Optical shape fraction measurements of submicrometre laboratory and atmospheric aerosols

Multi-angle azimuthal measurements of light scattering by individual submicrometre particles have been made with the DAWN-A multi-angle light scattering (MALS) instrument. Variability in azimuthal scattering at a polar angle of 55° has been used to determine spherical and nonspherical fractions of laboratory-produced and atmospheric aerosols classified at mobility diameters of 0.2-0.8 μm (size parameters of 1.3-5.1). Parameters used for shape fractionation were derived empirically by comparing scattering signatures from crystalline sodium chloride cubes and spherical dioctyl sebacate (DOS) droplets and then applied to test aerosols and atmospheric aerosols sampled at low (3-10%) and high (50-80%) values of relative humidity. DOS and NaCI contrast was great enough to permit their distinction with less than 5% uncertainty for 0.4-0.8 μm, but contrast was observed to decrease as the size parameter approached unity. Crystalline ammonium sulphate test aerosol appeared mildly aspherical, with nonspherical fraction values in the range 15-40%. Nonspherical fractions of atmospheric aerosols measured during the Southeastern Aerosol and Visibility Study (SEAVS), conducted during July and August of 1995, were generally small (<10%), providing confidence in our application of Mie theory for determination of aerosol optical properties. DAWN-A shape data have been compared with aerosol chemistry and hygroscopicity measured during SEAVS, revealing an overall strong correlation of nonspherical fractions to soil dust content and 'less hygroscopic' fractions.