Method

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).

Results
Fig. 1. Spectra measured at the MSF of Aso ash aerosol.

Fig. 2. Derived refractive index.

The sample used is a volcanic ash sample, collected from the Aso volcanic eruptions in 1993. This sample has been collected from a bomb-shelter where 1 m to 2 m of ash accumulated, and is hence in as fresh a state as possible. Once dispersed into the MSF aerosol cell, we obtained an absorption spectra. An example is given in Fig. 1. On applying the method of Thomas 2005 the refractive index and particle size distribution was obtained. And example of the refractive index derived in shown in Fig. 2.

Future work

 The above spectra will be applied to remote satellite retrievals of ash clouds, and early results are promising. Ideally we would also repeat these experiments for a number of different volcanic eruptions as the spectral signature will change as the chemical composition of the ash for each eruption is different. We would like to extend the measurements into the visible and ultra-violet as this would be of great use for instruments such as lidar. With the latest Icelandic eruption of Eyjafjallajökull and disruption air transport there is clearly a need to improve the detection and quantification of volcanic ash clouds.

Links and references

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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