Measuring the mass extinction efficiency of elemental carbon in rural aerosol

Previous measurements of the mass absorption efficiency of ambient elementa l carbon (EC) indicate that EC optical properties vary with location and im ply that the variations may be due to different particle size distributions and composition at different locations (Liousse et al. 1993). For this rea son, optical properties appropriate to regional characteristics of EC, dete rmined over the wavelengths of light significant for aerosol extinction, ar e needed to adequately model the radiative impact of this species. Here we present a method for measuring one of these properties, the mass extinction efficiency (m(2) g(-1)) of EC, as a function of particle size and waveleng th of light. In this method, size segregated atmospheric aerosol particles are collected on Nucleopore filters. The filter samples are extracted in a mixture of 30% isopropanol and 70% deionized distilled water to form a susp ension of insoluble EC particles. Transmission of light through the extract ion liquid is measured over wavelengths from 300 to 800 nm using a spectrop hotometer. The transmission measurements taken through the liquid extract a re mathematically converted to EC extinction coefficients in air. Although the conversion is a function of a parameter determined from Mie theory, whi ch assumes monodisperse, spherical particles with a known density and refra ctive index relative to the medium, the method is shown to be reasonably in sensitive to these assumptions. Using EC mass concentration obtained from a parallel sample, the EC mass extinction efficiency (in air) is calculated from the extinction coefficient (in air). This method is applied to a rural Midwestern, midcontinental aerosol. In general, the EC mass extinction eff iciency in air is highest at lower wavelengths and for smaller particles. F or particles with diameters between 0.09 and 2.7 mum and an assumed density of 1.9 g cm(-3), the measured EC mass extinction efficiency at 550 nm rang es from 7.3 to 1.7 m(2) g(-1).