The ash samples are re-dispersed to generate an ash aerosol. The resulting aerosol is passed into an optical cell where the spectral transmission is measured by a Fourier transform spectrometer. These measurements of the optical transmission are then inverted for the particles refractive index and size distribution. To enable this inversion to take place the spectral dependence of the refractive index must be simplified to reduce the number of parameters, and scattering is modelled using Mie theory. This is achieved by the use of a simple harmonic oscillator model of the refractive index. Details of this method can be found in Thomas 2005. This method has also been applied to derived the refractive index for salt aerosols (Irshad 2009).
Casadevall, T.J. (Ed.), Introduction to Volcanic Ash And Aviation Safety, Proceedings of the 1st International Symposium on Volcanic Ash and Aviation Safety, USGS Bulletin 2047, 1994.
Irshad R., R.G. Grainger, D.M. Peters, R.A. McPheat, K.M. Smith, G.E. Thomas, Laboratory measurements of the optical properties of sea salt aerosol, Atmos. Chem. Phys., 9(1),221-230, 2009.
Thomas, G. E.; Bass, S. F.; Grainger, R. G. & Lambert, A. Retrieval of aerosol refractive index from extinction spectra with a damped harmonic-oscillator band model APPLIED OPTICS, 2005
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