ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-8-4787-2008 Parameterization of sea-salt optical properties and physics of the associated radiative forcing Li J. 1 Ma X. 1 von Salzen K. 1 Dobbie S. 2 Canadian Centre For Climate Modelling and Analysis, Science and Technology Branch, Environment Canada, University of Victoria, Victoria, British Columbia, Canada Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK 19 08 2008 8 16 4787 4798 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/8/4787/2008/acp-8-4787-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/4787/2008/acp-8-4787-2008.pdf

The optical properties of sea-salt aerosol have been parameterized at shortwave 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 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 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, to our knowledge, this is the first published sea-salt parameterization to provide a parameterization for both shortwave and longwave wavelengths. <br><br> 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, including the dependency on sea-salt column loading, effective variance, solar angle, and surface albedo. From these sensitivity tests, it is found that sea-salt forcings for both shortwave 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. Forcing sensitivity studies show that the influence of effective variance for sea-salt is minor; therefore, only one value of effective variance is used in the parameterization. The dependence of sea-salt forcing with solar zenith angle illustrates an interesting result that sea-salt can generate a positive top-of-the-atmosphere result (i.e. warming) when the solar zenith angle is relatively small (i.e. <30&deg;). Finally, it is found that the surface albedo significantly affects the shortwave radiative forcing, with the forcing diminishing to zero as the surface albedo tends to unity.

 References Alley, R. B.: The Younger Dryas cold interval as viewed from central Greenland, Quaternary Sci. Rev., 19 213–226, 2000. Bäumer, D., Lohmann, U., Lesins, G., Li, J., and Croft, B.: Parameterizing the optical properties of carbonaceous aerosols in the Canadian Centre for Climate Modeling and Analysis Atmospheric General Circulation Model with impacts on global radiation and energy fluxes, J. Geophys. Res., 112, D10207, doi:10.1029/2006JD007319, 2007. Ch\'ylek, J. and Wong, J. G. D.: Effect of absorbing aerosols on global radiation budget, J. Geophys. Res. Lett., 22, 929–931, 1995. Haywood, J. M., Ramaswamy, V., and Donner, L. J.: A limited-area-model case study of the effects of sub-grid scale variations in relative humidity and cloud upon the direct radiative forcing of sulfate aerosol, Geophys. Res. Lett., 24, 143–146, 1997. Haywood, J. M. and Shine, K. P.: Mult-spectral calculations of the direct radiative forcing of tropospheric sulphate and soot aerosols using a column model, Q. J. R. Meteorol. Soc., 123, 1907–1930, 1997. Dobbie, J. S., Li, J., and Ch\'ylek, P.: Two and Four Stream Optical Properties for Water Clouds and Solar Wavelengths, J. Geophys. Res., 104, 2067–2079, 1999. Dobbie, J. S., Li, J. Harvey, R., and Ch\'ylek, P.: Sea-salt optical properties and GCM forcing at solar wavelengths, Atmos. Res., 65 (special issue for aerosol), 211–233, 2003. Fitzgerald, J. W.: Approximation formulas for the equilibrium size of an aerosol particle as a function of its dry size and composition and the ambient relative humidity, J. Appl. Meteor., 14, 1044–1049, 1975 Hale, G. M. and Querry, M. R.: Optical constants of water in the 200 nm to 200 um wavelength region, Appl. Opt., 12, 555–563, 1973. Holben B. N., Eck, T. F., Slutsker, I., Tanre, D., Buis, J. P., Setzer, A., Vermote, E., Reagan, J. A., Kaufman, Y., Nakajima, T., Lavenu, F., Jankowiak, I., and Smirnov, A.: AERONET – A federated instrument network and data archive for aerosol characterization, Rem. Sens. Environ., 66, 1–16, 1998. Koepke, P., Hess, M., Schult, I., and Shettle, E. P.: Global Aerosol Data Set. Report No. 243 of the Max-Planck-Institut für Meteorologie, Hamburg, ISSN 0937-1060, 44 pp., 1997. Lewis, E. R. and Schwartz, S. E.: Sea salt aerosol production: Mechanisms, methods, measurements and models – a critical review, 413 pp., Geophysical Monograph, 152, 2004. Lewis, E. R. and Schwartz, S. E.: Comment on size distribution of sea-salt emissions as a function of relative humidity, Atmos. Env., 40, 591–592, 2006. Lewis, E. R.: An examination of Köhler theory resulting in an accurate expression for the equilibrium radius ratio of a hygroscopic aerosol particle valid up to and including relative humidity 100%, J. Geophys. Res., 113, D03205, doi:10.1029/2007JD008590, 2008. Li, J., Wong, J. G. D., Dobbie, J. S., and Ch\'ylek, P.: Parameterization of the optical properties and growth of sulfate aerosols. J. Atmos. Sci., 58, 193–209, 2001. Li, J.: Accounting for unresolved clouds in a 1-D infrared radiative transfer model. Part I: Solution for radiative transfer, cloud scattering, and overlap, J. Atmos. Sci., 59, 3302–3320, 2002. Li, J. and Min, Q.: Parameterization of the optical properties of sulfate aerosols in the in the infrared, J. Atmos. Sci., 59, 3130–3140, 2002. Li, J. and Barker, H. W.: A radiation algorithm with correlated k-distribution. Part I: local thermal equilibrium, J. Atmos. Sci., 62, 286–309, 2005. Li, J., Scinocca, J., Lazare, M., McFarlane, N., von Salzen, K., and Solheim, L.: Ocean surface albedo and its impact on radiation balance in climate models, J. Climate, 19, 6314–6333, 2006. Ma, X. Y. and von Salzen, K.: Dynamics of the sulphate aerosol size distribution on a global scale, J. Geophys. Res., 111, D08206, doi:10.1029/2005JD006620, 2006. Ma, X. Y., von Salzen, K., and Li, J.: Modelling sea-salt aerosol and its direct and indirect effects, Atmos. Chem. Phys., 8, 1311–1327, 2008. McClatchey, R. A., Fenn, R. W., Selby, J. E. A., Volz, F. E., and Garing, J. S.: Optical properties of the atmosphere, 3rd ed., AFCRL-72-0497, 108 pp., [NTIS N7318412], 1972. O'Dowd, C. D., Lowe, J. A., Smith, M. H., and Kaye, A. D.: The relative importance of non-sea-salt sulphate and sea-salt aerosol to the marine cloud condensation nuclei population: An improved multi-component aerosol-cloud droplet parameterization, Quart. J. R. Meteorol. Soc., 125, 1295–1313, 1999. Penner, J. E., Andrea, M., Annegarn, H., Barrie, L., Feichter, J., Hegg, D., Jayaraman, A., Leaitch, R., Murphy, D., Nganga, J. and Pitari, G.: Aerosols, their direct and indirect effects: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, in: IPCC Third Assessment Report, edited by: Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M., van der Linden, P. J., Dai, X., Maskell, K., and Johnson, C. A., Cambridge Univ. Press, New York, NY, USA, Clim. Change, 289–348, 2001. Schulz, M., Textor, C., Kinne, S., Balkanski, Y., Bauer, S. Berntsen, T., Berglen, T., Boucher, O., Dentener, F., Grini, A., Guibert, S., Iversen, T., Koch, D., Kirkevag, A., Liu, X., Montanaro, V., Myhre, G., Penner, J., Pitari, G., Reddy, S., Seland, O., Stier, P., and Takemura, T.: Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations, Atmos. Chem. Phys., 6, 5225–5246, 2006. Shettle, E. P. and Fenn, R. W: Models for the Aerosols of the Lower Atmosphere and the Effects of Humidity Variations on Their Optical Properties, AFGL-TR-79-0214, ADA085951, 20 September 1979. von Salzen, K.: Piecewise log-normal approximation of size distributions for aerosol modelling, Atmos. Chem. Phys., 6, 1351–1372, 2006. Tang, I. N., Tridico, A. C., and Fung, K. H.: Thermodynamic and optical properties of sea-salt aerosols, J. Geophys. Res., 102, 23 269–23 275, 1997. Volz, F. E.: Infrared refractive index of atmospheric aerosol substance, Appl. Opt., 11, 755–759, 1972. Wang, J. and Martin, S. T. : Satellite characterization of urban aerosols: importance of including hygroscopicity and mixing state in the retrieval algorithms, J. Geophys. Res., 112, D17203, doi:10.1029/2006JD008078, 2006. Winter, B. and Ch\'ylek, P.: Contribution of sea-salt aerosol to the planetary clear-sky albedo Tellus Series B, 49, 72–79, 1997.