SULFUR dioxide is a transient atmospheric constituent that

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1 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING, VOL. 2, NO. 4, DECEMBER Applications of Satellite-Based Sulfur Dioxide Monitoring A. J. Krueger, Nickolay A. Krotkov, Kai Yang, S. Carn, Gilberto Vicente, and Wilfrid Schroeder Abstract Sulfur dioxide is emitted by volcanoes, produced by combustion of fossil fuels or smelting of ores, and is an intermediate product from organic sources in the ocean. It is rapidly oxidized to sulfuric acid, which causes acidic pollution of lakes and streams and forms an aerosol that is important in climate change. Volcanic sulfur dioxide is a useful marker for ash clouds that are a hazard to aircraft. Satellites offer the best platform to monitor SO 2 sources and to track volcanic clouds. UV remote sensing instruments have measured eruption plume masses since Newer instruments are sensitive enough to also measure volcanic degassing, emissions from power plants, refineries, smelters, and heavy air pollution episodes. New retrieval algorithms have improved the data quality. The observations are used to constrain models of eruption processes and to monitor activity of all volcanoes in a consistent manner. The practical applications of the satellite data include aviation safety, air quality, environmental control, climate modeling, and atmospheric dynamics modeling. Index Terms Air quality, aviation hazards, remote sensing, sulfur dioxide, volcanic emissions. I. INTRODUCTION SULFUR dioxide is a transient atmospheric constituent that is produced by volcanic eruptions, passive volcanic degassing, fossil fuel combustion, smelting of sulfate ores, and from dimethyl-sulfide (DMS) emissions from the ocean. Once released, the lifetime in air is short ranging from minutes or hours in the boundary layer, to days in the free troposphere, and to a month in the stratosphere [1]. It is converted to sulfate (H SO ) through oxidation in a catalytic cycle with OH. Fossil fuel combustion accounts for about 80% of the sulfur budget in the troposphere but in the lower troposphere much of the sulfate is removed quickly by surface deposition and rainout. Nevertheless, the continuous wide spread production of sulfur dioxide Manuscript received February 27, 2009; revised July 27, First published December 15, 2009; current version published January 20, This work was supported in part by the NASA OMI Science Team under Grant NNG06GJ02G and in part by the NASA Applications Program under Grant NNS06AA05G. A. J. Krueger is with the Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD USA ( akrueger@umbc.edu). N. A. Krotkov and K. Yang are with the Goddard Earth Sciences and Technology Center, University of Maryland Baltimore County, Baltimore, MD USA ( nickolay.a.krotkov@nasa.gov; kai.yang.1@gsfc.nasa.gov). S. Carn is with the Department of Geological and Mining Engineering, Michigan Tech University, Houghton, MI USA ( scarn@mtu.edu). G. Vicente is with the NOAA/NESDIS/OSDPD/SSD Product Implementation Branch, E/SP2, NOAA Science Center, Camp Springs, MD USA ( gilberto.vicente@noaa.gov). W. Schroeder is with the Earth System Science Interdisciplinary Center, College Park, MD USA ( wilfrid.schroeder@noaa.gov). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /JSTARS has increased the turbidity of the atmosphere and reduced insolation [2]. Volcanic eruptions can place sulfur dioxide in the stratosphere where sulfate aerosol remains for more than a year. Even though volcanoes are a weaker global source than fossil fuel burning, the longer lifetime of sulfate in the stratosphere makes the net climate effect stronger. Global monitoring of sulfur dioxide produces important information in five diverse fields: air quality, climate change, volcanology, volcanic hazards, and Global Circulation Model (GCM) validation. Sulfate in air pollution is well known for production of acid rain and environmental degradation, together with reduced visibility and health effects. Sulfate aerosol affects climate through the Earth s radiative balance by scattering incident sunlight back to space and through effects on cloud droplet sizes. The reduced insolation results in cooling that is easiest to detect after volcanic eruptions due to the transient nature of these events. For example, the eruption of 20 Tg of SO by Mt. Pinatubo in 1992 produced a 0.5 degree decrease in global temperature [3]. Volcanic SO applications, with eruption data collected over the past 25 years, are better developed than the air quality uses, with data available only for the past decade. In volcanology, information on the release of volatiles, like sulfur dioxide, helps in understanding of eruption processes. Conventional petrologic estimates of SO release in explosive eruptions were found to be low by an order of magnitude [4] when satellite data became available. Explosive volcanic eruptions are hazardous to local residents as well as to aircraft and passengers due to ash clouds at flight altitude. Monitoring of aviation hazards particularly requires satellites because of the large scale and global drift of volcanic clouds. Visualization of the dispersion of volcanic gases over hemispheric scales is instructive of atmospheric mixing processes and poses a severe test of GCM fidelity over time. To date, most development has been in aviation hazards and a practical example is provided. II. REMOTE SENSING OF SULFUR DIOXIDE Sulfur dioxide has strong near and middle UV absorption bands in the region nm which overlap the prominent ozone absorption bands. Absorbing volcanic clouds were first observed from space as anomalies in ozone retrievals from the Total Ozone Mapping Instrument (TOMS) [5] on the Nimbus 7 satellite during the eruption of El Chichon in The anomalous gas was identified as sulfur dioxide [6], [7] and a new algorithm was developed to discriminate sulfur dioxide from ozone by spectral differences in the backscattered UV sunlight [8]. A quantitative record of all volcanic eruption inputs to the atmosphere for over two decades has been produced from the TOMS /$ IEEE

2 294 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING, VOL. 2, NO. 4, DECEMBER 2009 Fig. 1. OMI image of the trails of SO across North America, the Atlantic Ocean and Western Europe on August 16, 2008 produced by the eruption of Kasatochi Volcano on August 8, measurements [9]. The SO retrievals were validated when a cloud from the eruption of Mt Spurr in September 1992 passed over a Brewer station in Toronto while TOMS instruments on both Nimbus 7 and Meteor-3 satellites orbited overhead [10]. New hyperspectral instruments, such as Global Ozone Monitoring Experiment (GOME) [11], Scanning Imaging Absorption Spectrometer for Atmospheric CartograpHY (SCIAMACHY), and Ozone Monitoring Instrument (OMI) [12] offer advantages for detection of SO because the complete spectrum is available instead of the six fixed wavelengths in TOMS. A resulting twenty-fold decrease in background noise from optimizing wavelengths adds volcanic degassing and air pollution to the sulfur sources that can be monitored. Valuable information has been produced from GOME, GOME-2, and SCIAMACHY in Europe using Differential Optical Absorption Spectroscopy (DOAS) algorithms [13, and references therein]. The capabilities of IR instruments, such as AIRS and IASI, for detection of high altitude SO clouds, have also been demonstrated [14]. The following discussion uses OMI data to illustrate applications because of its high ground resolution and daily global coverage for monitoring of small SO sources. The US OMI Science Team uses full radiative transfer retrieval algorithms that were first developed for the Backscatter UltraViolet (BUV) instrument on the Nimbus 4 satellite in 1970 and extended for the TOMS instrument in Two radiative transfer look-up-table retrieval techniques, the Band Residual Difference (BRD) method [15], [16] for small boundary layer SO loadings and the Linear Fit (LF) algorithm [17] for volcanic SO, are operationally used to produce SO vertical columns from OMI measurements. The BRD algorithm makes use of pronounced SO absorption band structure at nm. After retrieving the total ozone column at a pair of longer wavelengths [18] a forward radiative transfer model predicts radiances at the shorter wavelengths assuming no SO. The differences between predicted and observed radiances at the SO band maxima and minima are then used to compute SO column amounts. This algorithm is very sensitive for detection of small amounts near the surface but fails for volcanic loadings greater than 10 DU ( Dobson Unit molecules cm ). The LF algorithm can work with both discrete-wavelength and hyper-spectral measurements. The LF method uses longer wavelengths where SO absorption is weaker and holds for SO loadings up to DU [17]. When very large SO loading is present in large volcanic clouds, the LF equation must be solved iteratively with an updated linearization point and weighting functions at each step [19]. The linear algorithms have been quite successful in detecting and measuring a wide range of SO loadings encountered in global observations [20], [17], [16], and are fast enough to generate SO products in near real time for application in aviation hazard mitigation [21]. An example of the high sensitivity from optimized OMI wavelengths is shown in Fig. 1, the trail of SO from the eruption of Kasatochi volcano in the Aleutian Islands in August III. VOLCANIC DEGASSING AND AIR POLLUTION With the increased sensitivity now available from hyperspectral UV instruments, much smaller SO burdens are detectable, including passive degassing from volcanoes (i.e., noneruptive

3 KRUEGER et al.: APPLICATIONS OF SATELLITE-BASED SULFUR DIOXIDE MONITORING 295 northern Andes have been degassing rather vigorously in recent years. OMI measures the total mass of SO at the time of the overpass, an indication of the total emissions over the past day. For example, the volcanoes of Ecuador and Columbia are shown for April 26, 2007 in Fig. 3. Tungurahua, at 2 S in Ecuador, has emitted moderate amounts of SO, but Nevado del Huila at 3 N in Columbia is more active on this day. This degassing varies from day to day and is hard to measure from the ground due to varying wind directions. By continued monitoring we are building a picture of secular changes in volcanic regions. More than 30 volcanic eruptions have been monitored since the OMI launch in Examples include Manam, Papua New Guinea (October 2004), Sierra Negra, Galapagos (October 2005), Nyamuragira, Africa (December 2006), Jebel al-tair, Yemen (September 2007), Chaiten, Chile (May 2008), Okmok, AK (July 2008), and Kasatochi, AK (August 2008). The last, and largest, produced a 1.5 Tg SO cloud on August 8, This cloud was observed for weeks as it was sheared into banners that drifted across much of the northern hemisphere (Fig. 1). Fig. 2. Average sulfur dioxide column amounts from fossil fuel burning observed with OMI over China in activity), plumes from sulfate ore smelters, and emissions from power plants and industry. With the OMI contiguous coverage and small footprint, smelter emissions can be monitored on a daily basis and discriminated from volcanic degassing. Typical column amounts in eruptions range from 20 to as much as 1000 DU, while degassing and pollution sources produce 1 3 DU in most cases. In addition, the signal produced by SO in the boundary layer is significantly reduced compared with SO in the free troposphere. Thus, retrieval noise control is very important particularly for air pollution data. Nevertheless, clouds of SO are frequently seen as they are carried offshore from China by westerly winds, particularly if they are lofted from the boundary layer to higher altitudes [16]. By averaging over long periods, it is possible to determine locations of the sources in air pollution. Fig. 2 shows the sulfur dioxide emitted by Chinese power plants and industry observed by OMI for the three-year period from 2005 to The dominant sources are clearly located in South Eastern China. Other air pollution sources have been found over power plants in Eastern Europe [22], India, South Africa, Mexico, Russia, and the Ohio River valley in the United States. Smelters have historically produced large amounts of SO but new controls on emissions have reduced the number of sources. Nevertheless, plumes from smelters in Peru [20] and Russia have been measured on a daily basis from space. Volcanic SO emissions have been monitored for years at selected volcanoes using airborne and ground based spectrometers. With OMI nearly all volcanoes are monitored on a daily basis. This advance in volcanology is leading to a better understanding of eruption processes. Release of SO can only happen when fresh magma is near the surface. Thus, degassing is an indirect measure of magma movement. Some volcanoes in the IV. VOLCANIC ERUPTIONS AND AVIATION HAZARDS Ash is a serious hazard to jet aircraft because it melts below the operating temperatures of the engines, collects on turbine components, and disrupts air flow to the point that the engines may shut down. Furthermore, ash abrades the windshield, interferes with static pitot air speed data, and collects on avionics. Explosive magmatic eruptions are the dominant source of ash hazards to aircraft at cruise altitudes. Sulfur dioxide in the magma is coincident with ash in fresh explosive magmatic eruption clouds. Thus, sulfur dioxide serves as a marker for ash, which usually falls out in the first three or four days. Satellites provide the best vantage point to see entire eruption clouds. Ash clouds themselves are difficult to identify due of the lack of a unique spectral signature. Its broad absorption extends across the UV and visible to thermal IR (TIR) wavelengths where differences in plume transmissions between 10 and 12-micron channels on polar and geostationary operational satellites are used to locate ash clouds day or night using the Split Window technique [23]. The technique requires optically thin ash clouds which develop over time as the plume is sheared or the ash falls out. The differences are not unique to ash, and water vapor and ice can interfere with detection. It is generally necessary to manually inspect every suspected eruption cloud to confirm an ash cloud by its spatial appearance. Thus, alternative methods have been sought to detect the fresh ash clouds (less than a day old) that are most hazardous to aircraft, as well as to generate automated alerts with low false alarm rates. At UV wavelengths ash can also be detected by the spectral difference from Rayleigh scattered light [24]. This is measured in TOMS and OMI data by an Aerosol Index (AI). It is not susceptible to masking like the TIR channel differences, but is limited to daylight conditions. However, other absorbers, like dust and smoke, and sun glint have the same signature so that automated alarms are not possible. The same is not true for sulfur dioxide, which has a distinctive absorption spectrum

4 296 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING, VOL. 2, NO. 4, DECEMBER 2009 Fig. 3. Passive degassing from volcanoes in Ecuador and Columbia on April 26, This degassing is an indicator of magmatic activity and varies from day-today. that allows easy discrimination from other constituents. In addition, no other sources of concentrated SO exist with column amounts greater than 5 DU, so that automated alarms are possible for fresh eruption clouds that usually have column amounts between 50 and 500 DU. The SO remains at the injection altitude and can be detected until it is converted to sulfate. An open question is, How long are volcanic clouds hazardous to aviation? as SO clouds can be tracked long after most of the ash has fallen out. To date all major damage has occurred within the first two days after eruption. However, the effects of acidic sulfate on the aircraft are not presently established. V. NEAR REAL-TIME DISTRIBUTION OF RESEARCH SATELLITE DATA FOR AVIATION HAZARDS The OMI derived ash and SO daily concentrations are used to monitor volcanic clouds and to detect pre-eruptive volcanic degassing globally [21]. The data provide valuable information in support of the U.S. Federal Aviation Administration goal of a safe and efficient National Air Space. Near real-time (NRT, i.e., not older than 3 h) observations of SO and volcanic ash are incorporated into data products compatible with Decision Support Tools in use at Volcanic Ash Advisory Centers (VAACs) in Washington and Anchorage and at the United States Geological Survey (USGS) Volcano Observatories. An operational online NRT OMI SO image and data product distribution system was developed in a NASA sponsored collaboration between the University of Maryland, Baltimore County and the NOAA Office of Satellite Data Processing and Distribution. Automated volcanic eruption alarms, and the production of volcanic cloud subsets for multiple regions of interest are provided through the website: This site provides access to different graphical products derived from the OMI, GOME-2, and the Atmospheric Infrared Sounder (AIRS), intended to facilitate rapid access to global volcanic cloud data. Detailed maps of volcanic regions are provided to show degassing activity, which is useful for monitoring emissions that may be precursory to eruptive activity. In Europe, the Support to Aviation Control Service (SACS) provides similar NRT data products from SCIAMACHY and GOME-2 to the VAACs at the site:

5 KRUEGER et al.: APPLICATIONS OF SATELLITE-BASED SULFUR DIOXIDE MONITORING 297 Fig. 4. NOAA near real-time volcanic hazard website showing volcanic SO clouds during the past 24 h using OMI data ( OMI/OMISO2/index.html)The most recent orbit is marked by a yellow outline on a global map projection for tropical latitudes. High latitude eruption clouds are shown on polar projections. Links provide detailed data and images of SO and ash plumes from active volcanoes. VI. CONCLUSION Sulfur dioxide retrievals from satellites are valuable in climate change modeling, air quality assessment, volcanology, and characterization of aviation hazards. A nearly 3-decade record of volcanic eruption sizes is available from the TOMS and OMI instruments for modeling of climate impacts. These direct measurements of eruption mass demonstrated far larger sulfur output than previously estimated, thus forcing revisions in models of pre-eruptive processes. Increased sensitivity from hyperspectral instruments now allows monitoring of volcanic degassing, and adds smelter output and severe air pollution from fossil fuel combustion, in addition to eruption masses. For the first time an independent assessment of large SO releases is available globally on a daily basis and is measured on the same scale as volcanic releases. Capabilities for monitoring sulfur dioxide developed with data from research satellites are now being transitioned to operational satellites. Algorithms developed for OMI data can retrieve a large range of column amounts of SO ranging from fresh eruption clouds, to passive degassing plumes and smelter plumes, to concentrated air pollution clouds. The global budget of sulfur, previously estimated only from source inventory data, has become measurable in part from space. The volcanic data are used for aviation safety using SO as a distinctive marker of fresh volcanic ash clouds. ACKNOWLEDGMENT The authors would like to thank the KNMI OMI team for producing L1B radiance data and the U.S. OMI Science and Operations teams for continuing support. The Dutch-Finnish built OMI instrument is part of the NASA EOS Aura satellite payload. The OMI project is managed by NIVR and KNMI in The Netherlands. REFERENCES [1] G. J. S. Bluth, S. D. Doiron, C. C. Schnetzler, A. J. Krueger, and L. S. Walter, Global tracking of the SO clouds from the June, 1991 Mount Pinatubo eruptions, Geophys. Res. Lett., vol. 19, no. 2, pp , [2] V. Ramanathan, P. J. Crutzen, J. T. Kiehl, and D. Rosenfeld, Aerosols, climate, and the hydrological cycle, Science, vol. 294, no. DOI: /science , p. 2119, [3] B. J. Soden, R. T. Wetherald, G. L. Stenchikov, and A. Robock, Global cooling following the eruption of Mt. Pinatubo: A test of climate feedback by water vapor, Science, vol. 296, no. 5568, DOI: /science , pp , [4] H. Shinohara, Excess degassing from volcanoes and its role on eruptive and intrusive activity, Rev. Geophys., vol. 46, no. DOI: / 2007RG000244, p. RG4005, [5] D. F. Heath, 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., vol. 14, pp , [6] A. J. Krueger, Sighting of El Chichon sulfur dioxide clouds with the Nimbus 7 total ozone mapping spectrometer, Science, vol. 220, no. 4604, pp , [7] A. J. Krueger, N. Krotkov, and S. Carn, El Chichon: The genesis of volcanic sulfur dioxide monitoring from space, J. Volcanol. Geotherm. Res., vol. 175, no. 4, pp , [8] A. J. Krueger, 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 instruments, J. Geophys. Res., vol. 100, no. D7, pp , 1995.

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