The extinction coefficient of multicomponent aerosols

Abstract

The general problem of visibility reduction by aerosols is explored in order to identify the conditions necessary for determination of the roles of individual chemical species. The particle extinction coefficient depends on the distribution of chemical species with respect to particle size and from particle to particle within each size range. Single particle measurements of composition and optical properties would be required to determine directly the chemical species contributions to the extinction coefficient. Several special cases are identified which would allow exact calculations based on available data. The composition and optical properties of particles must be uniform within each size interval if composition distribution data are to be used. Corroborative data are necessary to ensure that important contributors are not neglected since many chemical species are not readily measured. Estimation of the species mass extinction efficiencies by statistical analysis of chemical data obtained from total filler samples also requires that the normalized species mass distribution be invariant. The results of two field studies of visibility reduction in arid regions of the southwestern United States are used to explore the validity of these assumptions. The two methods, estimation from size-composition distribution data and from total filter data, yield similar estimates of species contributions to the extinction coefficient.