Abstract

Most aerosol particles are of mixed composition. This raises the question of shifts and distortion of the IR bands of their constituents. Shifts of as much as −40 cm^-1 have been found in wide-spaced bands of dry submicrometer particles consisting of internal mixes of alkali salts that were obtained by spraying solutions. Furthermore, overlapping bands merged. This was not observed with weaker and broader bands of ammonia salts. Comparison of these observations with theory is difficult because observed bands of small particles, especially if they are spheres, are much wider than is predicted by theory. Because most of the particles are "roundish," it has been assumed that the anisotropic internal structures (crystallites, dendrites, and voids from solution evaporation) might permit averaging of the theoretical results. This explains the experimental observations much better than recently promoted continuous distribution of ellipsoids, especially if one also considers reflectance of powders pressed to bulk density. Spectra of particles containing insoluble compounds showed little distortion. In conclusion, band shifts in atmospheric aerosols might be negligible.

© 1990 Optical Society of America