Electro-optical Detection of External Mixtures in Aerosols

A measurement technique was developed that has the capability to resolve externally mixed particles of different refractive indices within the range expected for and observed in atmospheric aerosols. Measurements of laboratory aerosols showed that for a specified geometric size, absorbing particles could be sized either larger or smaller than nonabsorbing particles by an optical particle counter and that the difference was in reasonable agreement with theoretical calculations. Atmospheric measurements showed that there was a range of refractive index present in particles of a specified geometric size from which we conclude that the aerosol was externally mixed chemically with respect to light-absorbing compounds. On some occasions, primarily during times of strong combustion source emissions, the degree of external mixing was large; the percentage of particles in the externally mixed, absorbing subpopulation that we measured ranged from about 20% to 50% of the total at that size. We conclude that the external mixing was largely between soot (or other light absorbing compounds) and inorganic (sulfate and nitrate compounds).