SIZE SEGREGATED LIGHT-ABSORPTION COEFFICIENT OF THE ATMOSPHERIC AEROSOL

The light absorption coefficient of atmospheric aerosols in the visibl e can be determined by depositing the particles on a filter and measur ing its ''transmission'' in a special optical arrangement. With an imp actor with rotating impaction plates producing a homogeneous deposit, it is possible to extend this technique to size segregated aerosol sam ples. A simultaneous determination of the mass size distribution is po ssible. Test measurements with black carbon aerosol have shown the fea sibility of this method. Samples of the atmospheric aerosol have been taken in and near Vienna, in Naples and near Bologna. The light absorp tion of the aerosol is always highest for particle diameters between 0 .1 and 0.2 mu m. Only in the humid environment of the Po valley it had a slightly larger peak size, whereas the size of the nonabsorbing par ticles increased considerably. The light absorption of the atmospheric aerosol is always higher in an urban environment. The mass absorption coefficient of the aerosol at all four locations was very similar, an d completely different from values which could be expected using effec tive refractive indices which are frequently used in models. Using the data measured in this work two alternate models for the effective ref ractive index and black carbon content of the aerosol are suggested: ( a) a size-dependent refractive index, where the imaginary part varies from - 0.25 for particles smaller than 30 nm to - 0.003 for particles larger than 2 mu m; this could especially be applied if an internal mi xing of the aerosol is to be expected, or (2) a size-dependent fractio n of elemental carbon in the case of external mixing with 43% of carbo n particles for sizes below 30 nm decreasing to 10% for sizes up to 0. 4 mu m.