Measurements of the complex refractive index of volcanic ash
Keywords: colloid optical properties
Becke line
Complex Refractive index
volcanic ash measurements
optical measurements
satellite sensing
Aerosol Refractive Index Measurements
Remote Sensing
reflectance measurements
Aerosols
aerosol measurements
Becke line method
aerosol properties
Aerosol Refractive Index
Ash
satellite aerosol measurements
Refractive Index
colloidal reflectance
Volcanic Ash
aerosol remote sensing
satellite measurements
colloids
Fourier Transform Spectroscopy
ash measurements
scattering
Aerosol optical properties
Becke
Aerosol detection
aerosol scattering

This thesis describes laboratory measurements of the complex refractive index of volcanic ash particles. These measurements are needed to model the radiative impact of volcanic ash, vital for accurate satellite remote sensing. Three experimental methods have been developed, and the results for the complex refractive index and optical properties of a wide range of volcanic ash samples are presented.
Measurements were made of the spectral transmission of radiation through suspended volcanic ash particles inside an aerosol cell, using a Fourier transform spectrometer at infrared wavelengths and two diffraction grating spectrometers covering ultraviolet, visible, and near-infrared wavelengths. In addition to the optical measurements, a suite of sampling and sizing instruments were connected downstream of the aerosol cell to measure the particle size distribution. The method was calibrated using two quartz samples. Mass extinction coefficients for nine volcanic ash samples, at 0.3 – 14 μm, are presented and show considerable variation. These variations are linked to the composition of the samples, measured using X-ray fluorescence (XRF) analysis.
The complex refractive index, at 0.3 – 14 μm, of the two quartz samples and two samples of volcanic ash fromthe 2010 Eyjafjallajökull eruptionwere retrieved from the extinction measurements. The forward model used Mie theory and a classical damped harmonic oscillator (CDHO) model to represent the complex refractive index of the samples in terms of a finite set of band parameters, as well as the real refractive index of the sample in the small wavelength limit. Previous studies have shown that there is a redundancy in the retrievals between the band strength parameters and the real refractive index in the small wavelength limit, which can lead to spurious values for the retrieved complex refractive index. This problem was overcome by using an independent measurement of the real refractive index at a visible wavelength, to constrain the model parameter of the real refractive index in the short wavelength limit. Independent measurements of the complex refractive index at visible wavelengths are also important because the extinction produced at these wavelengths is highly sensitive to the particle size distribution, and any uncertainty in the measured size distribution will contribute to significant systematic error in the refractive index retrieved from extinction.
The retrieved spectral complex refractive index of Eyjafjallajökull ash was applied using the ORAC retrieval scheme to measurements of the 2010 Eyjafjallajökull eruptionmade by theMODIS instrument aboard NASA's Terra satellite. Significant difference were found in the retrieved plume parameters of optical path, effective radius, and plume altitude, compared to assuming a literature measurement for the refractive index of pumice.
For three discrete visible wavelengths (450, 546.7, and 650 nm) an optical microscope was used to make measurements of the complex refractive index of the volcanic ash samples. The long-established Becke line method was used to measure the real refractive index of the samples. For the imaginary refractive index, a new and novelmethod was developed involvingmeasurements of the attenuation of light in individual particles. A strong linear correlation was found between the SiO2 content of the samples and both their real and imaginary refractive indices at the visible wavelengths investigated. Furthermore, from the XRF compositional analysis of the samples valueswere calculated for the ratio of non-bridging oxygen atoms per tetrahedral cation (NBO/T), and it was found that NBO/T was an even stronger predictor of real refractive index at visible wavelengths.
The optical microscope measurements could only be applied to particles with a radius larger than 10 μm. A new refractometer method was investigated for retrieving the real refractive index of submicron particles from colloidal reflectance measurements close to the critical angle in an internal reflection configuration. A coherent scattering model (CSM) was used to model the coherent reflection from a half-space of monodisperse or polydisperse particles, and a simple extension of the model is presented to properly account for the modified size distribution at the interface in an internal reflection set-up. A rigorous sensitivity analysis was performed to determine how experimental uncertainties propagate into uncertainty associated with the retrieved real refractive index, and the uncertainty due to non-spherical effects was estimated using T-matrix methods. Experimental reflectance data at a wavelength of 635 nm were obtained for spherical monodisperse polystyrene calibration particles, a polydisperse sand sample, and a polydisperse volcanic ash sample. The retrieved values for the real refractive index agreed, within propagated uncertainties, with values measured using other techniques. The method is shown to be a viable technique for measuring the real refractive index of small quantities of submicron particles, and can also retrieve the concentration and size of particles.