Light absorption by samples of atmospheric aerosol particles as a function of size was studied using the integrating sphere method. In addition, the optical properties of fog and cloud-water residues were determined. The samples were taken at two locations in West Germany: one fairly remote from pollution sources, the other near the industrial area of Frankfurt. The results show that particles <0.4 μm volume equivalent radius significantly alter the values of the absorption coefficient σA. The absorption index k of atmospheric aerosol particles, in general, cannot be considered a property of the sample material. The absorption of light by samples of large particles from background areas exposed to urban influences can be explained quantitatively by the presence of soot particles. The soot content of samples of large particles is ∼−10% of the total particle number. The number of soot particles in samples of giant particles is very small. Starting with the highest absorption of solar radiation the following sequence was found: non-activated particles, large particles, giant particles, cloud-and fog-droplet residues and, finally, rainwater residues.