THE DEPENDENCE OF THE SPECIFIC ATTENUATION CROSS-SECTION ON BLACK CARBON MASS FRACTION AND PARTICLE-SIZE

The specific attenuation cross-section sigma(ATN) which relates the at tenuation of light by a particle-loaden quartz fiber filter to the bla ck carbon (BC) content of deposited fine dust is the most important pa rameter for optical BC determination using an aethalometer. A simple m odel is presented to describe the effects oflight scattering particles on the specific attenuation cross-section. It is found that sigma(ATN ) varies with the BC fraction of total mass f according to sigma(ATN)= sigma(abs)+(f(-1)-1) sigma(seat). The average specific scattering cros s-section sigma(seat) depends on the light scattering dry fraction of fine dust particles, while the specific absorption cross-section sigma (abs) is determined by the light absorbing BC particles. The applicati on of the model to an externally mixed BC-NaCl aerosol shows excellent agreement between the proposed theory and our experiments. The suitab ility of the outlined model is confirmed by analyses of attenuation da ta sets obtained from several measuring sites in Germany. For the exam ined data sets the average attenuation cross-section sigma(ATN) of amb ient BC varies between 8 m(2) g(-1) (rural sites) and 19 m(2) g(-1) (s treet crossing). These variations can be explained by assuming a site- independent BC absorption cross-section sigma(abs)=4 m(2)g(-1) which i s derived from experiments with a BC test aerosol, and by varying the sigma(seat) value from 0.45 m(2)g .(-1) (rural sites) to 1.2-1.4 m(2)g (-1) (urban areas). Finally, the particle size dependence of the g spe cific absorption cross-section sigma(abs) is examined for a particle s ize range from 0.05 mu m to 1.1 mu m. As it is suggested by the fracta l nature of BC particles no size dependence of sigma(abs) with respect to the used BC test aerosol is found. Copyright (C) 1996 Published by Elsevier Science Ltd.