To adequately assess the effects of atmospheric aerosols on climate, their optical constants (scattering and absorption coef cients) must be known. The absorption and scattering coef cients of the aerosols are derived from the real and imaginary parts of the complex refractive index and are dependent on their size and chemical composition. Because aerosol properties vary signi cantly with location, it is dif cult to assign values for the absorption and scattering of solar radiation by aerosols in models of global climate change. This study reports a new method of collecting size-fractionated atmospheric aerosol samples for the purpose of directly measuring their transmission and re ectance spectra followed by the determination of the complex refractive index across the entire atmospherically relevant spectral range. The samples were collected with a modi ed Sierra high-volume cascade impactor with the usual lter collection surfaces replaced with Te on sheets machined to hold quartz (ultraviolet [UV]/visible transparent) and/or silver chloride (infrared transparent) sample collection plates. Re ectance and transmission spectra can be obtained on the aerosol samples directly as a function of wavelength, from 280 nm to 2.5 m, with an integrating sphere coupled to an UV/visible or a Fourier transform infrared (FTIR) spectrophotometer. The effective real and imaginary components of the refractive index of the bulk sample material can then be approximated, as a function of wavelength, from the sample spectra. Preliminary results are presented for carbon soot samples generated in the laboratory and for standard diesel soot samples in the UV/visible spectral range. These are compared to results obtained for size-fractionated atmospheric aerosol samples collected near Pasco, WA, West Mesa, AZ, and Argonne, IL.

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