Sea-Salt Aerosol Number Size Distribution Schemes In Radiative Transfer Models

In this paper,the volume size distribution data from the AERONET are used and 12oceanic sites are chosen to investigate the characteristics of sea-salt aerosol number size distribution and its parameterization.To begin with,the lognormal distribution and Gamma distribution schemes are carefully scrutinized with the observation data.It is the first attempt to analyze biases from both the skewness and kurtosis perspective.Moreover,Mie calculation is conducted to assess the effects of different size distribution schemes on particle scattering and absorption abilities.Finally,sensitivity tests of size distribution schemes on sea-salt radiative forcing are conducted by SBDART radiative transfer model and CanAM4 climate model.The biases caused by size distribution schemes are quantitatively explored from the solar zenith angle as well as the global distribution perspective.Conclusions are as follows:(1)Sea-salt aerosols are mainly coarse particles with a single mode volume size distribution.Particle numbers simulated by the lognormal distribution are larger than the numbers of observations when 0.4?m<r<1?m and r>10?m.Adopting the Gamma distribution can improve the accuracy of parameterized number distribution by 2.3 and 3times at the radius mentioned above,respectively.(2)The smaller skewness value of the lognormal distribution causes a slight shift of the size distribution peak towards the relatively larger radius,which leads to the number overestimation in the mid-radius size.The smaller kurtosis contributes to a slower drop of the large-radius end,resulting in more aerosols falling in the large radius part.Those results apply for all daily data of the 12 chosen sites.(3)Assuming the lognormal distribution instead of the Gamma distribution leads to a significant overestimation of extinction coefficient,while the single scattering albedo and asymmetry factor remain unchanged.Different size distribution schemes do not change phase function.(4)There are negative sea-salt radiative forcing values both at the TOA and surface,indicating cooling effects in these regions.Using the Gamma distribution scheme to calculate the sea-salt radiative forcing can improve the accuracy by 2.5 and 2.6 times at the TOA and surface,respectively.With the growing solar zenith angle,the overestimation caused by the lognormal distribution can be magnified with a 91%relative error.(5)As indicated by the CanAM4 model,the global average sea-salt radiative forcing difference of the lognormal distribution to the Gamma distribution can be-0.9Wm~2and-1.3Wm~2at the TOA and surface,respectively.Regions with larger surface wind show more distinct biases in sea-salt radiative forcing calculated by the lognormal size distribution scheme.