The composition and optical absorption coefficient of atmospheric particulate matter

The optical absorption spectra of atmospheric dust as determined by transmission and diffuse reflectance spectroscopic methods is discussed in the 0.4 to 40 um wavelength range. Quantitative measurements are presented which show the imaginary refractive index to be about 0.007i, with little wavelength dependence, in the 0.4 to 1.3 μm spectral interval. The absorption coefficients of individual materials found in atmospheric dust are also given. This work suggests that atmospheric dust may be composed mainly of weakly absorbing particles contaminated with small amounts of very strongly absorbing materials such as free carbon. The implications of this are discussed from the point of view of laser beam attenuation and lidar return signals. Mie theory computations for ruby lidar wavelengths are shown which suggest that for some models of atmospheric dust, the concept of an average imaginary refractive index may be misleading. Thus, it may be necessary to consider the individual complex refractive indices and size distributions of more than one constituent material present in the dust. This implies that anthropogenic contributions to the atmospheric aerosol, such as free carbon and other strong absorbers, may be of greater optical significance than their relative concentrations might indicate.