1. "Validation of TOMS Volcanic Aerosol and SO2 Products Using MODIS and AVHRR"

2. Progress Report - March, 2003

3. Principal Investigator: Gregg J.S. Bluth

4. Period Covered: 7/1/02 - 6/30/03

5. Institution Address:

Department of Geological and Mining Engineering and Sciences

Michigan Technological University

Houghton, MI 49931

6. NASA SENH Grant NAG5-11062

Project Overview

This proposed research stems from the unique capability of our research group to validate the TOMS SO₂ and aerosol retrievals from other (infrared) satellite-based instruments, previously with AVHRR and GOES and now with Terra's MODIS sensor. We believe we are in position to provide the Earth Enterprise project with a valuable validation capability and complementary data products for the study of volcanogenic sulfur dioxide and aerosols.

The goal of this project is to provide a better understanding of the formation and fate of volcanogenic tropospheric aerosols, through improved detection, accuracy and interpretations of TOMS data. We will focus on several objectives to accomplish our goal: (1) using the recent eruption of Hekla (and for other eruptions) derive SO₂, ash and aerosol products from MODIS and other infrared satellite data to compare with TOMS measurements; (2) use the TOMS 20-year database and wind trajectory modeling to analyze cases of ash, aerosol and gas separation in drifting clouds, in order to compare IR/UV satellite retrievals; and (3) quantify chemical reaction rates for sulfur dioxide removal, and aerosol formation and removal, under a broad range of volcano-atmosphere conditions.

Recent improvements in the spectral and spatial resolution of polar orbiting satellites have precipitated the use of algorithms that use passive infrared (IR) radiation to observe volcanic emissions. Amongst the species we are now able to detect, from sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), are SO₂, SO₄^2-, ice and ash. However, these species physical, chemical and, most importantly, spectral interactions are currently poorly understood. The Total Ozone Mapping Spectrometer (TOMS), with a data archive that spans three decades, provides us with an opportunity to produce spectrally independent data that facilitates validation of our IR algorithms. TOMS data pertaining to volcanic emissions are the benchmark for future SO₂ and aerosol retrievals.

Research Progress, 7/02 - 6/03 (see Tables 1 and 2)

This year's work has focused on completion of case studies initiated in year 1, as well as our initial methodology efforts. Applications of TOMS and MODIS techniques to volcanic clouds are being done with careful effort to examine (as activity allows) a variety of environmental and volcanological conditions. Three new manuscripts have been submitted, with six others in progress and close to submission stage (Table 1). The specific progress on year 2 of our project is as follows:

¨MODIS methodology. Firstly, we have completed the basic methodology paper on MODIS volcanic cloud retrievals featuring the 8.6 micron absorption feature for SO2 (Watson et al., 2003 in review). Secondly, we have written a forward model, based in the thermal infrared, that allows us to test out MODIS and ASTER retrievals. Our current thrust is working towards quantifying spectral interactions between aerosol species and SO2 present in volcanic clouds. Current algorithms assume that only the species that the algorithm targets is present in the cloud. This is clearly a spectacular over-simplification. All four target species (ash, gas, aerosol, ice) have transmission features in the 7-12 micron window we use. The forward model, based about Mie scattering code embedded into a MODTRAN-based atmospheric radiative transfer model, allows us to assess the results of our retrievals, and particularly our choice of a priori assumptions. This work has been completed and a manuscript is in preparation (Watson et al., 2003 in prep.).

¨Algorithm development. A new SO2 algorithm has been developed, in conjunction with Dr. Prata of CSIRO, Australia (Prata et al., 2003 in review). Based about a much stronger absorption feature at 7.3 microns it uses emission of IR radiation from water vapor in the lower atmosphere (< 5 km) as a source that the SO2 in a cloud above absorbs. The algorithm appears to work very well for high, dry clouds but poorly for lower clouds in the tropics. We are currently comparing results from the 8.6 micron algorithm in order to better understand the interactions between species and the effects of water vapor. We are developing an understanding of how species interact spectrally, and more importantly, what are the strengths and weaknesses of each technique.

¨Hekla case study. The 2000 Hekla eruption was studied with TOMS, MODIS and AVHRR sensors. The eruption is an ash-poor stratospheric event which is marked mainly by a cirrus cloud which has a high SO2 content. This study included multiple retrieval algorithms using TOMS, MODIS, AVHRR and TOVS, as well as in-situ sampling. This work has been submitted for publication (Rose et al., 2003 in review).

¨MODIS-TOMS retrieval comparisons. A study of 2000-2001 Popocatepetl activity focuses on TOMS and MODIS retrievals in conjunction with a detailed COSPEC (ground-based SO2 sensor) survey. Some of the conditions for this study include starkly contrasting regions of ash-rich and ash-poor cloud masses at multiple altitudes. This work is expected to be completed in late 2003.

TOMS and MODIS analyses for both SO2 and AI have been completed for the 2001 eruption of Cleveland volcano, AK. This high latitude eruption is ash-rich, and provides abundant opportunities for algorithm comparisons. A manuscript is in preparation (Gu et al., 2003 in prep.).

Multispectral analysis of the August 2000 eruption clouds of Miyakejima, Japan is underway. This project is designed to systematically compare four retrieval algorithms available for volcanic cloud analysis using data from TOMS, MODIS, AVHRR, ASTER, and GMS. One of the key aspects of this study come from cloud retrievals in a temperate marine atmosphere. All the satellite data have been acquired data, except GMS, which will be accomplished during a July trip to Japan (supported under a Michigan Space Grant project).

The February 6, 2001 eruption of Nyamuragira volcano (Democratic Republic of Congo) has been observed by both TOMS and MODIS. This eruption involves a tropical (wet) atmosphere. Work to date has included sensitivity studies of the MODIS retrievals to inputs (cloud thickness, height, ground elevation, and background emissivity). TOMS and MODIS SO2 data are resampled to identical resolutions for comparison, and show systematic differences (MODIS >> TOMS SO2) which appear to be due to atmospheric water vapor interferences. The time involved in this study reflects the numerous difficulties in MODIS retrievals over a heterogeneous land surface and low altitude water vapor interferences in infrared signals.

¨Sandstorms. An interesting application of MODIS and TOMS data have been to study sandstorms. A Ph.D. student supported by this project, Yingxin Gu, has completed a study on quantitative analysis of a massive dust storm over China using MODIS and TOMS. This is the first quantitative study of dust clouds by satellite that we are aware of, and includes comparisons of MODIS ash retrievals to the TOMS Aerosol Index. This paper is being submitted for publication (Gu et al., in prep.).

Table 1. Manuscripts Related to This Project

Bluth, G.J.S., I.M. Watson, W.I. Rose, S. A. Carn, A.J. Krueger, V.J. Realmuto, and L.R. Lait (2003, in prep.) Multispectral Retrievals of Sulfur and Ash Emissions from Nyamuragira Volcano. For submission to Journal of Volcanology and Geothermal Research.

Carn, S.A., A.J. Krueger, G.J.S. Bluth, S.J. Schaefer, N.A. Krotkov, I.M. Watson, and S. Datta (2003, in review) Volcanic eruption detection by the Total Ozone Mapping Spectrometer (TOMS) instruments: a 22-year record of sulfur dioxide and ash emissions. In: C. Oppenheimer, J. Barclay & D. Pyle (eds.), Volcanic Degassing, Special Publication of the Geological Society of London.

Gu, Y., Rose, W.I., and Bluth, G.J.S. (2003, in prep.) Retrieval of Total Mass and Sizes of Particles in Sandstorms Using Two MODIS IR Bands: A Case Study of April 7 2001 Sandstorm in China. For submission to Geophysical Research Letters.

Gu, Y., Rose, W.I., Bluth, G.J.S, Watson, I.M., (2003, in prep.) The eruption of Cleveland volcano, February 2001 observed by satellite.

Matiella, M.A., Watson, I.M, Rose, W.I., Bluth, G.J.S (2003, in prep.) Moderate Resolution Imaging Spectroradiometer (MODIS) observations of Popocatepetl Volcano, in preparation

Prata A.J., Rose, W.I., Self., S. and O'Brien, D. (2003, in review) Global, long-term sulphur dioxide measurements from TOVS data: A new tool for studying explosive volcanism and climate, AGU Special Publication on Volcanism and the Atmosphere, ed. by A. Robock and C. Oppenheimer.

Rose, W.I., Gu, Y., Watson, I.M., Yu, T., Bluth, G.J.S., Prata, A.J., Krueger, A.J., Krotkov, N., Carn, S., Fromm, M.D., Hunton, D.E., Ernst, G.G.J., Viggiano, A.A., Miller, T.M., Ballentin, J.O., Reeves, J.M., Wilson, J.C., Anderson, B.E., and Flittner, D. (2003, in press) The February-March 2000 eruption of Hekla, Iceland from a satellite perspective, AGU Special Publication on Volcanism and the Atmosphere, ed. by A Robock and C Oppenheimer, in press.

Watson, I.M., Realmuto, V.J., Rose, W.I., Prata, A.J., Bluth, G.J.S., Gu, Y., Yu, T., (2003, in review) Thermal infrared remote sensing of volcanic emissions using the Moderate Resolution Imaging Spectroradiometer (MODIS), submitted to Journal of Volcanology and Geothermal Research

Watson, I.M., Realmuto, V.J., Rose, W.I., Bluth G.J.S., (2003, in prep.) Forward modeling of volcanic cloud transmissions through different atmospheres, for submission to Journal of Geophysical Research - Atmospheres.

Watson, I.M., Realmuto, V.J., Rose, W.I., Bluth, G.S.J., A.J. Prata, Y. Gu, L. Rodriguez (2003, in prep.) Observations of passive degassing from Pacaya Volcano using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), in preparation for Geophysical Research Letters.

Research plans for upcoming year (7/03 - 6/04)

Our plans for the third year of funding will focus on completing the several ongoing MODIS-TOMS retrieval comparisons, and will include new studies as available and personnel allow. Current studies of Nyamuragira, Cleveland, Miyakejima, Popocatepetl eruptions are on schedule for completion in late 2003/early 2004. These studies will allow us to develop as thorough a collection of TOMS/MODIS comparisons under the broadest range possible of atmospheric and volcanic cloud conditions. Algorithm testing and technique development includes algorithm comparisons and modeling studies of retrieval methodologies. The broad range of case studies we have undertaken are invaluable to testing and validating these retrieval schemes under robust scenarios.

Dr. G. Bluth, Project PI Date