Laser Light Backscattering from Laboratory Aerosols

Abstract

Laser light backscattering is being used increasingly to measure ambient particu-late concentrations. In this study, a small volume of aerosol confined by flowing filtered air was illuminated by a Ne-He laser, and the light scattered at 175° from the forward direction was measured. NaCI, KCI, and NaBr crystals were atomized from 1 and 8% (v/v) solutions to form size polydisperse aerosols whose light scattering was found to be more closely related to volume of aerosol per volume of air than to mass or surface area of aerosol per volume of air; the average back-scattering was 4 × 10^–15 watts per 10^–14 cm³ of aerosol illuminated by 2.6 × 10^–3 watts and scattered into 4.4 × 10^–3 steradians solid angle, for aerosols having mass mean diameters from 0.2 to 0.4 microns. The paper gives the aerosols’ size distributions as determined by electron microscopy, as well as their mass concentrations and backscattering. No simple relationship was noticed between scattering and index of refraction. Monodisperse polystyrene latex aerosols (0.796 microns in diameter; α = πd/λ = 4) were also tested and scattered approximately as much light as expected from the Mie theory.