VOL. 56, NO.2 JOURNAL OF THE ATMOSPHERIC SCIENCES 15 JANUARY 1999 SCIAMACHY: Mission Objectives and Measurement Modes H. BOVENSMANN,J.P.BURROWS,M.BUCHWITZ,J.FRERICK,S.NOEÈ L, AND V. V. R OZANOV Institute of Environmental Physics, University of Bremen, Bremen, Germany K. V. CHANCE Harvard±Smithsonian Center for Astrophysics, Cambridge, Massachusetts A. P. H. GOEDE SRON Ruimetonderzoek, Utrecht, the Netherlands (Manuscript received 5 September 1997, in ®nal form 16 June 1998) ABSTRACT SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) is a spectrometer designed to measure sunlight transmitted, re¯ected, and scattered by the earth's atmosphere or surface in the ultraviolet, visible, and near-infrared wavelength region (240±2380 nm) at moderate spectral resolution (0.2± 1.5 nm, l/Dl ø 1000±10 000). SCIAMACHY will measure the earthshine radiance in limb and nadir viewing geometries and solar or lunar light transmitted through the atmosphere observed in occultation. The extraterrestrial solar irradiance and lunar radiance will be determined from observations of the sun and the moon above the atmosphere. The absorption, re¯ection, and scattering behavior of the atmosphere and the earth's surface is determined from comparison of earthshine radiance and solar irradiance. Inversion of the ratio of earthshine radiance and solar irradiance yields information about the amounts and distribution of important atmospheric constituents and the spectral re¯ectance (or albedo) of the earth's surface. SCIAMACHY was conceived to improve our knowledge and understanding of a variety of issues of importance for the chemistry and physics of the earth's atmosphere (troposphere, stratosphere, and mesosphere) and potential changes resulting from either increasing anthropogenic activity or the variability of natural phenomena. Topics of relevance for SCIAMACHY are R tropospheric pollution arising from industrial activity and biomass burning, R troposphere±stratosphere exchange processes, R stratospheric ozone chemistry focusing on the understanding of the ozone depletion in polar regions as well as in midlatitudes, and R solar variability and special events such as volcanic eruptions, and related regional and global phenomena. Inversion of the SCIAMACHY measurements enables the amounts and distribution of the atmospheric con- 1 stituents O3,O2,O2( D), O4, BrO, OClO, ClO, SO2,H2CO, NO, NO2,NO3, CO, CO2,CH4,H2O, N2O, and aerosol, as well as knowledge about the parameters pressure p, temperature T, radiation ®eld, cloud cover, cloud- top height, and surface spectral re¯ectance to be determined. A special feature of SCIAMACHY is the combined limb±nadir measurement mode. The inversion of the combination of limb and nadir measurements will enable tropospheric column amounts of O3,NO2, BrO, CO, CH4,H2O, N2O, SO2, and H2CO to be determined. 1. Introduction man et al. 1997; MuÈller et al. 1997) resulting from the tropospheric emission of chloro¯uorocarbon com- Large and signi®cant changes in the composition and pounds (CFCs, halones, and HFCs) (WMO 95); (ii) the behavior of the global atmosphere have emphasized the global increase of tropospheric O3 (WMO 1995); (iii) need for global measurements of atmospheric constit- the observed increase of tropospheric ``greenhouse gas- uents. Examples are (i) the precipitous loss of Antarctic es'' such as CO2,CH4,N2O, and O3 (IPCC 1996); and (WMO 1995) and Arctic stratospheric ozone (O3) (New- (iv) the potential coupling between polar stratospheric ozone loss and increased greenhouse gas concentrations (Shindell et al. 1998). To assess the signi®cance of such changes a detailed Corresponding author address: Dr. Heinrich Bovensmann, Institute of Environmental Physics, University of Bremen (FB1), P.O. Box 33 understanding of the physical and chemical processes 04 40, D-28334 Bremen, Germany. controlling the global atmosphere is required. Similarly E-mail: [email protected] knowledge about the variability and temporal behavior q 1999 American Meteorological Society 127 128 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 56 of atmospheric trace gases is necessary to test the pre- (Barth et al. 1983) within one instrument. SCIAMA- dictive ability of the theories currently used to model CHY measures in the wavelength range from 240 nm the atmosphere. Consequently, the accurate assessment to 2380 nm the following: of the impact of current and future anthropogenic ac- tivity or natural phenomena on the behavior of the at- R The scattered and re¯ected spectral radiance in nadir mosphere needs detailed knowledge about the temporal and limb geometry, and spatial behavior of several atmospheric trace con- R the spectral radiance transmitted through the atmo- stituents (gases, aerosol, clouds) on a global scale, in- sphere in solar and lunar occultation geometry, and cluding the troposphere. R the extraterrestrial solar irradiance and the lunar ra- Over the past two decades pioneering efforts have diance. been made by the scienti®c community to establish both ground-based networks and satellite projects that will Limb, nadir, and occultation measurements are planned eventually result in an adequate global observing sys- to be made during every orbit. Trace gases, aerosols, tem. Examples of satellite borne elements of such pro- clouds, and the surface of the earth modify the light grams are the Solar Backscatter Ultraviolet (SBUV) and observed by SCIAMACHY via absorption, emission, Total Ozone Mapping Spectrometer (TOMS) on NASA's and scattering processes. Inversion of the radiance and Nimbus-7 satellite (Heath et al. 1975); the Stratospheric irradiance measurements enables the amounts and dis- Aerosol and Gas Experiment (SAGE) (McCormick et tributions of a signi®cant number of constituents to be al. 1979); the Upper Atmosphere Research Satellite retrieved from their spectral signatures and is discussed (UARS) (Reber et al. 1993) with the Microwave Limb in section 4. Figure 1 shows the wavelength range to Sounder (MLS), the Halogen Occultation Experiment be observed by SCIAMACHY and the position of spec- (HALOE), the Cryogenic Limb Array Etalon Spectrom- tral windows where atmospheric constituents are to be eter (CLAES), and the Improved Stratospheric and Me- retrieved. sospheric Sounder (ISAMS) instruments on board; and SCIAMACHY and GOME, which is a small-scale the Second European Remote Sensing satellite (ERS-2), version of SCIAMACHY (see Burrows et al. 1999 and which carries the Global Ozone Monitoring Experiment references therein), represent a new generation of space- (GOME) (Burrows et al. 1999). In the near future, sev- based remote sounding sensors, which rely on and uti- eral new missions will be launched and will contribute lize the simultaneous spectrally resolved measurement signi®cantly to research in the ®elds of atmospheric of light upwelling from the atmosphere to determine chemistry and physics: NASA's Earth Observing System amounts of atmospheric constituents. (EOS) satellites EOS-AM and EOS-CHEM, the Japa- Using data from GOME, which was launched on nese Advanced Earth Observing System (ADEOS), and board the European Remote Sensing satellite ERS-2 in the European Space Agency's (ESA) Environmental April 1995, the feasibility of the instrument and retrieval Satellite (ENVISAT). concepts have been successfully demonstrated for nadir The Scanning Imaging Absorption Spectrometer for observations. The trace gases O3,NO2, BrO, OClO, Atmospheric Chartography (SCIAMACHY) is part of SO2, and H2CO have been observed as predicted (Bur- the atmospheric chemistry payload onboard ENVISAT rows et al. 1999), and studies of ClO, NO, and aerosol being prepared by ESA. Following the call for earth retrieval are proceeding. The determination of O3 pro®le observation instrumentation in the Announcement of information, including tropospheric O3, from GOME Opportunity for the Polar Platform issued by ESA, the measurements (Burrows et al. 1999; Munro et al. 1998; SCIAMACHY proposal (Burrows et al. 1988) was sub- Rozanov et al. 1998) has a large number of potential mitted to ESA by an international team of scientists led applications. In addition, the retrieval of tropospheric by Principal Investigator J. P. Burrows. After peer re- column information of SO2,H2CO, NO2, and BrO from view SCIAMACHY was selected as part of the payload GOME measurements was demonstrated (Burrows et al. for the satellite now known as ENVISAT, which is 1999). planned to be launched in 2000. The goal of this paper is to provide a comprehensive The heritage of SCIAMACHY (Burrows et al. 1988) overview of the SCIAMACHY mission and instrument, lies in both the ground-based measurements using Dif- to summarize the retrieval strategies, to report on ferential Optical Absorption Spectroscopy (DOAS) planned data products and expected data quality, and to (Brewer et al. 1973; Platt and Perner 1980; Solomon et demonstrate the range of applications and the potential al. 1987) and previous satellite atmospheric remote that lies in the concept of this new generation of hy- sensing missions. SCIAMACHY combines and extends perspectral UV±VIS±NIR sensors. Section 2 provides the measurement principles and observational modes of details about the targeted constituents. In section 3 the the nadir scattered sunlight measuring instruments instrument design and observational modes are pre- SBUV and TOMS (Heath et al. 1975), the solar occul- sented. The proposed retrieval strategies are summa- tation instrument SAGE (McCormick et al. 1979; Maul-