5 samples were measured, and in addition the light absorption cross-section of the EC in PM2.5 was derived. On average, elemental carbon contributed 8.6 +/- 3.5 % to the PM2.5 mass. The aerosol light absorption coefficient was linearly proportional to the ambient EC concentration. Consequently, an average specific absorption cross-section of 10.8 +/- 2.6 m(2)/g was obtained for EC in the aerosols. Analyses of the variations in the absorption cross-section revealed that the cross-section converged towards the average level as the ambient EC concentration increased. The scattering in the low EC regime is proposed to be due to differences in the mixing state: internal mixing enhanced the cross-section, whereas external mixing resulted in a decreasing trend.