Analysis of sea-salt aerosol size distributions in radiative transfer

The characteristics of sea-salt aerosol number size distribution and its parameterization are investigated based on the AERONET measurements. The results of sea-salt volume size distributions are quasi mono-modal. With the effective radius and effective variance, the parameterization based on the Gamma and lognormal distributions can be determined. Using the Gamma distribution instead of the lognormal for sea-salt can improve the accuracy in relative errors of parameterized number distribution by 2.3 times at 0.1 mu m < r < 1 mu m and by 3 times at r > 10 mu m. The theory of skewness and kurtosis can be used to explain the better performance of the Gamma distribution. The smaller skewness of lognormal indicates the number overestimation in the mid-radius size and the smaller kurtosis contributes to the inaccuracy at the large-radius end. Radiative transfer model using the Gamma distribution can improve the accuracy in relative errors of sea-salt radiative forcing by 2.5 times at the top of the atmosphere and 2.6 times at the surface compared with using the lognormal distribution. CanAM4 model indicates that using the lognormal distribution can overestimate the global mean sea-salt radiative forcing by about - 0.9 W M-2 at TOA and -1.3 W M-2 at the surface by comparing with using the Gamma distribution.