Atmospheric particulate matter was examined to estimate the significance of free carbon as an absorber of near-ultraviolet, visible, and near-infrared radiation. Bulk and size-fractionated samples have been disassembled into acetone-soluble, water-soluble, and insoluble fractions. The absorption coefficients for these fractions, and for the insoluble material after removal of the free carbon by burning, have been measured. The results show that in the visible and near infrared, free carbon, although not a major component by mass, is by far the dominant absorbing material. These measurements in relation to other research suggest that geographic variations in and anthropogenic contributions to the free-carbon content cause much of the variation in the absorption coefficient of atmospheric particulate samples.
