POSSIBILITY OF REFRACTIVE-INDEX DETERMINATION OF ATMOSPHERIC AEROSOL-PARTICLES BY GROUND-BASED SOLAR EXTINCTION AND SCATTERING MEASUREMENTS

A method is developed to infer the optical properties of aerosol parti cles from ground-based solar extinction and scattering measurements un der cloudless conditions. On the basis of the spectral aerosol optical thickness (extinction measurement) radiative transfer calculations ar e carried out yielding the diffusely scattered radiances at the ground . These calculated radiances are compared with the scattering measurem ent. The unknown refractive index and consequently all the other optic al aerosol properties are varied in the model calculations until best agreement to the measurement is achieved. Thus a complete set of aeros ol characteristics is deduced describing the measurements in an optica lly equivalent sense. The method is examined by means of aerosol model s. These tests show that the method works well if many optically effec tive fine particles are present and in the case of slight absorption. The imaginary part of the refractive index cannot be obtained with the present version of the method. The application to real atmospheric me asurements stresses the need to consider the nonsphericity of the aero sol particles, especially if the relative humidity is low and the port ion of large particles is high. A comparison with the results of indep endent nucleopore filter measurements yields good agreement. Furthermo re typical values of the real part of the refractive index for differe nt geographical regions are given.