Development and Testing of a Tandem Integrating Sphere Absorption Meter

University Of Alabama In Huntsville, Huntsville AL

Investigators

Abstract

This project is motivated by the need for a better method of measuring the absorption properties of aerosol particles for solar radiation. Current methods consist of capturing an aerosol sample on filter paper or some other material, illuminating it, and measuring the effect of the aerosol film on transmission or reflection of the light. A problem is to distinguish between the light that is scattered from the beam and that which is actually absorbed. The problem is approached by using an integrating sphere, a cavity with a diffusely reflective inner surface that traps the photons scattered by the aerosol film. A small port in the cavity allows a probe to sample the photon density of the trapped light, from which the transmittance and reflectance of the aerosol film may be determined. This project improves upon the concept of an integrating cavity by use of a tandem integrating sphere (called TANINS), an absorption meter consisting of two integrating spheres, joined at the sample port, capable of determining the hemispheric transmittance and reflectance of the filter sample with high accuracy and sensitivity. The instrument performance is evaluated by comparison with measurements using a well-understood integrating cavity of standard design and by measurements on laboratory-generated aerosols with known optical properties. The measurements are then converted to characterize in situ or undisturbed aerosols by theoretical modeling to account for the effects of particle clumping and the filter substrate. The work provides fundamental information on aerosol optical properties that is needed for atmospheric remote sensing and for evaluating the effects of aerosol layers on regional and global climate.