Optical properties of the ash from El Chichon Volcano

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Abstract
We have measured visible wavelength optical properties of the ash from the 1982 El Chichón eruptions. These measurements were made on ash samples collected at three surface sites at distances between 12 and 80 km from the volcano. The most distant sample is taken as most representative of the silicate ash injected into the stratosphere.The measured optical properties are expressed as a complex refractive index n, with the aerosol absorption expressed as the imaginary component of the refractive index, nIM. Each of these samples showed quite low values of absorption, with nIMat 500 nm ranging from 1.5×10−3for the 12 km sample to 1.0×10−3for the 80 km sample. Based on these measurements, we estimate that n for the stratospheric silicate ash is given by n = 1.53 ‐ 0.001i.