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 particles from ground-based solar extinction and scattering measurements under cloudless conditions. On the basis of the spectral aerosol optical thickness (extinction measurement) radiative transfer calculations are carried out yielding the diffusely scattered radiances at the ground. These calculated radiances are compared with the scattering measurement. The unknown refractive index and consequently all the other optical aerosol properties are varied in the model calculations until best agreement to the measurement is achieved. Thus a complete set of aerosol characteristics is deduced describing the measurements in an optically equivalent sense. The method is examined by means of aerosol models. These tests show that the method works well if many optically effective 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 measurements stresses the need to consider the nonsphericity of the aerosol particles, especially if the relative humidity is low and the portion of large particles is high. A comparison with the results of independent nucleopore filter measurements yields good agreement. Furthermore typical values of the real part of the refractive index for different geographical regions are given.