The optical properties of sea-salt aerosol have been parameterized at solar and longwave wavelengths. The optical properties were parameterized in a simple functional form in terms of the ambient relative humidity based on Mie optical property calculations. The proposed parameterization is tested relative to Mie calculations and is found 5 to be accurate to within a few percent. In the parameterization, the effects of the size distribution on the optical properties are accounted for in terms of

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... effective radius of the sea-salt size distribution. This parameterization differs from previous works by being formulated directly with the wet sea-salt size distribution (and compared to AERONET results) and, to our knowledge, this is the first published sea-salt parameterization to 10 provide a parameterization for both solar and longwave wavelengths. We have used this parameterization in a set of idealized 1-D radiative transfer calculations to investigate the sensitivity of various attributes of sea-salt forcing, such as dependence with sea-salt column loading, effective variance, solar angle, and surface albedo. From these sensitivity tests, it is found that sea-salt forcings for both solar 15 and longwave spectra are linearly related to the sea-salt loading for realistic values of loadings. The radiative forcing results illustrate that the shortwave forcing is an order of magnitude greater than the longwave forcing results and opposite in sign, for various loadings. Studies of the sensitivity of forcing results to variations in effective variance show there to be minimal variation; therefore, only one value of effective vari-20 ance is used in the parameterization. The dependence of sea-salt forcing with solar angle illustrates an interesting result that sea-salt can generate a positive top-of-theatmosphere result (i.e. warming) when the solar zenith angle is relatively small 30 • . Finally, it is found that the surface albedo significantly affects the solar radiative forcing, with the forcing diminishing to zero as the surface albedo tends to unity. 25 We anticipate this new sea-salt optical property parameterization will be useful for GCM models due to its simplicity, computational efficiency, and that its sensitivities have been explored and summarized in this work. 5814 Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 25 2007). Dobbie et al.'s approach is based on a two-mode scheme which separates the sea-salt size distribution into separate particle size categories for accumulation mode and coarse mode sea-salt. Here, a new parameterization for sea-salt longwave and 5815 ACPD 8, 5813-5845, 2008 Abstract found in Lewis and Schwartz (2004). After injection, the solution droplets will either grow or evaporate until thermodynamic 5816 ACPD 8, 5813-5845, 2008 Abstract ACPD 8, 5813-5845, 2008 Abstract 8, 5813-5845, 2008 Abstract ACPD 8, 5813-5845, 2008 Abstract ACPD 8, 5813-5845, 2008 Abstract ACPD 8, 5813-5845, 2008 Abstract ACPD 8, 5813-5845, 2008 Abstract 8, 5813-5845, 2008 Abstract 8, 5813-5845, 2008 Abstract ACPD 8, 5813-5845, 2008 Abstract 8, 5813-5845, 2008 Abstract 8, 5813-5845, 2008 Abstract 8, 5813-5845, 2008 Abstract 8, 5813-5845, 2008 Abstract 25 the net solar flux at the surface diminishes as the surface albedo approaches unity. For the forcing at 200 mb, the reduction of forcing is mostly caused by a reduction in the 5830 ACPD 8, 5813-5845, 2008 Abstract 8, 5813-5845, 2008 Abstract 8, 5813-5845, 2008 Abstract 8, 5813-5845, 2008 Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion longwave wavelengths that is appropriate for implementation in GCMs. We note that the parameterization presented in the paper can be modified for other band structures. Parameterization coefficients in Eqs. (16-18) for the 25 single wavelength values between 0.2 to 4 µm and the coefficients in Eq. (20) for 39 single wavelength values between 4 to 50 µm are available from the authors. 5 Acknowledgements. We are grateful to C. Reader for her helpful comments. 8, 2008