1. Introduction
Large and significant changes in the composition and behavior of the global atmosphere have emphasized the need for global measurements of atmospheric constituents. Examples are (i) the precipitous loss of Antarctic (WMO 1995) and Arctic stratospheric ozone (O3) (Newman et al. 1997; Müller et al. 1997) resulting from the tropospheric emission of chlorofluorocarbon compounds (CFCs, halones, and HFCs) (WMO 95); (ii) the global increase of tropospheric O3 (WMO 1995); (iii) the observed increase of tropospheric “greenhouse gases” such as CO2, CH4, N2O, and O3 (IPCC 1996); and (iv) the potential coupling between polar stratospheric ozone loss and increased greenhouse gas concentrations (Shindell et al. 1998).
To assess the significance of such changes a detailed understanding of the physical and chemical processes controlling the global atmosphere is required. Similarly knowledge about the variability and temporal behavior of atmospheric trace gases is necessary to test the predictive ability of the theories currently used to model the atmosphere. Consequently, the accurate assessment of the impact of current and future anthropogenic activity or natural phenomena on the behavior of the atmosphere needs detailed knowledge about the temporal and spatial behavior of several atmospheric trace constituents (gases, aerosol, clouds) on a global scale, including the troposphere.
Over the past two decades pioneering efforts have been made by the scientific community to establish both ground-based networks and satellite projects that will eventually result in an adequate global observing system. Examples of satellite borne elements of such programs are the Solar Backscatter Ultraviolet (SBUV) and Total Ozone Mapping Spectrometer (TOMS) on NASA’s Nimbus-7 satellite (Heath et al. 1975); the Stratospheric Aerosol and Gas Experiment (SAGE) (McCormick et al. 1979); the Upper Atmosphere Research Satellite (UARS) (Reber et al. 1993) with the Microwave Limb Sounder (MLS), the Halogen Occultation Experiment (HALOE), the Cryogenic Limb Array Etalon Spectrometer (CLAES), and the Improved Stratospheric and Mesospheric Sounder (ISAMS) instruments on board; and the Second European Remote Sensing satellite (ERS-2), which carries the Global Ozone Monitoring Experiment (GOME) (Burrows et al. 1999). In the near future, several new missions will be launched and will contribute significantly to research in the fields of atmospheric chemistry and physics: NASA’s Earth Observing System (EOS) satellites EOS-AM and EOS-CHEM, the Japanese Advanced Earth Observing System (ADEOS), and the European Space Agency’s (ESA) Environmental Satellite (ENVISAT).
The Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) is part of the atmospheric chemistry payload onboard ENVISAT being prepared by ESA. Following the call for earth observation instrumentation in the Announcement of Opportunity for the Polar Platform issued by ESA, the SCIAMACHY proposal (Burrows et al. 1988) was submitted to ESA by an international team of scientists led by Principal Investigator J. P. Burrows. After peer review SCIAMACHY was selected as part of the payload for the satellite now known as ENVISAT, which is planned to be launched in 2000.
The heritage of SCIAMACHY (Burrows et al. 1988) lies in both the ground-based measurements using Differential Optical Absorption Spectroscopy (DOAS) (Brewer et al. 1973; Platt and Perner 1980; Solomon et al. 1987) and previous satellite atmospheric remote sensing missions. SCIAMACHY combines and extends the measurement principles and observational modes of the nadir scattered sunlight measuring instruments SBUV and TOMS (Heath et al. 1975), the solar occultation instrument SAGE (McCormick et al. 1979; Mauldin et al. 1985), and the limb scattered sunlight measuring instrument Solar Mesospheric Explorer (SME) (Barth et al. 1983) within one instrument. SCIAMACHY measures in the wavelength range from 240 nm to 2380 nm the following:
The scattered and reflected spectral radiance in nadir and limb geometry,
the spectral radiance transmitted through the atmosphere in solar and lunar occultation geometry, and
the extraterrestrial solar irradiance and the lunar radiance.
SCIAMACHY and GOME, which is a small-scale version of SCIAMACHY (see Burrows et al. 1999 and references therein), represent a new generation of space-based remote sounding sensors, which rely on and utilize the simultaneous spectrally resolved measurement of light upwelling from the atmosphere to determine amounts of atmospheric constituents.
Using data from GOME, which was launched on board the European Remote Sensing satellite ERS-2 in April 1995, the feasibility of the instrument and retrieval concepts have been successfully demonstrated for nadir observations. The trace gases O3, NO2, BrO, OClO, SO2, and H2CO have been observed as predicted (Burrows et al. 1999), and studies of ClO, NO, and aerosol retrieval are proceeding. The determination of O3 profile information, including tropospheric O3, from GOME measurements (Burrows et al. 1999; Munro et al. 1998;Rozanov et al. 1998) has a large number of potential applications. In addition, the retrieval of tropospheric column information of SO2, H2CO, NO2, and BrO from GOME measurements was demonstrated (Burrows et al. 1999).
The goal of this paper is to provide a comprehensive overview of the SCIAMACHY mission and instrument, to summarize the retrieval strategies, to report on planned data products and expected data quality, and to demonstrate the range of applications and the potential that lies in the concept of this new generation of hyperspectral UV–VIS–NIR sensors. Section 2 provides details about the targeted constituents. In section 3 the instrument design and observational modes are presented. The proposed retrieval strategies are summarized in section 4. Section 5 focuses on the expected data precision and section 6 summarizes the current status of operational data products.
2. Scientific objectives and targeted constituents
The main objective of the SCIAMACHY mission is to improve our knowledge of global atmospheric change and related issues of importance to the chemistry and physics of our atmosphere (cf. WMO 1995 and IPCC 1996) such as
the impact of tropospheric pollution arising from industrial activity and biomass burning,
exchange processes between the stratosphere and troposphere,
stratospheric chemistry in the polar regions (e.g., under “ozone hole” conditions) and at midlatitudes, and
modulations of atmospheric composition resulting from natural phenomena such as volcanic eruptions, solar output variations (e.g., solar cycle), or solar proton events.
a. Tropospheric chemistry
SCIAMACHY will measure the backscattered sunlight that reaches the earth’s surface (λ ≥ 280 nm). The retrieval of tropospheric constituents is influenced and limited by clouds. SCIAMACHY is the only atmospheric chemistry sensor on ENVISAT capable of determining trace gases and aerosol abundances in the lower troposphere including the planetary boundary layer under cloud-free conditions. From the SCIAMACHY nadir and limb measurements tropospheric columns of O3, NO2, BrO, CO, CH4, H2O, N2O, SO2, and H2CO (cf. Fig. 2) will be retrieved. In addition, surface spectral reflectance, aerosol and cloud parameters (cover and cloud-top height), and the tropospheric flux from 280 to 2380 nm will be retrieved. These data are required for studies of the oxidizing capacity of the troposphere, photochemical O3 production and destruction, and tropospheric pollution (biomass burning, industrial activities, aircraft).
b. Stratosphere–troposphere exchange
For the investigation of stratosphere–troposphere exchange (Holton et al. 1995) SCIAMACHY measurements of the height-resolved profiles of the tracers O3, H2O, N2O, CH4, and aerosol will be of primary significance. These measurements enable investigations of the downward transport of stratospheric O3 and upward transport of important species (e.g., aerosol, CH4, H2O, and N2O). The CH4 and N2O molecules are emitted into the planetary boundary layer. Their long tropospheric lifetime results in being transported to the stratosphere, where they are the dominant source of the ozone-destroying HOx and NOx radicals. Studies of relatively small-scale features such as tropopause folding at midlatitudes require a high spatial resolution and are unlikely to be unambiguously observed by SCIAMACHY. However, larger-scale stratosphere–troposphere exchange as envisaged by Holton et al. (1995) will be readily observed.
In the neighborhood of the tropopause the different measurements modes of SCIAMACHY will have different vertical and horizontal resolutions. Solar and lunar occultation modes yield measurements with a vertical resolution of 2.5 km and a horizontal resolution of 30 km across track, determined by the solar diameter, and extending roughly 400 km along track. For the limb measurements the geometrical spatial resolution is approximately 3 km vertically and typically 240 km horizontally across track, determined by scan speed and integration time, and extending roughly 400 km along track (see Table 3). More details about the geometrical resolution of the different measurement modes will be given in section 3b.
c. Stratospheric chemistry and dynamics
The study of the stratospheric chemistry and dynamics will utilize the simultaneous retrieval of total columns from nadir measurements and vertical stratospheric profiles from limb and occultation measurements of O3, NO2, BrO, H2O, CO, CH4, and N2O (and OClO and possibly ClO under ozone hole conditions), as well as aerosol and stratospheric cloud information. Temperature and pressure profiles can be determined from limb and occultation observations of the well-mixed gases CO2 and O2 assuming local thermal equilibrium.
SCIAMACHY will be making measurements when halogen loading of the stratosphere maximizes around the turn of the century (WMO 1995). It has recently been pointed out by Hofmann (1996) that the springtime polar lower-stratospheric O3, specifically the layer from 12 to 20 km, will be the first region to show a response to the international control measures on chlorofluorocarbon compounds (CFCs) defined in the Montreal Protocol of 1987 and its Copenhagen and London amendments. SCIAMACHY will enable this preposition to be studied in detail.
In general, SCIAMACHY measurements will yield detailed information about the development of stratospheric O3 above the Arctic and Antarctica, the global stratospheric active halogen species (BrO, ClO, OClO), and the global O3 budget as a function of the height in the atmosphere. As SCIAMACHY measures simultaneously the backscattered radiation field and constituent profiles, an important objective is to test the accuracy of current stratospheric photochemical models and their predictive capability.
d. Mesospheric chemistry and dynamics
In the upper stratosphere and lower mesosphere SCIAMACHY measurements yield profiles of O3, H2O, N2O, NO, O2, and O2(1Δ). These measurements will be used to study the distribution of H2O and O3 and their global circulation. There has recently been much discussion of upper-stratospheric and mesospheric chemistry in the context of the “ozone deficit problem” (Crutzen at al. 1995; Summers et al. 1997). It has also been suggested that monitoring of H2O in the lower mesosphere may offer an opportunity for the early detection of climate change (Chandra et al. 1997). The O3 destruction by mesospheric and upper-stratospheric NO will be investigated. Finally, the mesospheric source of stratospheric NOx will be quantified.
In contrast to the retrieval of the majority of trace gases from SCIAMACHY data, NO and O2(1Δ) profiles are to be determined from their emission features rather than their absorptions. Satellite measurements of NO via the γ-band emission had been demonstrated by SME to determine profile information from the limb scan (Barth et al. 1983, 1988) and by SBUV to determine column amounts above 45 km from nadir measurements (McPeters 1989). NO can be detected above 40 km via the emission from the excited A2Σ+ state into the ground state X2Π1/2,3/2 (NO γ-band transitions, 200–300 nm) as determined in a model sensitivity study by Frederick and Abrams (1982). SCIAMACHY will be able to detect several bands in the 240–300-nm spectral region of the γ-band emissions of NO in limb as well as in nadir observation mode.
O2(1Δ) can be detected using its emission around 1.27 μm as shown by results from the SME (Thomas et al. 1984). The combination of height-resolved O3, O2(1Δ), and UV radiance products from SCIAMACHY provides detailed information about the photolysis of O3 in the upper stratosphere and mesosphere. This will provide an excellent opportunity to test our current photochemical knowledge of the mesosphere.
e. Climate research
For use in climate research, SCIAMACHY measurements will provide the distributions of several important greenhouse gases (O3, H2O, CH4, N2O, and CO2), aerosol and cloud data, surface spectral reflectance (280–2380 nm), the incoming solar spectral irradiance and the outgoing spectral radiance (240–2380 nm), and profiles of p and T (via O2 and CO2). As it is intended that SCIAMACHY observations are to be made for many years, this long-term dataset will provide much unique information useful for the study of the earth–atmosphere system and variations of the solar output and its impact on climate change. To reach continuity with other spectrometers measuring solar spectral irradiance such as SBUV or GOME, it is foreseen that SCIAMACHY will be calibrated with standard methods also applied to the GOME or SBUV calibration (Weber et al. 1998).
3. The instrument
Details of the instrument concept and design have been given by Burrows and Chance (1991), Goede et al. (1994), Burrows et al. (1995), and Mager et al. (1997). The design is summarized in the following subsections. Since the development of the design of SCIAMACHY two significant changes have occurred.
The original concept (Burrows and Chance 1991; Burrows et al. 1995) used an active Stirling cooler to maintain the infrared detectors of SCIAMACHY at their operational temperature of 150 K. During the development phase it was found that a passive cooler could be used for this purpose. This has the advantage of reducing the electrical power consumption and potentially extending the lifetime of the mission.
As an outcome of phase B studies an additional seventh polarization measurement device (PMD), measuring the 45° component of the incoming radiance, was added to the spectrometer, to improve the radiometric accuracy for the limb mode.
a. Design and performance
The SCIAMACHY instrument is a passive remote sensing moderate-resolution imaging spectrometer. It comprises a mirror system, a telescope, a spectrometer, and thermal and electronic subsystems. A schematic view of the light path within the instrument is depicted in Fig. 3. The incoming radiation enters the instrument via one of three ports.
For nadir measurements the radiation from the earth’s scene is directed by the nadir mirror into a telescope (off-axis parabolic mirror), which focuses the beam onto the entrance slit of the spectrometer.
For limb and solar/lunar occultation measurements the radiation is reflected by the limb (elevation) mirror to the nadir (azimuth) mirror and then into the telescope, which focuses the beam onto the entrance slit of the spectrometer.
For internal and subsolar calibration measurements the radiation of internal calibration light sources or the solar radiation is directed by the nadir mirror into the telescope.
The spectral and radiometric characteristics of the SCIAMACHY spectrometer are summarized in Table 1. The spectral resolution of the spectrometer varies between 0.24 and 1.48 nm depending on channel number (see Table 1). For DOAS retrieval (see section 4) a high spectral stability is required. The instrument is designed to have a spectral stability of 1/50 of a detector pixel, which requires a temperature stability of the spectrometer of better than 250 mK over one orbit in combination with dedicated calibration measurements. The second relevant retrieval strategy (see section 4), the Full Retrieval Method (FURM) based on optimal estimation (Rodgers 1976), requires in addition to high spectral stability a high radiometric accuracy of the SCIAMACHY measurements. Knowledge of the state of polarization of the incoming light and the polarization response of the instrument determines the radiometric accuracy of the radiance, irradiance, and higher-level data products. To achieve the required radiometric accuracy of 2%–4% (depending on the spectral region), dedicated on-ground and in-flight radiometric calibration measurements have to be performed in combination with measurements of the polarization properties of the atmosphere. For the latter purpose SCIAMACHY is equipped with seven polarization measurement devices. Six of these devices (PMD 1–6) measure light polarized perpendicular to the SCIAMACHY optical plane, generated by a Brewster angle reflection at the second face of the predispersing prism. This polarized beam is split into six different spectral bands, as described in Table 1. The spectral bands are quite broad and overlap with spectral regions of channels 2, 3, 4, 5, 6, and 8. The PMDs and the light path to the array detectors (including the detectors) have different polarization responses. Consequently, the appropriate combination of PMD data, array detector data, and on-ground polarization calibration data enables the polarization of the incoming light for the nadir measurements (Kruizinga et al. 1994;Frerick et al. 1997) to be determined. For atmospheric limb measurements, where both limb and nadir mirrors are used, the light is off the optical plane of the spectrometer. This requires the measurement of additional polarization information of the incoming light. A seventh PMD (PMD 7) will therefore measure the 45° component of the light extracted from the channels 3–6 light path, as depicted in Fig. 3. All PMDs are read out every 1/40 s and they observe the same atmospheric volume as channels 1–8. In addition to these PMD data being used for the determination of the polarization characteristics of the incoming light, they are also planned to be used to determine the fractional cloud cover of the observed ground scene.
Additional information about the polarization of the incoming light can be obtained from the diode array overlap regions 1/2 (309–314 nm), 2/3 (394–405 nm), 3/4 (604–620 nm), 4/5 (785–805 nm), and 5/6 (1000–1050 nm). The polarization efficiency is different for the measurements of the same wavelength in the different channels. Inversion of these measurements yields the ratio of plane to parallel polarization components of the incoming light in a manner similar to that used for the array and PMD detectors. The advantage of the overlap regions is that they are in small wavelength bands, having the same spectral resolution as the corresponding channel.
SCIAMACHY aims to retrieve trace gas amounts of relatively weak absorbers. For example, the differential optical density due to the BrO absorption around 350 nm detected with GOME (Burrows et al. 1999) is in the order of 10−3 and below. Therefore, to achieve a high retrieval precision, a high signal-to-noise ratio (S/N) is required for the scattered radiance as well as for the solar irradiance and lunar radiance from the UV to the NIR. The predicted instrumental S/N values as a function of wavelength are depicted in Fig. 4. These S/N values are calculated for an individual detector pixel, for example, of nadir, limb, and occultation measurements. In most cases the predicted S/N is well above 103. Exceptions are found in channels 1, 7, and 8. In channel 1 S/N decreases toward the UV primarily because the sun is weaker and ozone absorption increases strongly from 320 to 250 nm. In the IR channels 7 and 8 the lower S/N values arise from the higher noise of the InGaAs detectors. For these channels the S/N is limited by the detector noise. The apparent missing S/N in Fig. 4c for channel 1 is the result of the almost complete absorption of the solar photons by the ozone layer when observing the tangent height of 15 km. In general, higher S/N values can be obtained by averaging measurements either temporally or spectrally at the cost of losing temporal (and consequently spatial) or spectral resolution. This strategy enables the optimal set of radiance and irradiance data to be generated for a given inversion. Summation of succeeding measurements on board (so-called onboard co-adding) is to be used to match optimally the amount of down-linked data to the ENVISAT data rate allowed for SCIAMACHY. In order to cope with the large dynamic range of the input signals (limb scattered radiance vs solar irradiance), which is of six to eight orders of magnitude, the exposure time of each channel can be selected independently over a wide range of values from 0.03125 to 80 s. In addition, an arrangement involving an aperture stop and a neutral density filter is used to limit the intensity of the incoming light during solar occultation measurements. To optimize S/N over the orbit, exposure times are varied as a function of the solar zenith angle.
To calibrate the instrument in flight and to monitor the instrument performance, SCIAMACHY is equipped with a Pt/Cr/Ne hollow cathode (spectral calibration), a 5-W UV-optimized tungsten-halogen lamp with an equivalent blackbody temperature of approximately 3000 K (relative radiometric calibration), an extra reference mirror (monitoring of the mirror reflectivity), and a calibrated onboard diffuser (radiometric calibration via the sun).
b. Observational modes, spatialcoverage, and resolution
Measurements of SCIAMACHY are performed in three different viewing geometries: nadir, limb, and solar/lunar occultation. In nadir mode the atmospheric volume beneath the spacecraft is observed. The nadir mirror scans across the satellite track and each full scan covers an area on ground of approximately 30 km along track by 960 km across track. The spatial resolution in the nadir scan mode is determined by the combination of the scan speed and the integration time of the detectors. The scan speed along track is determined by the spacecraft speed of approximately 7 km s−1. The across-track speed is determined by the nadir scan mirror rate, resulting in a scan speed of approximately 240 km s−1 on ground. The effective spatial resolution varies along the orbit for the different data products, influenced by S/N requirements and by data rate limitations. From 60°N to 60°S the data rate is the limiting factor due to the high backscattered radiance. Onboard co-adding results in a typical spatial resolution in nadir of approximately 30 km along track by 240 km across track for all 8000 detector pixels (cf. Table 2). To minimize the loss of spatial information due to onboard co-adding, selected spectral windows, so-called clusters, can be processed on board with reduced co-adding. This will result in a higher nadir spatial resolution for important constituents such as O3, NO2, H2O, aerosols, and clouds of 30 km along track by 60 km across track, as shown in Fig. 5. The orbit dependence of the spatial resolution of selected clusters and the PMDs is shown in Fig. 5. As a consequence of their high sampling frequency, the PMDs yield measurements having the highest spatial resolution. For 50-ms effective temporal resolution of the PMD detectors and electronics, the PMD spatial resolution is approximately 30 × 15 km2 (along × across). As mentioned above, PMD data are used for polarization correction and cloud detection. The increase in the spatial resolution in the across-track direction from midlatitudes to equatorial regions results from a trade-off between the spatial resolution of the non-co-added spectral regions and that for the co-added spectral regions. This is because the high earthshine radiance in the equatorial regions allows the use of rather short exposure times for a given S/N value, thus generating more data than originally foreseen for SCIAMACHY by ENVISAT.
In limb mode the spectrometer slit is projected parallel to the horizon in-flight direction by the combination of the limb and the nadir mirror. The instrument observes the atmosphere tangentially to the earth’s surface taking into account effects such as the curvature of the earth’s surface. In this measurement mode the limb mirror scans the atmosphere in horizontal (azimuth) direction, whereas appropriate movement of the nadir mirror results in a vertical (elevation) scan direction. A typical limb scan cycle comprises 34 horizontal scans (azimuth) at different tangent heights (elevation), starting 3 km below the horizon. After each horizontal scan, which has a duration of 1.5 s, a step of 3 km in elevation is made by the elevation mirror. No measurements are to be made during this repositioning, which takes approximately 150 ms. Each horizontal scan of the atmosphere in limb covers 960 km in the horizontal (across-track direction). The vertical resolution is approximately 3 km. The latter is determined by the geometrical field of view (FOV), the pointing stability of the instrument and the stability of the satellite, and the multiple scattering of light in the atmosphere, which is dependent on the tangent height. The individual spatial resolution within a 960-km scan in the horizontal direction across track is typically 240 km, being determined by the integration time. The 960-km total swath and the 240-km scenes in limb viewing match explicitly the total swath and individual scenes observed in nadir. For limb observation the optical light path through the atmosphere limits the resolution in horizontal direction along track to approximately 400 km. Due to the spacecraft velocity the atmospheric volumes observed during successive horizontal scans are slightly displaced. Within one limb cycle with a typical duration of 60 s the spacecraft moves roughly 400 km in the along-track direction. This displacement has to be taken into account by the inversion algorithm.
The ability of the SCIAMACHY instrument to observe the same atmospheric volume first in limb and thereafter in nadir viewing geometries within about 7 min is perhaps one of its most important and novel features. This requires the synchronization of integration times, scan ranges, and viewing directions for all limb and nadir measurements. For the dedicated limb–nadir scanning strategy and a total swath width of 960-km global coverage at the equator is achieved after 6 days.
Occultation measurements are performed using the elevation and azimuth scan mirrors in a manner similar to that for the limb mode, but with the sun or moon in the instrument’s field of view. The instrument tracks the sun or the moon actively, resulting in a better pointing performance of the instrument. Solar occultation measurements are planned to be performed for each orbit during sunrise. For the ENVISAT orbit (cf. Table 3), solar occultation is restricted to latitudes between 65° and 90°N. Lunar occultation will be performed from half moon to full moon. For periods of 5–8 days per month lunar occultation measurements will provide latitudinal coverage from 30° to 90°S. The solar scanning strategy is similar to the SAGE II scanning (Mauldin et al. 1985): during sunrise SCIAMACHY scans several times over the full solar disc. When the sun is above the atmosphere several calibration measurements can be performed. However, for the lunar measurements longer integration times are necessary and a “stare” mode similar to that used on the Halogen Occultation Experiment (HALOE) (Russell et al. 1993) is implemented.
A typical orbit starts with a limb measurement of the twilit atmosphere, followed by the solar occultation measurement during sunrise over the North Pole and an optimized limb–nadir sequence. If the moon is visible in the Southern Hemisphere (30°–90°S), lunar occultation measurements every second orbit are performed. Note that all these measurements, with the exception of lunar occultation, are done every orbit, that is, 14 orbits per day.
In addition to the aforementioned measurements a number of calibration measurements will be performed on a regular basis (Frerick et al. 1997). These include
solar irradiance measurements,
spectral calibration,
relative radiometric calibration, and
dark current measurements.
4. Measurement principle and retrieval methods
In order to determine the amounts of trace constituents from SCIAMACHY’s measurements a number of different retrieval techniques are to be applied. For trace gas retrieval two basic approaches are recommended.
a. DOAS
SCIAMACHY uses the retrieval technique known as Differential Optical Absorption Spectroscopy. The basic idea behind the use of DOAS to detect atmospheric constituents can be traced back to Brewer et al. (1973), who determined NO2 concentrations from the measurements of zenith sky scattered light and transmitted solar light. Platt and Perner (1980) used DOAS for long path measurements of tropospheric gases. The method has been further improved for zenith sky absorption measurements of tropospheric and stratospheric constituents from the ground (Solomon et al. 1987; Richter et al. 1998). Recently the application of DOAS to the retrieval of trace gases from spaceborne spectral measurements has been successfully demonstrated for the first time by GOME for O3, NO2, BrO, OClO, SO2, and H2CO (Burrows et al. 1999 and references therein). In the following the basic idea of DOAS for spaceborne spectral measurements will be described and in the appendix a derivation of the DOAS equation for two simple cases is presented. A detailed overview about the algorithm is out of the scope of this paper. An example of an operational implementation of the satellite DOAS method is given in Burrows et al. (1999).
Modified DOAS was first used for the interpretation of ground-based zenith sky observations (Richter et al. 1998). An application of modified DOAS to O3 retrieval from GOME satellite data is presented in Burrows et al. (1999). In the appendix some more details about the physical basics are presented in the form of two instructive examples for the nadir geometry for multiple absorbers: (i) light scattered at the earth’s surface only and (ii) light singly scattered in the atmosphere.
b. FURM full retrieval method
The FURM algorithm consists of two parts: (i) a forward model and (ii) an inversion scheme. SCIATRAN, a significant extension of the radiation transfer model GOMETRAN (Rozanov et al. 1997; Rozanov et al. 1998), will be used as the forward RTM to calculate for the relevant geometry the radiance for a given state of the atmosphere. The optimal estimation scheme (Rodgers 1976) is used to invert the data. It matches in iterative steps the calculated radiance to the measured radiance by modifying model atmospheric parameters such as the vertical distribution of O3 using appropriate weighting functions provided by the radiative transfer model (Rozanov et al. 1998). The successful retrieval of O3 profile information with FURM from GOME nadir measurements has already been demonstrated (De Beek et al. 1997; Munro et al. 1998). Details about FURM are presented in Burrows et al. (1999). Currently, limb and occultation retrieval algorithms, based on the forward model SCIATRAN and an inversion approach, are being developed at the University of Bremen.
c. Retrieval of tropospheric column information
For the inversion from radiance and irradiance to tropospheric column information two methods are to be used.
1) The residual approach
For this method the difference between a total column determined from nadir observations and a stratospheric column derived from limb (or occultation) measurements yields tropospheric column densities. This approach is known for tropospheric O3 retrieval as the tropospheric residual method (Fishman et al. 1990). The feasibility of this method has been investigated for tropospheric O3 by subtracting SAGE stratospheric O3 columns from TOMS total O3 columns (Fishman et al. 1990) or by subtracting SBUV stratospheric O3 columns from TOMS total O3 columns (Fishman et al. 1996). In the past one of the main limitations of the method has been the inadequate spatial and temporal overlap between the measurement pair. SCIAMACHY overcomes this problem by using a dedicated limb–nadir observation strategy, described in section 3, where subsequent limb and nadir measurements of the same air volume are made within 7 min of one another. Using SCIAMACHY data, the residual method is to be applied to tropospheric column retrieval of O3, NO2, BrO, SO2, H2O, H2CO, N2O, CO, CH4, and aerosol (cf. Fig. 2).
2) Tropospheric data fromnadir measurements
For the special case of O3, and possibly also for H2O and aerosol, it is feasible to derive tropospheric distributions from the analysis of saturated and unsaturated atmospheric absorptions (Chance et al. 1996; Munro et al. 1998; Burrows et al. 1999 and references therein). The strong variation of the optical thickness with height and wavelength enables the retrieval of height-resolved information with a vertical resolution of 6–10 km, depending on the altitude. This approach was first proposed by Singer and Wentworth (1957) and has been successfully applied to data from SBUV instruments (Bhartia et al. 1996) to derive vertical stratospheric O3 profiles from nadir measurements. In addition, it has been pointed out that the simultaneous observation of the T-dependent Huggins bands and the T-independent Chappuis bands provides additional information about tropospheric O3 from nadir measurements (Chance et al. 1996). Auxiliary information about temperature and pressure profiles can be determined from retrievals of the well-mixed gases CO2 and O2. Explicitly, temperature will be retrieved from the Boltzmann distribution of the vibrational–rotational structure of molecular absorption bands. Their number density will be determined from the T-invariant effective oscillator strength.
5. Precision estimates of SCIAMACHY tracegas measurements
From the inception of the SCIAMACHY project sensitivity studies have been performed by members of the original SCIAMACHY science team (Burrows et al. 1988; Chance et al. 1991) and later the Science Advisory Group and their institutes (Burrows et al. 1992; Burrows and Chance 1992; Burrows et al. 1994; Guyenne and Readings 1993; Rozanov et al. 1992; Schrijver et al. 1995). In the aforementioned studies, with the exception of Burrows et al. (1992) (see section 5a), noise and instrument performance was estimated using different variants of a SCIAMACHY instrument model. The radiation entering SCIAMACHY was simulated using a radiative transfer model coupled with a line by line model of the atmospheric absorptions where necessary. In this section estimates of the expected precision of SCIAMACHY data are summarized with respect to the current instrument performance. Table 4 provides an updated summary of the estimated precision of trace gas absorption and emission measurements in the different measurement modes, combining the results from the different sensitivity studies. The estimation of the absorption or emission precision requires the inversion of the precision with a forward radiative transfer model describing the viewing geometry. The simplest RTM is for occultation measurements. The estimated precision for the limb and occultation retrievals depends strongly on the tangent height. This is because the concentration of gases, but not necessarily their mixing ratio, usually falls off with altitude and because for the limb measurements the signal is dependent on the amount of scattered light, which depends on the pressure at a given tangent height. The retrieval precisions for occultation measurements in the UV–VIS as well as nadir retrieval precision in the NIR will be discussed in more detail below.
a. Retrieval precisions for solar and lunar occultation measurements
For SCIAMACHY occultation measurements Burrows et al. (1992) reported on the precision to be expected for the “minor” trace gases BrO, OClO, and NO3 (Table 4). Here the results of the retrieval precision of O3 and NO2 from solar and lunar occultation measurements under background stratospheric aerosol condition are summarized and compared to retrieval precisions of SAGE II (Chu et al. 1989) and OZON-M (Timofeyev et al. 1986). Using a radiative transfer model for occultation, appropriate assumptions about the instrument error, and an optimal estimation retrieval scheme (Timofeyev et al. 1986) the precision of O3 and NO2 for a typical midlatitude scenario at the spring equinox was investigated. Figure 6 shows the retrieval precision for both trace gases as a function of altitude. For O3 retrieval both the Hartley–Huggins and the Chappuis spectral bands were used. The instrument noise was assumed to be constant with altitude. The total error combines instrument and shot noise. For solar occultation the combined noise to signal ratio is in the order of 10−4 (cf. S/N in Fig. 4c) resulting in a theoretical retrieval precision on the absorption measurements of better than 0.1% between 15 and 60 km (cf. Fig. 6a). Below 22 km the retrieval precision decreases with decreasing altitude due to the presence of the stratospheric aerosol layer, which scatters light and results in fewer photons reaching the instrument. For the SCIAMACHY lunar occultation measurements significantly lower light fluxes will be detected and the combined noise to signal ratio is larger, being approximately 10−3. As a consequence the retrieval precision for absorption is less than 0.3% from 15 to 60 km. In Table 4 typical theoretical retrieval precisions are included (cf. Fig. 6).
Figure 6 also includes a precision estimate for the instruments SAGE II and OZON-M. SAGE II uses seven spectral bands between 385 nm and 1.02 μm (Mauldin et al. 1985). The Russian OZON-M experiment measures solar occultation by continuously scanning in four spectral intervals: 260–300 nm, 360–420 nm, 600–690 nm, and 910–1040 nm at a spectral resolutions of 0.2, 0.3, 0.5, and 0.8 nm, respectively. Estimated retrieval precisions were simulated by the same method as for SCIAMACHY and are shown in Fig. 6. The estimated SAGE II precision is in reasonable agreement with estimates of Chu et al. (1989), who predicted a precision of about 10% between cloud-top height and 60 km.
The theoretical simulations indicate that the SCIAMACHY precision for ozone absorption measurements is potentially very high. There are two main reasons for this predicted high precision: (i) the simultaneous observation of Hartley–Huggins and Chappuis absorption and (ii) the intrinsically low instrumental noise of the diode array detectors.
However, in practice, the precision is expected to be limited by a variety of effects: (i) the knowledge of the molecular absorption cross sections, (ii) the pointing error, and (iii) the conversion of a slant column absorption or emission to a tangent height or pressure profile (concentration or mixing ratio).
For the NO2 retrieval precision, a behavior similar to that for O3 is obtained and is shown in Fig. 6b. In lunar occultation a retrieval precision of better than 4% between 15 and 45 km can be achieved, whereas for the solar occultation this precision is well below 1%. The reduction in the precision above 45 km is mainly due to the lower abundance of NO2 in this region. In comparison to SAGE II and OZON-M, the precision of NO2 from SCIAMACHY is predicted to be about a factor of more than 4 better.
It is interesting to point out here that molecular scattering can also be determined from SCIAMACHY’s occultation measurements with a high precision and yields estimates of the molecular number density, which can directly be inverted to yield pressure profiles.
b. Nadir CO, CH4, H2O, N2O precisions
One important goal of SCIAMACHY is to retrieve CO and CH4 (beside other absorbers like O3, NO2, H2O, N2O, and CO2) down to the ground or cloud top. The retrieval precisions of CO, CH4, N2O, and H2O in the NIR channel 8 (2265–2380 nm) from nadir measurements are discussed below. This report updates, using the current instrument and detector performance, the findings of previous studies (Schrijver et al. 1995). For this study the radiance reaching the SCIAMACHY entrance slit was simulated by using the radiative transfer model FASCODE/MODTRAN (Clough et al. 1981). The instrument response including detector noise was calculated with the SCIAMACHY instrument simulator software (Slijkhuis 1994) for an integration time of 1 s. The computed noise was used to estimate uncertainties in the determination of concentrations of the relevant trace gases using the Cramer–Rao formalism (Kendall and Stuart 1979). The relative and absolute 1σ precisions were calculated as a function of albedo, season, and latitude for a single SCIAMACHY observation. Figure 7 summarizes the outcome of these simulations as a function of albedo. The origin of the difference between the 50°N and 40°S scenario is the higher CO total column in the Northern Hemisphere (for details see Schrijver et al. 1995). The estimated retrieval precision of CO varies little between the different scenarios when expressed in absolute amounts: the standard deviation σ is mostly of the order of 1017 mol cm−2. Variations in the relative precisions of CO are determined predominately by the variation of the CO concentrations around the globe. For the Northern Hemisphere the retrieval precision of CO varies in the range 3%–10%, whereas for the Southern Hemisphere for a clean remote troposphere it can be higher than 20%. For CH4, studies yield minimum detectable columns of the order of 5 × 1016 mol cm−2 resulting in a precision of the order of 1% for a single measurement. Variations with latitude are much less pronounced for CH4 because of the smoother variation of the model column with latitude.
In addition, Schrijver et al. (1995) studied the retrieval precision of N2O and H2O. N2O can be measured with a precision of 3%–10%, nearly independent of latitude, and the H2O precision lies well below the 1% limit. As previously reported by Ashcroft and Morel (1995), there is a strong correlation between H2O and CH4 in the 2.3-μm wavelength region, which may make it necessary to retrieve H2O from an independent spectral window where no CH4 structures appears. Nevertheless it is expected that the retrieved amounts of CH4 from the relatively high spectral resolution of 0.26 nm around 2.3 μm in channel 8 of SCIAMACHY measurements have insignificant interference from the H2O absorptions.
6. Operational data products
An operational data product is defined for SCIAMACHY and ENVISAT purposes as being a regularly generated, quality controlled, and archived data product. Currently it is foreseen that all limb and nadir measurement segments will be processed operationally within the ENVISAT ground segment. Scientific products for process studies from lunar and solar occultation measurements as well as special tropospheric products are planned to be generated for the atmospheric research and related communities by specific scientific groups.
SCIAMACHY operational data products are divided into two categories: the fast delivery (FD) products and the offline (OL) products. The FD products are to be generated directly after downlink at the ESA ground stations in near–real time, a few hours after the spectrum is acquired. The OL products comprise a set of consolidated and fully calibrated products. The latter are to be processed offline at the German Processing and Archiving Center as part of the ENVISAT ground segment.
SCIAMACHY operational data products are defined in several levels. The level 1 product results from the application of calibration parameters, orbit, and attitude information to the raw data and yields geolocated and calibrated spectral radiance and irradiance. These data comprise the extraterrestrial solar irradiance and the spectral radiance of scattered solar radiation modified by absorption on its path through the earth’s atmosphere. From these level 1 products the profiles and column densities of various atmospheric constituents are retrieved to yield geolocated, calibrated concentrations of trace gases, as well as aerosol and cloud information (level 2 products). At present the level 2 FD products (third row in Table 5) are limited to nadir observations. However, they may be extended to include limb products. These products are to be retrieved with the well-established and computationally effective DOAS algorithm. These are essentially the algorithms developed for GOME but are extended to include windows in the SCIAMACHY IR regions. It is foreseen that the level 2 OL processor uses either DOAS or FURM in nadir, depending on the retrieved constituents. It is proposed that level 2 products for limb observations will use a technique similar to FURM. The level 2 OL products serve as a basis for all higher level and/or value added products, for example, interpolated and temporally averaged data and multisensor products based on SCIAMACHY and other ENVISAT instruments.
To ensure high data quality over the mission lifetime, instrument performance will be monitored continuously and an intensive validation campaign will be performed at the beginning of the mission. Additionally, a longer-term validation by regularly performed balloon, aircraft, and satellite measurements over the whole mission lifetime is planned.
7. Conclusions
SCIAMACHY is a novel atmospheric remote sounding instrument, which will be launched as part of the ESA ENVISAT platform in 2000. SCIAMACHY will measure the extraterrestrial solar irradiance and the earthshine radiance in a variety of viewing geometries:(i) alternate nadir and limb viewing and (ii) solar and lunar occultation.
The inversion of SCIAMACHY measurements yields
the amounts and distributions of trace gases (O3, O2, O2(1Δ), O4, BrO, OClO, ClO, SO2, H2CO, NO, NO2, NO3, CO, CO2, CH4, H2O, N2O) and aerosol,
pressure and temperature profiles,
cloud parameters (cloud cover, cloud-top height), and
surface spectral reflectance.
SCIAMACHY is intended to provide new insight into the global behavior of the troposphere and the stratosphere and is a good candidate instrument for any future global monitoring system. Its measurements and the data products retrieved from them are to be used for the study of a wide range of applications such as the impact of pollution, the response of the stratospheric chemistry to control strategies, and the assessment of global change.
Acknowledgments
SCIAMACHY is a national contribution to the ENVISAT mission funded by the German Aerospace Center (DLR) and the Netherlands Agency for Aerospace Programs (NIVR), including a contribution of the Belgian Institute for Space Aeronomy (BIRA-IASB). The SCIAMACHY industrial consortium comprises the prime contractors Dornier Satellite Systems (Germany) and Fokker Space (the Netherlands) and the subcontractors OHB-Systems (Germany), Jenoptik (Germany), Space Research Organisation Netherlands SRON (the Netherlands), TPD-TNO (the Netherlands), OIP (Belgium), and Epitaxx, Inc. (United States). The instrument and algorithm development is supported by the activities of the SCIAMACHY Science Advisory Group (SSAG), a team of scientists from various international institutions: University of Bremen (Germany), SRON (the Netherlands), Smithonian Astrophysical Observatory SAO (United States), BIRA-IASB (Belgium), Max-Planck Institute for Chemistry in Mainz (Germany), KNMI (the Netherlands), University of Heidelberg (Germany), Institute for the study of Geophysical and Environmental Methodologies IMGA (Italy), and the Centre National pour la Recherche Scientifique CNRS-LPMA (France). Operational data processing is to be performed by the German Remote Sensing Data Center (DLR-DFD) within the ENVISAT ground segment. The mission operation is supported by the SCIAMACHY Operations Support Team SOST, which prepares mission scenarios, measurement strategies, and instrument calibration activities. The authors would like to thank H. Schrijver and I. Aben (SRON) for the provision of an update of the CO/CH4 sensitivity study (Schrijver et al. 1995) prior to publication. The authors would also like to thank two anonymous reviewers for their valuable comments. This work has been funded as part of the SCIAMACHY Scientific Support Study by the German Ministry of Education and Research (BMBF) under 50EP9207 and the University of Bremen.
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APPENDIX
The Satellite-Modified DOAS Equation forTwo Instructive Cases
For an understanding of the DOAS approach applied to nadir viewing geometry it is useful to consider the different rays reaching the instrument. To outline the physical principle behind DOAS two cases will be calculated explicitly in this appendix: (a) a single ray only reflected at the earth’s surface and (b) rays of light scattered only once in the atmosphere, neglecting surface reflection. For simplicity an infinitely narrow instrument FOV is assumed and emission processes are neglected.
Calculation of SODmodi
Wavelength range covered by SCIAMACHY and absorption windows of the targeted constituents.
Citation: Journal of the Atmospheric Sciences 56, 2; 10.1175/1520-0469(1999)056<0127:SMOAMM>2.0.CO;2
Altitude ranges of atmospheric constituents targeted by SCIAMACHY. Retrieval from the occultation measurements yields information over a wider altitude range than the limb measurements, due to its higher S/N ratio.
Citation: Journal of the Atmospheric Sciences 56, 2; 10.1175/1520-0469(1999)056<0127:SMOAMM>2.0.CO;2
Schematic view of the SCIAMACHY optical layout. All imaging optical components (mirrors, redirecting prisms, lenses, etc.) are omitted. All used gratings are in a fixed position. Each detector contains a 1024-pixel photo diode array.
Citation: Journal of the Atmospheric Sciences 56, 2; 10.1175/1520-0469(1999)056<0127:SMOAMM>2.0.CO;2
Typical signal-to-noise (S/N) ratio as seen by SCIAMACHY simulated with MODTRAN 3 (Anderson et al. 1994) and the SCIAMACHY instrument simulator software (Dobber 1996) for (a) nadir measurement, albedo = 0.3, 64° solar zenith angle, integration times 1 s (channels 1, 2a, 8), 0.5 s (channel 7), 0.25 (channel 2b), 0.125 s (channels 3–6); (b) limb measurement, 15 km tangent height, albedo = 0.3, 67° solar zenith angle, integration times 1.5 s (channel 1a), 0.375 s (channels 1b, 2, 5–8), 0.1875 s (channels 3 and 4); and (c) solar occultation measurement, 15 km tangent height, integration times 0.03125 s for channels for channels 2 and 6, 0.0625 s for all other channels. For occultation no signal is expected in channel 1 because of the strong O3 absorption at this tangent height.
Citation: Journal of the Atmospheric Sciences 56, 2; 10.1175/1520-0469(1999)056<0127:SMOAMM>2.0.CO;2
(Continued) Other parameters identical for all simulations: spring season, observed atmospheric volume 55°N, U.S. Standard Atmosphere (Anderson et al. 1994), background stratospheric aerosol, no clouds. The jump in S/N ratio around 1650 nm (channel 6) results from the different detector material used below and above 1650 nm.
Citation: Journal of the Atmospheric Sciences 56, 2; 10.1175/1520-0469(1999)056<0127:SMOAMM>2.0.CO;2
Nadir pixel size across/along track for different spectral regions (clusters) as function of geographic latitude at the time of s/c equinox (spring/fall). In addition to O3, the interval from 425 to 530 nm also contains the spectral window for NO2 retrieval (cf. Fig. 1). Also shown is the spatial resolution of the broadband PMDs, which is independent of geographic latitude.
Citation: Journal of the Atmospheric Sciences 56, 2; 10.1175/1520-0469(1999)056<0127:SMOAMM>2.0.CO;2
Theoretical precision estimates for O3 and NO2 from occultation measurements under background stratospheric aerosol conditions and comparison with SAGE II and OZON-M. SCIAMACHY lunar occultation instrumental error: straight line; SCIAMACHY solar occultation instrumental error: dotted line; SAGE II instrumental error: dashed line; OZON-M instrumental error: dashed–dotted line. (a) O3 retrieval precision and (b) NO2 retrieval precision. All instrument errors are assumed to be independent of height. Pointing errors and errors in the absorption cross section are neglected.
Citation: Journal of the Atmospheric Sciences 56, 2; 10.1175/1520-0469(1999)056<0127:SMOAMM>2.0.CO;2
Theoretical retrieval precision of tropospheric CO and CH4 concentrations for four combinations of month and latitude as a function of albedo for 1-s integration time in channel 8 (2265–2380 nm): (a) precision in percent and (b) precision in column density (mol cm−2).
Citation: Journal of the Atmospheric Sciences 56, 2; 10.1175/1520-0469(1999)056<0127:SMOAMM>2.0.CO;2
Optical parameters of the spectrometer from the design analysis.
Spatial coverage and resolution of SCIAMACHY measurement modes. In solar occultation mode the spatial resolution in azimuth direction is defined by the diameter of the sun.
Summary of technical instrument data and parameters of the ENVISAT sun-synchronous polar orbit.
Summary of theoretical precision estimates of targeted trace gases. Note that for limb profiles the precision depends strongly on altitude. A question mark (?) indicates feasibility study necessary to determine retrieval precision.
SCIAMACHY level 2 operational data products. Fast delivery (FD) products will be available within a few hours of spectrum acquisition. FD processing will employ lookup tables for radiative transfer rather than operational running of the models. Offline (OL) data products will be produced using improved ancillary data that become available after spectrum acquisition, for example, analyzed temperature and pressure fields.