A polar photometer was used to investigate phase functions of glass fiber filters with varying aerosol particle loadings. Angle-resolved analysis revealed that the shape of the normalized phase function S-n(theta) in the transmitted hemisphere is independent of aerosol composition and determined exclusively by the fibrous filter matrix, while S-n(theta) in the reflected hemisphere depends on the scattering properties of the aerosol sample. Cross sensitivities of the specific attenuation cross section with respect to scattering components were examined with well-defined laboratory generated aerosol samples. The specific attenuation cross section at theta = 165 degrees (reflected hemisphere) was found considerably less dependent of the aerosol composition than any angular position in the transmitted hemisphere, and is therefore best suited for the determination of black carbon (BC). The angular position theta = 165 degrees was calibrated with an almost pure BC test aerosol. BC mass loadings of ambient aerosol samples (0.15-20 mu g cm(-2)) determined with this calibration function were in good agreement with values analyzed by coulometry.