Enhanced monitoring of sulfur dioxide sources with hyperspectral UV sensors

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1 Enhanced monitoring of sulfur dioxide sources with hyperspectral UV sensors Arlin Krueger* 1, Kai Yang 2, and Nickolay Krotkov 2 ; 1 Joint Center for Earth Sciences Technology 2 Goddard Earth Sciences and Technology Center University of Maryland, Baltimore County Baltimore, MD USA ABSTRACT Sulfur dioxide, a short-lived atmospheric constituent, is oxidized to sulfate aerosols, a climate agent. Main sources are volcanoes, smelters, and fossil fuel combustion. Satellite monitoring of SO 2 began with TOMS data in 1978 that detected volcanic eruption clouds. Hyperspectral instruments, like OMI and GOME, have a twenty-fold improvement in sensitivity. Degassing volcanoes, smelters, and large power plants are now monitored for a database of SO 2 emission to the atmosphere. SO 2 is a distinctive marker for volcanic ash clouds, a hazard to aircraft. Keywords: Hyperspectral, Remote Sensing, UV spectrometer, Sulfur Dioxide, Volcanic emissions, Air quality, Aviation hazards. INTRODUCTION Satellite remote sensing of volcanic clouds began with the 1982 eruption of El Chichon in southern Mexico. Its globe-circling plume was tracked for three weeks as a sulfur dioxide cloud in the Nimbus 7 UV Total Ozone Mapping Spectrometer (TOMS) data 1,2. The total mass of the sulfur dioxide cloud was determined by integrating over the cloud area. This ability to measure the mass of eruption clouds was later applied to all the eruptions since 1978 in the TOMS dataset to produce a 30-year climate record of volcanic input. Only months after the El Chichon eruption, two Boeing 747 passenger jet airplanes were disabled after they flew into ash clouds from eruptions of Galunggung volcano in West Java, Indonesia. This has led to development of satellite techniques to rapidly locate volcanic ash clouds directly and through a surrogate, sulfur dioxide. TOMS VOLCANIC CLOUD DATA The TOMS instrument 3,4 was designed to map the spatial distribution of total ozone using a retrieval scheme developed with the Nimbus 4 Backscatter UltraViolet (BUV) instrument. With the telemetry bandwidth of the Nimbus 7 satellite only six UV wavelength bands could be monitored and the nadir spatial resolution was limited to 50 km. Full contiguous daily global mapping was obtained by scanning over a 2700 km swath. When El Chichon erupted, an unexpected UV absorbing cloud appeared over the volcano. The absorbing gas was identified as sulfur dioxide 1. A new algorithm was required to discriminate sulfur dioxide from ozone 5. Figure 1 shows the cloud on the fifth day after the major eruption on April 4, *akrueger@umbc.edu; phone ; fax Remote Sensing of Clouds and the Atmosphere XIV, edited by Richard H. Picard, Klaus Schäfer, Adolfo Comeron, Evgueni Kassianov, Christopher J. Mertens, Proc. of SPIE Vol. 7475, 74750Y 2009 SPIE CCC code: X/09/$18 doi: / Proc. of SPIE Vol Y-1

2 Figure 1: The sulfur dioxide cloud produced by the eruption of El Chichon volcano in southern Mexico on April 4, 1982 as measured with the TOMS instrument on the Nimbus 7 satellite. This cloud expanded westward, passing completely around the Earth until the leading edge reached Mexico three weeks later, while the trailing edge remained anchored over Mexico. Integration of the SO 2 over the cloud area for several days yielded an initial mass of 7.5 million tons. The El Chichon SO 2 cloud was sheared into a banner that extended completely around the world after 3 weeks. Based on wind data, the cloud was in the lower stratosphere, between 20 and 25 km. The rate of loss of sulfur dioxide implied a lifetime of about a month, consistent with a dry stratosphere. This confirmed that volcanic eruptions produce global cooling through production of a long-lived sulfate layer in the stratosphere rather than an ash cloud, which falls out rapidly. Scores of volcanic eruptions were found in the TOMS data record from a series of four missions between 1978 and The largest eruption, Mt Pinatubo in 1991, produced 20 million tons (or Tg) of SO 2 while El Chichon with 7 Tg was the second largest. Every eruption since October 1978 was captured in the TOMS database except for an 18-month gap in between TOMS missions. The contiguous daily coverage is important in monitoring volcanic eruptions because of the short (~ 1 day) lifetime of sulfur dioxide at low altitudes and the small size of some eruption clouds. Time series of the volcanic input from major eruptions were produced 6 for input to climate models (Figure 2). Proc. of SPIE Vol Y-2

3 Figure 2: The time series of the major volcanic eruption input of sulfur dioxide into the atmosphere derived from TOMS data. The smallest detectable eruption clouds contain about 5 kt of SO 2. The limit is set by the standard deviation of the background noise, which is determined by the inversion matrix properties that depend on the instrument wavelengths 7. A less than optimum TOMS wavelength selection limited the detection to large volcanic eruptions. IMPROVED SO 2 DETECTION WITH HYPERSPECTRAL SATELLITE INSTRUMENTS In the 1990 s satellite memory and telemetry advances allowed collection of far more information through higher spatial resolution or greater spectral content. For the atmosphere, better spatial resolution allows detection of smaller sources and improved resolution of horizontal structure in absorbing or scattering clouds. With more spectral bands more absorbing gases can be measured using discrete wavelength retrieval algorithms, while collection of complete spectra permits use of band structure in absorption spectra, new wavelength calibration techniques, and fitting of Ring scattering in Frauenhofer lines of the solar spectrum. The first hyperspectral UV satellite instrument, the Global Ozone Monitoring Experiment (GOME), was launched in 1995 on the ERS-2 satellite 8. This instrument had broad spectral coverage that permitted detection of several atmospheric species that absorb in the near UV and blue (NO 2, OClO, BrO, OCHO) but had very limited spatial resolution (320 x 40 km) with only 3 pixels per cross-track scan. With a continuous spectrum a Differential Optical Absorption Spectrometer (DOAS) algorithm 9 could be employed for detection of trace gas species with weak absorption. In addition, with a better selection of wavelengths a twenty-fold improvement in SO 2 sensitivity over TOMS was achieved 10. Strong sources of air pollution sulfur dioxide could be detected, especially in averages over time, in addition to volcanic clouds. The next hyperspectral instrument was the Scanning Imaging Absorption Spectrometer for Atmospheric CartograpHY (SCIAMACHY) instrument 11 on ENVISAT in This instrument added nadir mapping and limb observations to the GOME capability. The ground resolution of 60 x 30 km was a moderate improvement over the TOMS 50 x 50 km resolution. The discontinuous along track spatial coverage by interruption for limb soundings Proc. of SPIE Vol Y-3

4 posed a problem in coverage of volcanic eruption clouds that rapidly grow larger than the contiguous coverage segments. As a result, the total mass of sulfur dioxide in eruptions could not be determined on a daily basis. The first hyperspectral instrument with contiguous spatial coverage is the OMI instrument 12 launched on the EOS Aura platform in This Dutch/ Finnish instrument provides continued monitoring of volcanic SO 2 cloud masses after the TOMS series ended in In addition to a twenty-fold increase in sensitivity, the OMI 13 x 24 km nadir ground resolution is improved over TOMS by a factor of eight to permit detection SO 2 clouds two orders of magnitude smaller 13,14. The SO 2 retrieval characteristics for the Nimbus 7 TOMS and the OMI and GOME-2 instruments are shown in Table 1. Both hyperspectral instruments have large improvements in retrieval noise but OMI s small nadir footprint allows detection of much smaller sources. This performance degrades away from the nadir due to footprint area growth, while GOME-2 footprints remain constant in area, and the detection limit is independent of cross track position. Nimbus 7 TOMS EOS Aura OMI MetOp GOME-2 Nadir footprint (km) 50 x x x 40 Scenes / scan Spectral resolution (nm) Bands 6 discrete bands UV 2 Ch 2: Wavelength span, nm SO 2 retrieval noise, 1σ (DU) for a cloud at 15 km SO 2 detection limit (tons) 5 adjacent pixels > 5σ Table 1: Comparison of sulfur dioxide retrieval noise and nadir cloud detection limits between OMI, TOMS, and GOME-2. The OMI improvement is due to optimum wavelengths and smaller footprint size. The GOME-2 improvement is due to retrieval noise decrease from optimum wavelengths. The decrease in retrieval noise results in detection of smaller extended areas clouds. The decrease in detection limit results in detection of smaller point sources. VOLCANIC DEGASSING AND AIR POLLUTION As with the GOME and SCIAMACHY instruments, OMI is able to detect passive degassing from volcanoes and from sulfate ore smelters. With their increased sensitivity emissions as small as a few hundred tons can be detected, while the TOMS instrument required at least 5000 tons. An example of strong degassing from volcanoes in Ecuador and Colombia is shown in Figure 3. Tungurahua in Ecuador (the more southern source) showed activity through frequent degassing through 2006 until July 14 when the emissions increased to 1.5 kt, leading to an explosion on July 15 produced the 3.8 kt plume shown in the lower right hand panel. These are examples of daily images from many active volcanic regions that are available at the UMBC Sulfur Dioxide Group web site: Proc. of SPIE Vol Y-4

5 Figure 3. OMI observations of daily sulfur dioxide emissions from volcanoes in northern South America during July 12 15, Tungurahua in Ecuador (lower source) emitted a few hundred tons each day until July 14 and 15 when an eruption released a 3800 ton cloud. A second volcano, Galeras, just north of the Ecuador - Colombia border was intermittently degassing during this time period. None of these plumes would have been detected with the TOMS instrument. Sources of air pollution are much harder to detect than SO 2 from tall volcanoes because they are at low altitudes. Clouds may frequently obscure these sources and even with clear skies the signal is weak because of Rayleigh scattering which limits sunlight from reaching the planetary boundary layer. Nevertheless, it is possible to regularly detect anthropogenic emissions. Smelters have been strong sources of sulfur dioxide that are comparable to degassing volcanoes. Environmental efforts to capture the sulfur dioxide to prevent acid rain, smog, and other environmental degradations have been largely successful. However, a number of smelters are still producing large emissions that are detectable with hyperspectral instruments. One example of sources that can be detected frequently, shown in Figure 4, are the Norilsk, Russia nickel smelters. Proc. of SPIE Vol Y-5

6 Figure 4: An example of daily OMI observations of sulfur dioxide emissions from smelters in Norilsk, Russia on May 7, 8, and 9, VOLCANIC ERUPTION: Kasatochi An example of the drift and shearing of a volcanic cloud is from the explosive eruption of Kasatochi volcano on the Aleutian Islands on August 9, The eruption produced 1.2 Tg of SO 2 in the largest eruption since the 20 Tg Mt. Pinatubo eruption of 1991, The Kasatochi cloud was detectable for weeks due to the sensitivity of the OMI retrievals. This cloud was sheared into three fingers that were stretched into distinct trails across much of the northern hemisphere. The first four days (August 9-13, 2008) of the dispersal of the cloud are shown in Figure 6. Figure 7 illustrates the distribution across North America, the Atlantic Ocean and Western Europe on August 16. An application of the SO 2 images is in aviation safety. Sulfur dioxide is a marker for volcanic ash clouds because both are contained in explosive magmatic eruption plumes 15. The ash is co-located with the SO 2 in fresh eruption clouds, but falls out within days. Ash clouds are difficult to detect under some circumstances while SO 2 clouds are unique as there are no other sources with comparable concentrations. The OMI and GOME-2 data are made available in near real-time in the US by the National Oceanographic and Atmospheric Administration at the web site: In Europe the SCIAMACHY and GOME-2 data are provided in near real-time through the Support to Aviation Control Service hosted by BIRA/-IASB, Brussels at: Proc. of SPIE Vol Y-6

7 Figure 6: The sulfur dioxide cloud from the eruption of Kasatochi volcano in the Aleutian Islands on August 8, In the four days shown, August 9 13, the cloud was sheared into banners that drifted across Canada and the United States. The initial sulfur dioxide mass is about 1,200 kt (1.2 Tg), the largest eruption since the 20 Tg cloud from Mt Pinatubo in Most of the ash from the eruption fell out in the first few days, leaving a plume of SO 2, which remained detectable until the SO 2 was converted to sulfate. CONCLUSIONS Quantitative mapping of volcanic eruption clouds was initiated in retrievals of sulfur dioxide with data from the Nimbus 7 TOMS instrument, a six-channel UV spectrometer designed to measure ozone. This led to a 25- year record of volcanic eruption sizes of interest to climate modelers. The introduction of hyperspectral instruments including GOME, SCIAMACHY, OMI, and GOME-2 has improved the sensitivity for detection of SO 2 by an order of magnitude so that volcanic degassing and air pollution from smelters and fossil fuel burning can also be detected. The volcanic SO 2 images also find application in aviation safety as an easily detected proxy for ash clouds that can disable aircraft in flight. The data are now being provided in near real-time to operational agencies responsible for aviation safety and volcano monitoring. Proc. of SPIE Vol Y-7

8 ACKNOWLEDGMENTS This work was supported by NASA TOMS and OMI Science Teams and by the NASA Applications Program Cooperative Agreement Grant NNS06AA05G (NASA research volcanic emissions data for Aviation Hazards). In-kind support has been provided by NOAA/NESDIS and USGS. REFERENCES [1] Krueger A. J. Sighting of El Chichon sulfur dioxide clouds with the Nimbus 7 total ozone mapping spectrometer. Science 220, (1983) [2] Krueger A. J., Krotkov, N, and Carn, S., El Chichon: The genesis of volcanic sulfur dioxide monitoring from space. J. Volcanol. Geotherm. Res. 175, (2008) [3] Heath, D. F., A.J. Krueger, H.R. Roeder, and B.D. Henderson, The Solar Backscatter Ultraviolet and Total Ozone Mapping Spectrometer (SBUV/TOMS) for Nimbus G, Opt. Eng., 14, (1975) [4] Krueger, A, J., UV Remote Sensing of the Earth s Atmosphere, [Diagnostic Tools in Atmospheric Physics], IOS Press, Amsterdam, (1995) [5] Krueger, A. J., L.S. Walter, P.K. Bhartia, C.C. Schnetzler, N.A. Krotkov, I. Sprod, and G.J.S. Bluth. Volcanic sulfur dioxide measurements from the Total Ozone Mapping Spectrometer (TOMS) Instruments. J. Geophys. Res., 100, 14,057-14,076 (1995) [6] Carn S.A., Krueger A.J., Bluth G.J.S., Schaefer S.J., Krotkov N.A., Watson I.M., Datta S., Volcanic eruption detection by the Total Ozone Mapping Spectrometer (TOMS) instruments: a 22-year record of sulfur dioxide and ash emissions, In: Oppenheimer C, Pyle DM, Barclay J (eds) Volcanic degassing. Geological Society, London, Special Publications, 213, (2003) [7] Gurevich, G. S. and Krueger, A. J. Optimization of TOMS wavelength channels for ozone and sulfur dioxide retrievals Geophys. Res. Lett., 24, 17, (1997) [8] Burrows, J.P., Weber, M., Buchwitz, M., Rozanov, V.V., Ladstädter-Weissenmayer, A., Richter, A., de Beek, R., Hoogen, R., Bramstedt, K.,-U. Eichmann, K., Eisinger M. and Perner, D., The Global Ozone Monitoring Experiment (GOME): Mission Concept and First Scientific Results. J. Atm. Sciences, 56, (1999) [9] Platt, U.: Differential optical absorption spectroscopy (DOAS), [Monitoring by Spectroscopic Techniques], Wiley & Sons, New York, USA, 27 84, (1994) [10] Eisinger, M., and J. P. Burrows, Tropospheric sulfur dioxide observed by the ERS-2 GOME instrument, Geophys. Res. Lett., 25, (1998) [11] Bovensmann, H., J.P. Burrows, M. Buchwitz, J. Frerick, S. Noël, V.V. Rozanov, K.V. Chance, and A.P.H. Goede, SCIAMACHY: Mission Objectives and Measurement Modes. J. Atmos. Sci., 56, (1999) [12] Levelt P.F., van den Oord GHJ, Dobber MR, Ma lkki A, Visser H, De Vries J, Stammes P, Lundell JOV, Saari H The ozone monitoring instrument. IEEE Trans Geosci Remote Sens 44(5): , doi: /tgrs (2006) Proc. of SPIE Vol Y-8

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