ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-221-2009 Laboratory measurements of the optical properties of sea salt aerosol Irshad R. 1 Grainger R. G. 1 Peters D. M. 1 McPheat R. A. 2 Smith K. M. 2 Thomas G. 1 Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, University of Oxford, UK Space Science and Technology Department, Rutherford Appleton Laboratory, Didcot, UK 13 01 2009 9 1 221 230 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/9/221/2009/acp-9-221-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/221/2009/acp-9-221-2009.pdf

The extinction spectra of laboratory generated sea salt aerosols have been measured from 1 μm to 20 μm using a Bruker 66v/S FTIR spectrometer. Concomitant measurements include temperature, pressure, relative humidity and the aerosol size distribution. The refractive indices of the sea salt aerosol have been determined using a simple harmonic oscillator band model (Thomas et al., 2004) for aerosol with relative humidities at eight different values between 0.4% to 86%. The resulting refractive index spectra show significant discrepancies when compared to existing sea salt refractive indices calculated using volume mixing rules (Shettle and Fenn, 1979). Specifically, an additional band is found in the refractive indices of dry sea salt aerosol and the new data shows increased values of refractive index at almost all wavelengths. This implies that the volume mixing rules, currently used to calculate the refractive indices of wet sea salt aerosols, are inadequate. Furthermore, the existing data for the real and imaginary parts of the refractive indices of dry sea salt aerosol are found not to display the Kramers-Kronig relationship. This implies that the original data used for the volume mixing calculations is also inaccurate.

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