EVALUATION OF A SECTIONAL REPRESENTATION OF SIZE DISTRIBUTIONS FOR CALCULATING AEROSOL OPTICAL-PROPERTIES

A sectional method for representing the atmospheric aerosol size distr ibution is evaluated for calculating the optical properties used in at mospheric radiative transfer calculations. Many current atmospheric ae rosol models and measurement analysis methods are based on a lognormal representation of the aerosol size distribution. The sectional method offers the advantage over the lognormal size distribution that as the aerosol size distribution evolves with time, for example, as in the c ase of the Mount Pinatubo stratospheric aerosol, regional haze aerosol , or stack plume particles, the optical properties are computed only o nce for each size section and do not need to be recomputed as the aero sol size distribution changes. Thus the computational costs are consid erably reduced. The sectional approach was evaluated by comparing the aerosol optical properties calculated with a continuous distribution t o those calculated with a sectional distribution. The sectional approa ch was shown to be most accurate when the optical properties were inte grated assuming a constant surface area distribution within each secti on. For a sectional size distribution with nine sections between 0.01 and 10 mu m in diameter, the results show a maximum error of 6% for th e scattering coefficient, 10% for the phase function, and 4% in the sk y radiance. The results indicate that the sectional approach offers an excellent compromise between accuracy and computational efficiency.