Integrating sandwich: a new method of measurement of the light absorption coefficient for atmospheric particles

Abstract

A new technique has been developed for determination of optical depths δ_a as low as 0.0005 for thin layers of absorbing materials or particles. The measurement involves optical amplification of the absorption and is not affected by the scattering properties of the absorber. This is accomplished by introducing the absorber into the virtually isotropic radiation field between two high-reflectance diffusing wafers and measuring the resultant attenuation of the transmitted light. The technique has been directed toward determination of the absorption coefficient of atmospheric aerosols in remote and relatively unpolluted locations. Provided appropriate collection filters and sampling conditions are used, the method can establish an absorption coefficient for the aerosol as low as 5 × 10⁻⁹ m⁻¹ within a 10-h sampling period. A proportionally higher absorption coefficient requires proportionally less sample time. This paper discusses instrument design, the theoretical optical model, laboratory calibration, and a field test of the technique.