OPTICAL SHAPE FRACTION MEASUREMENTS OF SUBMICROMETER LABORATORY AND ATMOSPHERIC AEROSOLS

Multi-angle azimuthal measurements of light scattering by individual s ubmicrometre particles have been made with the DAWN-A multi-angle ligh t scattering (MALS) instrument. Variability in azimuthal scattering at a polar angle of 55 degrees has been used to determine spherical and nonspherical fractions of laboratory-produced and atmospheric aerosols classified at mobility diameters of 0.2-0.8 mu m (size parameters of 1.3-5.1). Parameters used for shape fractionation were derived empiric ally by comparing scattering signatures from crystalline sodium chlori de cubes and spherical dioctyl sebacate (DOS) droplets and then applie d to test aerosols and atmospheric aerosols sampled at low (3-10%) and high (50-80%) values of relative humidity. DOS and NaCl contrast was great enough to permit their distinction with less than 5% uncertainty for 0.4-0.8 mu m, but contrast was observed to decrease as the size p arameter approached unity. Crystalline ammonium sulphate test aerosol appeared mildly aspherical, with nonspherical fraction values in the r ange 15-40%. Nonspherical fractions of atmospheric aerosols measured d uring the Southeastern Aerosol and Visibility Study (SEAVS), conducted during July and August of 1995, were generally small (<10%), providin g confidence in our application of Mie theory for determination of aer osol optical properties. DAWN-A shape data have been compared with aer osol chemistry and hygroscopicity measured during SEAVS, revealing an overall strong correlation of nonspherical fractions to soil dust cont ent and 'less hygroscopic' fractions.