Ultraviolet optical model of volcanic clouds for remote sensing of ash and sulfur dioxide

Abstract

Abstract. The total ozone mapping spectrometer (TOMS) instruments have detected every significant volcanic eruption from November 1978 to December 1994 on the Nimbus 7 and Meteor 3 satellites and since July 1996 on the new satellites, TOMS-Earth Probe and ADEOS. We apply a radiative transfer model to simulate the albedos of these fresh eruption clouds to study the limitations of the present algorithm which assumes an absorbing cloud above a scattering atmosphere. The conditions are found to be approximated when the total absorption optical depth is less than 2 (i.e., 100 Dobson units (DU) SO2 at 312 nm or 300 DU SO2 at 317 nm). The spectral dependence of the albedo of a nonabsorbing Rayleigh atmosphere can be specified by only two parameters which are uniquely different when ash or sulfate aerosols are present in the stratosphere. However, the interaction between ash scattering and SO 2 absorption within a volcanic cloud produces a nonlinear effect at strongly absorbing wavelengths that accounts for overestimation of sulfur dioxide in ash-laden volcanic clouds by the Krueger et al. [1995] algorithm. Correction of this error requires knowledge of the ash properties. A method for determining two of the ash parameters from the longer TOMS wavelengths is described. Given the altitude of the cloud, surface reflectivity, and an estimate of effective variance of the ash size distribution, the optical thickness and either the effective radius or the index of refraction can be deduced. The ash retrievals are also needed to evaluate the tephra/gas ratio of eruptions and to compare the ash properties of different volcanoes. IntroductionThe purposes of this paper are to describe an optical model of volcanic clouds at ultraviolet (UV) wavelengths, to describe the simulated albedos used to test retrieval algorithms, to study limitations of remote sensing of thick volcanic clouds, to examine the interaction between ash scattering and sulfur dioxide absorption within a volcanic cloud, and to show the possibility of measuring ash properties from space. The sulfur dioxide results in the various volcanic cloud papers are based on the algorithm described by Krueger et al. [1995] using the level-2 Nimbus 7 data and calibration available prior to 1995. This algorithm was tested under lowlatitude conditions with simulated volcanic cloud albedos and found to produce SO2 retrievals with errors less than _+10%. However, when ash is present in the volcanic cloud, the retrieval overestimated sulfur dioxide by 15-25% depending on the particle size and composition. Because of this unexpected Copyright 1997 by the American Geophysical Union. Paper number 97JD01690.0148-0227/97/97JD-01690509.00 relation between sulfur dioxide and ash we initiated a study of the optical properties of volcanic clouds. A proposed optical model which includes ozone and SO2 gas absorption and scattering by volcanic ash is described in this paper.Given the optical model of a volcanic plume, we solve the radiative transfer equation to obtain the albedo at the TOMS...

A multi-sensor comparison of sulphur dioxide emissions from the 2005 eruption of Sierra Negra volcano, Galápagos Islands

“…The SO 2 retrieval is based on the Residual Model of Krueger (1983) which was later replaced by the Linear Model (Kerr Algorithm) (Kerr et al, 1980;Krotkov et al, 1997;Krueger et al, 2000). The Linear Model assumes that solar radiation is attenuated by absorbing species according to Beers law so that:…”

“…TOMS detects backscattered radiation in the near-ultraviolet region of the electromagnetic-spectrum and measures in six wavelength bands from 308 nm to 360 nm (Krotkov et al, 1997). A Sulphur Dioxide Index (SOI) is computed to indicate the presence or absence of SO 2 by comparison with pre calculated radiances (Bhartia, 2003;McPeters et al, 1998).…”

Re‐evaluation of SO2 release of the 15 June 1991 Pinatubo eruption using ultraviolet and infrared satellite sensors

“…[28] Krotkov et al [1997] suggest that TOMS SO 2 can be overestimated by 15-25% when significant ash is present in the SO 2 cloud. Guo et al (submitted manuscript, 2004) concluded that ash fell out very rapidly, so this overestimate would be expected to be strongest on the first day.…”

Quantitative shape measurements of distal volcanic ash

“…Despite their irregular shape, ash particles are most commonly modeled as spheres in both transport modeling experiments [ Brazier et al , 1982; Carey and Sigurdsson , 1982; Suzuki , 1983; Armienti et al , 1988; Glaze and Self , 1991; Sparks et al , 1992; Jarzemba et al , 1997] and remote sensing algorithms [ Wen and Rose , 1994; Krotkov et al , 1997] primarily because no quantitative description of particle shape has been made. Numerous qualitative SEM studies (summarized by Heiken and Wohletz [1985]) have shown that volcanic particles are generally quite angular and/or irregular and include parachute‐shaped bubble wall shards, equant mineral grains, and subrounded vesicular pumice clasts (Figure 1).…”