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Retrievals of sulfur dioxide

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Retrievals of sulfur dioxide from the Global Ozone MonitoringExperiment 2 (GOME‐2) using an optimal estimationapproach: Algorithm and initial validationC. R. Nowlan,1X. Liu,1K. Chance,1Z. Cai,2T. P. Kurosu,1C. Lee,3and R. V. Martin1,4Received 15 February 2011; revised 9 May 2011; accepted 7 June 2011; published 16 September 2011.[1] We apply an optimal estimation algorithm originally developed for retrieving ozoneprofiles from the Global Ozone Monitoring Experiment (GOME) and the OzoneMonitoring Instrument (OMI) to make global observations of sulfur dioxide from theGlobal Ozone Monitoring Experiment 2 (GOME‐2) on the MetOp‐A satellite. Ourapproach combines a full radiative transfer calculation, retrieval algorithm, and trace gasclimatologies to implicitly include the effects of albedo, clouds, ozone, and SO2profilesin the retrieval. Under volcanic conditions, the algorithm may also be used to directlyretrieve SO2plume altitude. Retrieved SO2columns over heavy anthropogenic pollutiontypically agree with those calculated using a two‐step slant column and air mass factorapproach to within 10%. Retrieval uncertainties are quantified for GOME‐2SO2amounts;these are dominated by uncertainty contributions from noise, surface albedo, profileshape, correlations with other retrieved parameters, atmospheric temperature, choice ofwavelength fitting window, and aerosols. When plume altitudes are also simultaneouslyretrieved, additional significant uncertainties result from uncertainties in the a priorialtitude, the model’s vertical layer resolution, and instrument calibration. Retrieved plumeheight information content is examined using the Mount Kasatochi volcanic plume on9 August 2008. An a priori altitude of 10 km and uncertainty of 2 km produce degreesof freedom for signal of at least 0.9 for columns >30 Dobson units. GOME‐2 estimatesof surface SO2are compared with in situ annual means over North America in 2008from the Clear Air Status and Trends Network (CASTNET; r = 0.85, N = 65) andAir Quality System (AQS) and National Air Pollution Surveillance (NAPS; r = 0.40,N = 438) networks.Citation: Nowlan, C. R., X. Liu, K. Chance, Z. Cai, T. P. Kurosu, C. Lee, and R. V. Martin (2011), Retrievals of sulfur dioxidefrom the Global Ozone Monitoring Experiment 2 (GOME‐2) using an optimal estimation approach: Algorithm and initialvalidation, J. Geophys. Res., 116, D18301, doi:10.1029/2011JD015808.1. Introduction[2] Sulfur dioxide (SO2) in the atmosphere has importantimpacts on chemistry and climate at both local and globallevels. Anthropogenic sources account for roughly 70% ofglobal emissions [Faloona, 2009]. These are primarily fossilfuel burning, with smaller contributions from smelting andbiomass burning. Natural sources account for the remainderof SO2emissions. These include contributions from marinephytoplankton through the production and oxidation ofdimethyl sulfide (∼20%), volcanic activity (∼7%–10%), anda small contribution from soil and vegetation decay throughthe production of H2S.[3]SO2oxidizes in the atmosphere to sulfate, and both areremoved by wet or dry deposition. In the boundary layer andlower troposphere, SO2has a lifetime of 1–3 days [Faloona,2009], while sulfate has a lifetime of 3–7 days. The effects ofSO2emissions include impacts on air quality and mortalityrates [Pope and Dockery, 2006], regional radiative forcing[Haywood and Boucher, 2000], and acid rain through wetdeposition. Tropospheric SO2emissions that result in aerosolformation in the upper troposphere may also influencestratospheric humidity levels [Notholt et al., 2005].[4] When SO2is injected into the stratosphere during par-ticularly explosive volcanic eruptions, the resulting sulfateaerosols can remain in the atmosphere for over a year [Forsteret al., 2007], affecting global climate through the scattering ofsolar radiation and absorption of longwave radiation. Theinduced radiative forcing can influence surface temperatureand stratospheric circulation patterns, and modify internal1Atomic and Molecular Physics Division, Harvard‐Smithsonian Centerfor Astrophysics, Cambridge, Massachusetts, USA.2Key Laboratory of Middle Atmosphere and Global EnvironmentObservation, Institute of Atmospheric Physics, Chinese Aca demy ofSciences, Beijing, China.3National Institute of Meteoro logical Research, Korea MeteorologicalAdministration, Seoul, South Korea.4Department of Physics and Atmospheric Science, Dalhousie University,Halifax, Nova Scotia, Canada.Copyright 2011 by the American Geophysical Union.0148‐0227/11/2011JD015808JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D18301, doi:10.1029/2011JD015808, 2011D18301 1of20climate modes [Forster et al., 2007]. Stratospheric ozone isalso affected through the supply of heterogeneous reactionsurfaces for ozone loss [World Meteorological Organization,2007].[5] Satellite measurements of SO2are useful for providingglobal coverage of emission sources and for observing theoften remote and unpredictable locations of volcanic erup-tions. The strong absorption of solar radiation by SO2in theultraviolet was first used to observe SO2from space with theTotal Ozone Mapping Spectrometer (TOMS) on the Nimbus7 spacecraft during the El Chichón volcanic eruption in 1982[Krueger, 1983]. SO2has since been measured from space inthe ultraviolet using the Solar Backscatter Ultravioletinstrument (SBUV/2) [McPeters, 1993], the Global OzoneMonitoring Experiment (GOME) on ERS‐2[Eisinger andBurrows, 1998; Khokhar et al., 2005; Thomas et al., 2005],the Scanning Imaging Absorption Spectrometer for Atmo-spheric Chartography (SCIAMACHY) on Envisat [Afe et al.,2004; Lee et al., 2008], the Ozone Monitoring Instrument(OMI) on Aura [Krotkov et al., 2006; Carn et al., 2007; Yanget al., 2007; Carn et al., 2008; Yang et al., 2009a], and theGlobal Ozone Monitoring Experiment 2 (GOME‐2) onMetOp‐A[Bobrowski et al., 2010; Heue et al., 2010].Infrared sounders are also able to measure SO2from satellites,including, but not limited to, the Atmospheric InfraredSounder (AIRS) [Carn et al., 2005], Tropospheric EmissionSpectrometer (TES) [Clerbaux et al., 2008], and InfraredAtmospheric Sounding Interferometer (IASI) [Clarisse et al.,2008; Karagulian et al., 2010], but with less sensitivity toboundary layer SO2than those instruments measuring back-scattered UV radiation.[6] GOME‐2[Munro et al., 2006] is the most recentlydeployed satellite instrument able to measure SO2in theUV. GOME‐2 is a


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