Direct measurement of suspended particulate volume concentration and far-infrared extinction coefficient with a laser-diffraction instrument.

A laser-diffraction instrument is described for measuring directly and in situ the absolute particulate volume concentration or the infrared optical extinction coefficient of a polydisperse aerosol. Diffraction and Mie theory are used to write integrals that describe the interaction of light scattered by the particles and the optics of the instrument. These integrals are inverted to yield variable transmission filters that interact with the scattered light to give the desired instrument response. The particle-size dependency of the instrument's performance is established, and the effect of the refractive index of the particles is evaluated. The design, major components, and absolute and field calibrations of the instrument are described. Examples of measurements in the field and laboratory are given.