Ash particle refractive index model for simulating the brightness temperature spectrum of volcanic ash clouds from satellite infrared sounder measurements

Using data from the Infrared Atmospheric Sounding Interferometer (IASI) measurements of volcanic ash clouds (VACs) and radiative transfer calculations, we attempt to simulate the measured brightness temperature spectra (BTS) of volcanic ash aerosols in the infrared region. In particular, the dependence on the ash refractive index (RI) model is investigated. We apply 21 RI models for the optical properties of ash particles, including recently published models. The results of BTS simulations are evaluated using the root mean square (rms) of the brightness temperature difference between measurements and simulations for channels in the wavenumber range of 750–1400 cm−1. The results of numerical simulations for 156 pixels of IASI measurements for ash plumes from seven volcanoes indicated that some newly established RI models have smaller rms than those obtained with the conventional ash RI model. For some eruption events, the RI models deduced from the chemical composition of ash samples for the same volcanic eruption events simulate the measured BTS well. The results suggest that infrared sounder measurements and precise BTS simulations of the VACs contribute to estimating the appropriate RI model in satellite VAC analysis.