Articles de revues : « SCIAMACHY measurements »

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Schlundt, C., A. A. Kokhanovsky, W. von Hoyningen-Huene, T. Dinter, L. Istomina et J. P. Burrows. « Synergetic cloud fraction determination for SCIAMACHY using MERIS ». Atmospheric Measurement Techniques Discussions 3, n^o 4 (19 août 2010) : 3601–42. http://dx.doi.org/10.5194/amtd-3-3601-2010.

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Abstract. Since clouds play an essential role in the Earth's climate system, it is important to understand the cloud characteristics as well as their distribution on a global scale using satellite observations. The main scientific objective of SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) onboard the ENVISAT satellite is the retrieval of vertical columns of trace gases. On the one hand, SCIAMACHY has to be sensitive to low variations in trace gas concentrations which means the ground pixel size has to be large enough. On the other hand, such a large pixel size leads to the problem that SCIAMACHY spectra are often contaminated by clouds. SCIAMACHY spectral measurements are not well suitable to derive a reliable sub-pixel cloud fraction that can be used as input parameter for subsequent retrievals of cloud properties or vertical trace gas columns. Therefore, we use MERIS/ENVISAT spectral measurements with its high spatial resolution as sub-pixel information for the determination of MerIs Cloud fRation fOr Sciamachy (MICROS). Since MERIS covers an even broader swath width than SCIAMACHY, no problems in spatial and temporal collocation of measurements occur. This enables the derivation of a SCIAMACHY cloud fraction with an accuracy much higher as compared with other current cloud fractions that are based on SCIAMACHY's PMD (Polarization Measurement Device) data. We present our new developed MICROS algorithm, based on the threshold approach, as well as a qualitative validation of our results with MERIS satellite images for different locations, especially with respect to bright surfaces such as snow/ice and sands. In addition, the SCIAMACHY cloud fractions derived from MICROS are intercompared with other current SCIAMACHY cloud fractions based on different approaches demonstrating a considerable improvement regarding geometric cloud fraction determination using the MICROS algorithm.

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Schlundt, C., A. A. Kokhanovsky, W. von Hoyningen-Huene, T. Dinter, L. Istomina et J. P. Burrows. « Synergetic cloud fraction determination for SCIAMACHY using MERIS ». Atmospheric Measurement Techniques 4, n^o 2 (22 février 2011) : 319–37. http://dx.doi.org/10.5194/amt-4-319-2011.

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Abstract. Since clouds play an essential role in the Earth's climate system, it is important to understand the cloud characteristics as well as their distribution on a global scale using satellite observations. The main scientific objective of SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) onboard the ENVISAT satellite is the retrieval of vertical columns of trace gases. On the one hand, SCIAMACHY has to be sensitive to low variations in trace gas concentrations which means the ground pixel size has to be large enough. On the other hand, such a large pixel size leads to the problem that SCIAMACHY spectra are often contaminated by clouds. SCIAMACHY spectral measurements are not well suitable to derive a reliable sub-pixel cloud fraction that can be used as input parameter for subsequent retrievals of cloud properties or vertical trace gas columns. Therefore, we use MERIS/ENVISAT spectral measurements with its high spatial resolution as sub-pixel information for the determination of MerIs Cloud fRation fOr Sciamachy (MICROS). Since MERIS covers an even broader swath width than SCIAMACHY, no problems in spatial and temporal collocation of measurements occur. This enables the derivation of a SCIAMACHY cloud fraction with an accuracy much higher as compared with other current cloud fractions that are based on SCIAMACHY's PMD (Polarization Measurement Device) data. We present our new developed MICROS algorithm, based on the threshold approach, as well as a qualitative validation of our results with MERIS satellite images for different locations, especially with respect to bright surfaces such as snow/ice and sands. In addition, the SCIAMACHY cloud fractions derived from MICROS are intercompared with other current SCIAMACHY cloud fractions based on different approaches demonstrating a considerable improvement regarding geometric cloud fraction determination using the MICROS algorithm.

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Noël, S., M. Buchwitz et J. P. Burrows. « First retrieval of global water vapour column amounts from SCIAMACHY measurements ». Atmospheric Chemistry and Physics Discussions 3, n^o 6 (11 novembre 2003) : 5659–88. http://dx.doi.org/10.5194/acpd-3-5659-2003.

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Abstract. Global water vapour column amounts have been derived for the first time from measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on the European environmental satellite ENVISAT. For this purpose, two different existing retrieval algorithms have been adapted, namely the Air Mass Corrected Differential Absorption Spectroscopy (AMC-DOAS) which was originally designed for GOME and the Weighting Function Modified Differential Absorption Spectroscopy (WFM-DOAS) which was mainly designed for the retrieval of CH4, CO2 and CO from SCIAMACHY near-infrared spectra. Here, both methods have been applied to SCIAMACHY's nadir measurements in the near-visible spectral region around 700 nm. The results of these two methods agree within a scatter of ±0.5 g/cm2 with corresponding SSM/I and ECMWF water vapour data. This deviation includes contributions from the temporal and spatial variability of water vapour. In fact, the mean deviation between the SCIAMACHY and the correlative data sets is much smaller: the SCIAMACHY total water vapour columns are typically about 0.2 g/cm2 lower than the SSM/I values and less than 0.1 g/cm2 lower than corresponding ECMWF data. The SCIAMACHY water vapour results agree well with correlative data not only over ocean but also over land, thus showing the capability of SCIAMACHY to derive water vapour concentrations on the global scale.

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Noël, S., M. Buchwitz et J. P. Burrows. « First retrieval of global water vapour column amounts from SCIAMACHY measurements ». Atmospheric Chemistry and Physics 4, n^o 1 (27 janvier 2004) : 111–25. http://dx.doi.org/10.5194/acp-4-111-2004.

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Abstract. Global water vapour column amounts have been derived for the first time from measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on the European environmental satellite ENVISAT. For this purpose, two different existing retrieval algorithms have been adapted, namely the Air Mass Corrected Differential Absorption Spectroscopy (AMC-DOAS) which was originally designed for GOME and the Weighting Function Modified Differential Absorption Spectroscopy (WFM-DOAS) which was mainly designed for the retrieval of CH4, CO2 and CO from SCIAMACHY near-infrared spectra. Here, both methods have been applied to SCIAMACHY's nadir measurements in the near-visible spectral region around 700 nm. Taking into account a systematic offset of 10%, the results of these two methods agree within a scatter of about ±0.5 g/cm2 with corresponding SSM/I and ECMWF water vapour data. This deviation includes contributions from the temporal and spatial variability of water vapour. In fact, the mean deviation between the SCIAMACHY and the correlative data sets is much smaller: the SCIAMACHY total water vapour columns are typically about 0.15 g/cm2 lower than the SSM/I values and less than 0.1 g/cm2 lower than corresponding ECMWF data. The SCIAMACHY water vapour results agree well with correlative data not only over ocean but also over land, thus showing the capability of SCIAMACHY to derive water vapour concentrations on the global scale.

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Krijger, J. M., P. Tol, L. G. Istomina, C. Schlundt, H. Schrijver et I. Aben. « Improved identification of clouds and ice/snow covered surfaces in SCIAMACHY observations ». Atmospheric Measurement Techniques Discussions 4, n^o 1 (22 février 2011) : 1113–38. http://dx.doi.org/10.5194/amtd-4-1113-2011.

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Abstract. An improved version is presented of the SCIAMACHY PMD Identification of Clouds and Ice/snow method (SPICI). SPICI uses the SCIAMACHY measurements in the wavelength range between 450 nm and 1.6 μm to make a distinction between clouds and ice/snow covered surfaces, specifically developed to identify cloud-free SCIAMACHY observations. For this purpose the SCIAMACHY Polarisation Measurement Devices (PMDs) are used because they provide higher spatial resolution compared to the main spectrometer measurements. The improvements (compared to Krijger et al., 2005) include a snow over vegetation detection and correction for SCIAMACHY PMD degradation.

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Bramstedt, Klaus, Thomas C. Stone, Manfred Gottwald, Stefan Noël, Heinrich Bovensmann et John P. Burrows. « Improved pointing information for SCIAMACHY from in-flight measurements of the viewing directions towards sun and moon ». Atmospheric Measurement Techniques 10, n^o 7 (5 juillet 2017) : 2413–23. http://dx.doi.org/10.5194/amt-10-2413-2017.

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Abstract. The SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on Envisat (2002–2012) performed nadir, limb, solar/lunar occultation and various monitoring measurements. The pointing information of the instrument is determined by the attitude information of the Envisat platform with its star trackers together with the encoder readouts of both the azimuth and the elevation scanner of SCIAMACHY. In this work, we present additional sources of attitude information from the SCIAMACHY measurements itself. The basic principle is the same as used by the star tracker: we measure the viewing direction towards celestial objects, i.e. sun and moon, to detect possible mispointings. In sun over limb port observations, we utilise the vertical scans over the solar disk. In horizontal direction, SCIAMACHY's sun follower device (SFD) is used to adjust the viewing direction. Moon over limb port measurements use for both the vertical and the horizontal direction the adjustment by the SFD. The viewing direction is steered towards the intensity centroid of the illuminated part of the lunar disk. We use reference images from the USGS Robotic Lunar Observatory (ROLO) to take into account the inhomogeneous surface and the variations by lunar libration and phase to parameterise the location of the intensity centroid from the observation geometry. Solar observations through SCIAMACHY's so-called sub-solar port (with a viewing direction closely to zenith) also use the SFD in the vertical direction. In the horizontal direction the geometry of the port defines the viewing direction. Using these three type of measurements, we fit improved mispointing parameters by minimising the pointing offsets in elevation and azimuth. The geolocation of all retrieved products will benefit from this; the tangent heights are especially improved. The altitudes assigned to SCIAMACHY's solar occultation measurements are changed in the range of −130 to −330 m, the lunar occultation measurements are changed in the range of 0 to +130 m and the limb measurements are changed in the range of −50 to +60 m (depending on season, altitude and azimuth angle). The horizontal location of the tangent point is changed by about 5 km for all measurements. These updates are implemented in version 9 of the SCIAMACHY Level 1b products and Level 2 version 7 (based on L1b version 9).

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Chehade, W., V. Gorshelev, A. Serdyuchenko, J. P. Burrows et M. Weber. « Revised temperature dependent ozone absorption cross section spectra (Bogumil et al.) measured with the sciamachy satellite spectrometer ». Atmospheric Measurement Techniques Discussions 6, n^o 2 (8 mars 2013) : 2449–81. http://dx.doi.org/10.5194/amtd-6-2449-2013.

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Abstract. Ozone absorption cross section spectra and other trace gases had been measured using the Scanning Imaging Absorption spectroMeter for Atmospheric ChartograpHY (SCIAMACHY) satellite instrument at relevant atmospheric conditions. The measured cross sections were relative cross sections and were converted to absolute values using published data. Using the SCIAMACHY's FM cross sections as published by Bogumil et al. (2003) in the SCIAMACHY retrievals of total ozone leads to an overestimation in the total ozone by 5% compared to collocated GOME data. This work presents the procedures followed to correct the ozone cross section data as published in Bogumil et al. (2003) starting from original raw data (optical density spectra) from the original measurements. The revised data agrees well within 3% with other published ozone cross-sections and preserves the correct temperature dependence in the Hartley, Huggins, Chappuis and Wolf bands. SCIAMACHY's total ozone columns retrieved using the revised cross section data are shown to be within 1% compared to the ozone amounts retrieved routinely from SCIAMACHY.

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de Laat, A. T. J., A. M. S. Gloudemans, H. Schrijver, I. Aben, Y. Nagahama, K. Suzuki, E. Mahieu et al. « Validation of five years (2003–2007) of SCIAMACHY CO total column measurements using ground-based spectrometer observations ». Atmospheric Measurement Techniques Discussions 3, n^o 4 (9 juillet 2010) : 2891–930. http://dx.doi.org/10.5194/amtd-3-2891-2010.

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Abstract. This paper presents a validation study of SCIAMACHY CO total column measurements from the IMLM algorithm using ground-based spectrometer observations from twenty surface stations for the five year time period of 2003–2007. Overall we find a good agreement between SCIAMACHY and ground-based observations for both mean values as well as seasonal variations. For high-latitude Northern Hemisphere stations absolute differences between SCIAMACHY and ground-based measurements are close to or fall within the SCIAMACHY CO 2σ precision of 0.2×1018 molecules/cm2 (~10%) indicating that SCIAMACHY can observe CO accurately at high Northern Hemisphere latitudes. For Northern Hemisphere mid-latitude stations the validation is complicated due to the vicinity of emission sources for almost all stations, leading to higher ground-based measurements compared to SCIAMACHY CO within its typical sampling area of 8×8°. Comparisons with Northern Hemisphere mountain stations are hampered by elevation effects. After accounting for these effects, the validation provides satisfactory results. At Southern Hemisphere mid- to high latitudes SCIAMACHY is systematically lower than the ground-based measurements for 2003 and 2004, but for 2005 and later years the differences between SCIAMACHY and ground-based measurements fall within the SCIAMACHY precision. The 2003–2004 bias is consistent with a previously reported Southern Hemisphere bias based on comparisons with MOPITT CO and is currently under investigation. No other systematic spatial or temporal biases could be identified based on the validation presented in this paper. Validation results are robust with regard to the choices of the instrument-noise error filter, sampling area, and time averaging required for the validation of SCIAMACHY CO total column measurements. Finally, our results show that the spatial coverage of the ground-based measurements available for the validation of the 2003–2007 SCIAMACHY CO columns is sub-optimal for validation purposes, and that the recent and ongoing expansion of the ground-based network by carefully selecting new locations may be very beneficial for SCIAMACHY CO and other satellite trace gas measurements validation efforts.

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de Laat, A. T. J., A. M. S. Gloudemans, H. Schrijver, I. Aben, Y. Nagahama, K. Suzuki, E. Mahieu et al. « Validation of five years (2003–2007) of SCIAMACHY CO total column measurements using ground-based spectrometer observations ». Atmospheric Measurement Techniques 3, n^o 5 (20 octobre 2010) : 1457–71. http://dx.doi.org/10.5194/amt-3-1457-2010.

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Abstract. This paper presents a validation study of SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) carbon monoxide (CO) total column measurements from the Iterative Maximum Likelihood Method (IMLM) algorithm using ground-based spectrometer observations from twenty surface stations for the five year time period of 2003–2007. Overall we find a good agreement between SCIAMACHY and ground-based observations for both mean values as well as seasonal variations. For high-latitude Northern Hemisphere stations absolute differences between SCIAMACHY and ground-based measurements are close to or fall within the SCIAMACHY CO 2σ precision of 0.2 × 1018 molecules/cm2 (∼10%) indicating that SCIAMACHY can observe CO accurately at high Northern Hemisphere latitudes. For Northern Hemisphere mid-latitude stations the validation is complicated due to the vicinity of emission sources for almost all stations, leading to higher ground-based measurements compared to SCIAMACHY CO within its typical sampling area of 8° × 8°. Comparisons with Northern Hemisphere mountain stations are hampered by elevation effects. After accounting for these effects, the validation provides satisfactory results. At Southern Hemisphere mid- to high latitudes SCIAMACHY is systematically lower than the ground-based measurements for 2003 and 2004, but for 2005 and later years the differences between SCIAMACHY and ground-based measurements fall within the SCIAMACHY precision. The 2003–2004 bias is consistent with previously reported results although its origin remains under investigation. No other systematic spatial or temporal biases could be identified based on the validation presented in this paper. Validation results are robust with regard to the choices of the instrument-noise error filter, sampling area, and time averaging required for the validation of SCIAMACHY CO total column measurements. Finally, our results show that the spatial coverage of the ground-based measurements available for the validation of the 2003–2007 SCIAMACHY CO columns is sub-optimal for validation purposes, and that the recent and ongoing expansion of the ground-based network by carefully selecting new locations may be very beneficial for SCIAMACHY CO and other satellite trace gas measurements validation efforts.

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Krijger, J. M., I. Aben et H. Schrijver. « Distinction between clouds and ice/snow covered surfaces in the identification of cloud-free observations using SCIAMACHY PMDs ». Atmospheric Chemistry and Physics Discussions 5, n^o 1 (14 février 2005) : 815–45. http://dx.doi.org/10.5194/acpd-5-815-2005.

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Abstract. SCIAMACHY on ENVISAT allows measurement of different trace gases including those most abundant in the troposphere (e.g. CO2, NO2, CH4). However, clouds in the observed scenes can severely hinder the observation of tropospheric gases. Several cloud detection algorithms have been developed for GOME on ERS-2 which can be applied to SCIAMACHY. The GOME cloud algorithms, however, suffer from the inadequacy of not being able to distinguish between clouds and ice/snow covered surfaces because GOME only covers the UV, VIS and part of the NIR wavelength range (240–790 nm). As a result these areas are always flagged as clouded, and therefore often not used. Here a method is presented which uses the SCIAMACHY measurements in the wavelength range between 450 nm and 1.6 µm to make a distinction between clouds and ice/snow covered surfaces. The algorithm is developed using collocated MODIS observations. The algorithm presented here is specifically developed to identify cloud-free SCIAMACHY observations. The SCIAMACHY Polarisation Measurement Devices (PMDs) are used for this purpose because they provide higher spatial resolution compared to the main spectrometer measurements.

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Kokhanovsky, A. A., W. von Hoyningen-Huene, V. V. Rozanov, S. Noël, K. Gerilowski, H. Bovensmann, K. Bramstedt, M. Buchwitz et J. P. Burrows. « The semianalytical cloud retrieval algorithm for SCIAMACHY II. The application to MERIS and SCIAMACHY data ». Atmospheric Chemistry and Physics 6, n^o 12 (18 septembre 2006) : 4129–36. http://dx.doi.org/10.5194/acp-6-4129-2006.

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Abstract. The SemiAnalytical CloUd Retrieval Algorithm (SACURA) is applied to the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) data. In particular, we derive simultaneously cloud optical thickness (COT) and cloud top height (CTH), using SCIAMACHY measurements in the visible (442 nm, COT) and in the oxygen A-band (755–775 nm, CTH). Some of the results obtained are compared with those derived from the Medium Resolution Imaging Spectrometer (MERIS), which has better spatial resolution and observes almost the same scene as SCIAMACHY. The same cloud algorithm is applied to both MERIS and SCIAMACHY data. In addition, we perform the vicarious calibration of SCIAMACHY at the wavelength 442 nm, using MERIS measurements at the same wavelength. Differences in the retrieved COT for the same cloud field obtained using MERIS and SCIAMACHY measurements are discussed.

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Krijger, J. M., I. Aben et H. Schrijver. « Distinction between clouds and ice/snow covered surfaces in the identification of cloud-free observations using SCIAMACHY PMDs ». Atmospheric Chemistry and Physics 5, n^o 10 (18 octobre 2005) : 2729–38. http://dx.doi.org/10.5194/acp-5-2729-2005.

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Abstract. SCIAMACHY on ENVISAT allows measurement of different trace gases including those most abundant in the troposphere (e.g. CO2, NO2, CH4, BrO, SO2). However, clouds in the observed scenes can severely hinder the observation of tropospheric gases. Several cloud detection algorithms have been developed for GOME on ERS-2 which can be applied to SCIAMACHY. The GOME cloud algorithms, however, suffer from the inadequacy of not being able to distinguish between clouds and ice/snow covered surfaces because GOME only covers the UV, VIS and part of the NIR wavelength range (240-790 nm). As a result these areas are always flagged as clouded, and therefore often not used. Here a method is presented which uses the SCIAMACHY measurements in the wavelength range between 450 nm and 1.6 µm to make a distinction between clouds and ice/snow covered surfaces. The algorithm is developed using collocated MODIS observations. The algorithm presented here is specifically developed to identify cloud-free SCIAMACHY observations. The SCIAMACHY Polarisation Measurement Devices (PMDs) are used for this purpose because they provide higher spatial resolution compared to the main spectrometer measurements.

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Wang, P., P. Stammes et R. Mueller. « Surface solar irradiance from SCIAMACHY measurements : algorithm and validation ». Atmospheric Measurement Techniques 4, n^o 5 (16 mai 2011) : 875–91. http://dx.doi.org/10.5194/amt-4-875-2011.

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Abstract. Broadband surface solar irradiances (SSI) are, for the first time, derived from SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY) satellite measurements. The retrieval algorithm, called FRESCO (Fast REtrieval Scheme for Clouds from the Oxygen A band) SSI, is similar to the Heliosat method. In contrast to the standard Heliosat method, the cloud index is replaced by the effective cloud fraction derived from the FRESCO cloud algorithm. The MAGIC (Mesoscale Atmospheric Global Irradiance Code) algorithm is used to calculate clear-sky SSI. The SCIAMACHY SSI product is validated against globally distributed BSRN (Baseline Surface Radiation Network) measurements and compared with ISCCP-FD (International Satellite Cloud Climatology Project Flux Dataset) surface shortwave downwelling fluxes (SDF). For one year of data in 2008, the mean difference between the instantaneous SCIAMACHY SSI and the hourly mean BSRN global irradiances is −4 W m−2 (−1 %) with a standard deviation of 101 W m−2 (20 %). The mean difference between the globally monthly mean SCIAMACHY SSI and ISCCP-FD SDF is less than −12 W m−2 (−2 %) for every month in 2006 and the standard deviation is 62 W m−2 (12 %). The correlation coefficient is 0.93 between SCIAMACHY SSI and BSRN global irradiances and is greater than 0.96 between SCIAMACHY SSI and ISCCP-FD SDF. The evaluation results suggest that the SCIAMACHY SSI product achieves similar mean bias error and root mean square error as the surface solar irradiances derived from polar orbiting satellites with higher spatial resolution.

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Krijger, J. M., P. Tol, L. G. Istomina, C. Schlundt, H. Schrijver et I. Aben. « Improved identification of clouds and ice/snow covered surfaces in SCIAMACHY observations ». Atmospheric Measurement Techniques 4, n^o 10 (19 octobre 2011) : 2213–24. http://dx.doi.org/10.5194/amt-4-2213-2011.

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Abstract. In the ultra-violet, visible and near infra-red wavelength range the presence of clouds can strongly affect the satellite-based passive remote sensing observation of constituents in the troposphere, because clouds effectively shield the lower part of the atmosphere. Therefore, cloud detection algorithms are of crucial importance in satellite remote sensing. However, the detection of clouds over snow/ice surfaces is particularly difficult in the visible wavelengths as both clouds an snow/ice are both white and highly reflective. The SCIAMACHY Polarisation Measurement Devices (PMD) Identification of Clouds and Ice/snow method (SPICI) uses the SCIAMACHY measurements in the wavelength range between 450 nm and 1.6 μm to make a distinction between clouds and ice/snow covered surfaces, specifically developed to identify cloud-free SCIAMACHY observations. For this purpose the on-board SCIAMACHY PMDs are used because they provide higher spatial resolution compared to the main spectrometer measurements. In this paper we expand on the original SPICI algorithm (Krijger et al., 2005a) to also adequately detect clouds over snow-covered forests which is inherently difficult because of the similar spectral characteristics. Furthermore the SCIAMACHY measurements suffer from degradation with time. This must be corrected for adequate performance of SPICI over the full SCIAMACHY time range. Such a correction is described here. Finally the performance of the new SPICI algorithm is compared with various other datasets, such as from FRESCO, MICROS and AATSR, focusing on the algorithm improvements.

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Wang, P., P. Stammes et R. Mueller. « Surface solar irradiance from SCIAMACHY measurements : algorithm and validation ». Atmospheric Measurement Techniques Discussions 4, n^o 1 (2 février 2011) : 873–912. http://dx.doi.org/10.5194/amtd-4-873-2011.

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Abstract. Broadband surface solar irradiances (SSI) are, for the first time, derived from SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY) satellite measurements. The retrieval algorithm, called FRESCO (Fast REtrieval Scheme for Clouds from Oxygen A band) SSI, is similar to the Heliosat method. In contrast to the standard Heliosat method, the cloud index is replaced by the effective cloud fraction derived from the FRESCO cloud algorithm. The MAGIC (Mesoscale Atmospheric Global Irradiance Code) algorithm is used to calculate clear-sky SSI. The SCIAMACHY SSI product is validated against the globally distributed BSRN (Baseline Surface Radiation Network) measurements and compared with the ISCCP-FD (International Satellite Cloud Climatology Project Flux Dataset) surface shortwave downwelling fluxes (SDF). For one year of data in 2008, the mean difference between the instantaneous SCIAMACHY SSI and the hourly mean BSRN global irradiances is −4 W m−2(−1%) with a standard deviation of 101 W m−2 (20%). The mean difference between the globally monthly mean SCIAMACHY SSI and ISCCP-FD SDF is less than −12 W m−2 (−2%) for every month in 2006 and the standard deviation is 62 W m−2 (12%). The correlation coefficient is 0.93 between SCIAMACHY SSI and BSRN global irradiances and is greater than 0.96 between SCIAMACHY SSI and ISCCP-FD SDF. The evaluation results suggest that the SCIAMACHY SSI product achieves similar mean bias error and root mean square error as the surface solar irradiances derived from polar orbiting satellites with higher spatial resolution.

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Hilbig, Tina, Klaus Bramstedt, Mark Weber, John P. Burrows et Matthijs Krijger. « Optimised degradation correction for SCIAMACHY satellite solar measurements from 330 to 1600 nm by using the internal white light source ». Atmospheric Measurement Techniques 13, n^o 7 (20 juillet 2020) : 3893–907. http://dx.doi.org/10.5194/amt-13-3893-2020.

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Abstract. SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) on-board the European Environmental Satellite (Envisat) provided spectrally resolved measurements in the wavelength range from 0.24 to 2.4 µm by looking into the Earth's atmosphere using different viewing geometries (limb, nadir, solar, and lunar occultation). These observations were used to derive a multitude of parameters, in particular atmospheric trace gas amounts. In addition to radiance measurements solar spectral irradiances (SSIs) were measured on a daily basis. The instrument was operating for nearly a decade, from August 2002 to April 2012. Due to the harsh space environment, it suffered from continuous optical degradation. As part of recent radiometric calibration activities an optical (physical) model was introduced that describes the behaviour of the scanner unit of SCIAMACHY with time (Krijger et al., 2014). This model approach accounts for optical degradation by assuming contamination layers on optical surfaces in the scanner unit. The variation in layer thicknesses of the various optical components is determined from the combination of solar measurements from different monitoring light paths available for SCIAMACHY. In this paper, we present an optimisation of this degradation correction approach, which in particular improves the solar spectral data. An essential part of the modification is the use of measurements from SCIAMACHY's internal white light source (WLS) in combination with direct solar measurements. The WLS, as an independent light source, therefore, gives an opportunity to better separate instrument variations and natural solar variability. However, the WLS emission depends on its burning time and changes with time as well. To use these measurements in the optimised degradation correction, the change in the WLS emission in space needs to be characterised first. The changes in the WLS with accumulated burning time are in good agreement with detailed laboratory lamp studies by Sperling et al. (1996). Although the optimised degradation-corrected SCIAMACHY SSIs still show some instrumental issues when compared to SSI measurements from other instruments and model reconstructions, our study demonstrates the potential for the use of an internal WLS for degradation monitoring.

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Hilboll, A., A. Richter, A. Rozanov, Ø. Hodnebrog, A. Heckel, S. Solberg, F. Stordal et J. P. Burrows. « Retrieval of tropospheric NO2 ; columns from SCIAMACHY combining measurements from limb and nadir geometries ». Atmospheric Measurement Techniques Discussions 5, n^o 4 (23 juillet 2012) : 5043–105. http://dx.doi.org/10.5194/amtd-5-5043-2012.

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Abstract. Satellite measurements of atmospheric trace gases have proved to be an invaluable tool for monitoring the Earth system. When these measurements are to be used for assessing tropospheric emissions and pollution, as for example in the case of nadir measurements of nitrogen dioxide (NO2), it is necessary to separate the stratospheric from the tropospheric signal. The SCIAMACHY instrument offers the unique opportunity to combine its measurements in limb and nadir viewing geometries into a tropospheric data product, using the limb measurements of the stratospheric NO2 abundances to correct the nadir measurements' total columns. In this manuscript, we present a novel approach to limb/nadir matching, calculating one stratospheric NO2 value from limb measurements for every single nadir measurement, abandoning global coverage for the sake of spatial accuracy. As a comparison, modelled stratospheric NO2 columns from the Oslo CTM2 are evaluated as stratospheric correction, and both datasets are confronted with the originally used reference sector method. Our study shows that stratospheric NO2 columns from SCIAMACHY limb measurements very well reflect stratospheric conditions. The zonal variability of stratospheric NO2 is captured by our matching algorithm, and the quality of the resulting tropospheric NO2 columns improves considerably. Modelled stratospheric NO2 columns from the Oslo CTM2 agree remarkably well with the measurements. Both datasets need to be matched to the level of the nadir measurements, however, because a time and latitude dependent bias between both stratospheric datasets and the measured nadir columns can be observed over clean regions. After accounting for this systematic bias between SCIAMACHY nadir observations and the stratospheric columns, both new stratospheric correction methods provide a significant improvement to the retrieval of tropospheric NO2 columns from the SCIAMACHY instrument.

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Azam, F., K. Bramstedt, A. Rozanov, K. Weigel, H. Bovensmann, G. P. Stiller et J. P. Burrows. « SCIAMACHY lunar occultation water vapor measurements : retrieval and validation results ». Atmospheric Measurement Techniques 5, n^o 10 (24 octobre 2012) : 2499–513. http://dx.doi.org/10.5194/amt-5-2499-2012.

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Abstract. SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) lunar occultation measurements have been used to derive vertical profiles of stratospheric water vapor for the Southern Hemisphere in the near infrared (NIR) spectral range of 1350–1420 nm. The focus of this study is to present the retrieval methodology including the sensitivity studies and optimizations for the implementation of the radiative transfer model on SCIAMACHY lunar occultation measurements. The study also includes the validation of the data product with the collocated measurements from two satellite occultation instruments and two instruments measuring in limb geometry. The SCIAMACHY lunar occultation water vapor measurement comparisons with the ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) instrument have shown an agreement of 5% on the average that is well within the reported biases of ACE in the stratosphere. The comparisons with HALOE (Halogen Occultation Experiment) have also shown good results where the agreement between the instruments is within 5%. The validations of the lunar occultation water vapor measurements with MLS (Microwave Limb Sounder) instrument are exceptionally good, varying between 1.5 to around 4%. The validations with MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) are in the range of 10%. A validated dataset of water vapor vertical distributions from SCIAMACHY lunar occultation measurements is expected to facilitate the understanding of physical and chemical processes in the southern mid-latitudes and the dynamical processes related to the polar vortex.

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Borsdorff, T., P. Tol, J. E. Williams, J. de Laat, J. aan de Brugh, P. Nédélec, I. Aben et J. Landgraf. « Carbon monoxide total columns from SCIAMACHY 2.3 µm atmospheric reflectance measurements : towards a full-mission data product (2003–2012) ». Atmospheric Measurement Techniques 9, n^o 1 (26 janvier 2016) : 227–48. http://dx.doi.org/10.5194/amt-9-227-2016.

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Abstract. We present a full-mission data product of carbon monoxide (CO) vertical column densities using the 2310–2338 nm SCIAMACHY reflectance measurements over clear-sky land scenes for the period January 2003–April 2012. The retrieval employs the SICOR algorithm, which will be used for operational data processing of the Sentinel-5 Precursor mission. The retrieval approach infers simultaneously carbon monoxide, methane and water vapour column densities together with a Lambertian surface albedo from individual SCIAMACHY measurements employing a non-scattering radiative transfer model. To account for the radiometric instrument degradation including the formation of an ice-layer on the 2.3 µm detector array, we consider clear-sky measurements over the Sahara as a natural calibration target. For these specific measurements, we spectrally calibrate the SCIAMACHY measurements and determine a spectral radiometric offset and the width of the instrument spectral response function as a function of time for the entire operational phase of the mission. We show that the smoothing error of individual clear-sky CO retrievals is less than ±1 ppb and thus this error contribution does not need to be accounted for in the validation considering the much higher retrieval noise. The CO data product is validated against measurements of ground-based Fourier transform infrared spectrometers at 27 stations of the NDACC-IRWG and TCCON network and MOZAIC/IAGOS aircraft measurements at 26 airports worldwide. Overall, we find a good agreement with TCCON measurements with a mean bias b = −1.2 ppb and a station-to-station bias with σ = 7.2 ppb. The negative sign of the bias means a low bias of SCIAMACHY CO with respect to TCCON. For the NDACC-IRWG network, we obtain a larger mean station bias of b = −9.2 ppb with σ = 8.1 ppb and for the MOZAIC/IAGOS measurements we find b = −6.4 ppb with σ = 5.6 ppb. The SCIAMACHY data set is subject to a small but significant bias trend of 1.47 ± 0.25 ppb yr−1. After trend correction, the bias with respect to MOZAIC/IAGOS observation is 2.5 ppb, with respect to TCCON measurements it is −4.6 ppb and with respect to NDACC-IRWG measurements −8.4 ppb. Hence, a discrepancy of 3.8 ppb remains between the global biases with NDACC-IRWG and TCCON, which is confirmed by directly comparing NDACC-IRWG and TCCON measurements. Generally, the scatter of the individual SCIAMACHY CO retrievals is high and dominated by large measurement noise. Hence, for practical usage of the data set, averaging of individual retrievals is required. As an example, we show that monthly mean SCIAMACHY CO retrievals, averaged separately over Northern and Southern Africa, reflect the spatial and temporal variability of biomass burning events in agreement with the global chemical transport model TM5.

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Borsdorff, T., P. Tol, J. E. Williams, J. de Laat, J. aan de Brugh, P. Nédélec, I. Aben et J. Landgraf. « Carbon monoxide total columns from SCIAMACHY 2.3 μm atmospheric reflectance measurements : towards a full-mission data product (2003–2012) ». Atmospheric Measurement Techniques Discussions 8, n^o 9 (17 septembre 2015) : 9731–83. http://dx.doi.org/10.5194/amtd-8-9731-2015.

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Abstract. We present a full-mission data product of carbon monoxide (CO) vertical column densities using the 2310–2338 nm SCIAMACHY reflectance measurements over clear sky land scenes for the period January 2003–April 2012. The retrieval employs the SICOR algorithm, which will be used for operational data processing of the Sentinel-5 Precursor mission, combined with a SCIAMACHY specific radiometric soft-calibration to mitigate instrumental issues. The retrieval approach infers simultaneously carbon monoxide, methane and water vapour column densities together with a Lambertian surface albedo from individual SCIAMACHY measurements employing a non-scattering radiative transfer model. To account for the radiometric instrument degradation including the formation of an ice-layer on the 2.3 μm detector-array, we consider clear sky measurements over the Sahara as a natural calibration target. For these specific measurements, we spectrally calibrate the SCIAMACHY measurements and determine a spectral radiometric offset and the width of the instrument spectral response function as a function of time for the entire operational phase of the mission. We show that the smoothing error of individual clear sky CO retrievals is less than ±1 ppb and thus this error contribution has not to be accounted for in the validation considering the much higher retrieval noise. The CO data product is validated against measurements of ground-based Fourier transform infrared spectrometers at 27 stations of the NDACC-IRWG and TCCON network and MOZAIC/IAGOS aircraft measurements at 26 airports worldwide. Overall, we find a good agreement with TCCON measurements with a mean bias b = −1.2 ppb and a station-to-station bias with σ = 7.2 ppb. For the NDACC-IRWG network, we obtain a larger mean station bias of b = −9.2 ppb with σ = 8.1 ppb and for the MOZAIC/IAGOS measurements we find b = −6.4 ppb with σ = 5.6 ppb. The SCIAMACHY data set is subject to a small but significant trend of 1.47 ± 0.25 ppb yr−1. After trend correction, the bias with respect to MOZAIC/IAGOS observation is 2.5 ppb, with respect to TCCON measurements it is −4.6 ppb and with respect to NDACC-IRWG measurements −8.4 ppb. Hence, a discrepancy of 3.8 ppb remains between the global biases with NDACC-IRWG and TCCON, which is confirmed by directly comparing NDACC-IRWG and TCCON measurements. Generally, the scatter of the individual SCIAMACHY CO retrievals is high and dominated by large measurement noise. Hence, for practical usage of the dataset, averaging of individual retrievals is required. As an example, we show that monthly mean SCIAMACHY CO retrievals, averaged separately over Northern and Southern Africa, reflect the spatial and temporal variability of biomass burning events in agreement with the global chemical transport model TM5.

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Ovigneur, B., J. Landgraf et I. Aben. « Retrieval of stratospheric aerosol density profiles from SCIAMACHY limb radiance measurements in the O2 ; A-band ». Atmospheric Measurement Techniques Discussions 4, n^o 2 (15 mars 2011) : 1795–823. http://dx.doi.org/10.5194/amtd-4-1795-2011.

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Abstract. In this paper we present an approach to retrieve stratospheric aerosol number densities in the altitude range 10–40 km from SCIAMACHY limb radiance measurements in the spectral range of the O2 A absorption band, near 760 nm. Here, the characteristic light paths differ for the measured light in the O2 A band and in the spectral continuum next to the absorption band. This difference is used to distinguish the effect of stratospheric aerosol scattering and ground reflection on the limb measurement. The capability to disentangle both effects is illustrated for SCIAMACHY limb observations over the Libyan desert, where the measurements are not affected by tropospheric clouds. Comparison of the SCIAMACHY retrieval and the SAGE II aerosol extinction product between 75 degrees Southern and Northern latitude shows the clear need for prior knowledge of the mean size of the stratospheric aerosol for the SCIAMACHY retrieval. We found best agreement between SCIAMACHY and SAGE II aerosol extinction for the period 2003–2005 for a prior choice of the mean aerosol size radius of 0.2 μm. The overall agreement between both data sets is in the range <50% root mean square difference at 14–30 km with a minimum of 30% at 22 km.

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Ovigneur, B., J. Landgraf, R. Snel et I. Aben. « Retrieval of stratospheric aerosol density profiles from SCIAMACHY limb radiance measurements in the O2 ; A-band ». Atmospheric Measurement Techniques 4, n^o 11 (2 novembre 2011) : 2359–73. http://dx.doi.org/10.5194/amt-4-2359-2011.

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Abstract. In this paper we present an approach to retrieve stratospheric aerosol number densities in the altitude range 10–40 km from SCIAMACHY limb radiance measurements in the spectral range of the O2 A absorption band, near 760 nm. Here, the characteristic light paths differ for the measured light in the O2 A-band and in the spectral continuum next to the absorption band. This difference is used to distinguish the effect of stratospheric aerosol scattering and ground reflection on the limb measurement. The capability to disentangle both effects is illustrated for SCIAMACHY limb observations over the Libyan desert, where the measurements are not affected by tropospheric clouds. Comparison of the SCIAMACHY retrieval and the SAGE II aerosol extinction product between 75° southern and northern latitude shows the clear need for prior knowledge of the mean size of the stratospheric aerosol for the SCIAMACHY retrieval. We found best agreement between SCIAMACHY and SAGE II aerosol extinction for the period 2003–2005 for a prior choice of the mean aerosol size radius of 0.2 μm. The overall agreement between both data sets is in the range <50% root mean square difference at 14–30 km with a minimum of 30% at 22 km.

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Houweling, S., W. Hartmann, I. Aben, H. Schrijver, J. Skidmore, G. J. Roelofs et F. M. Breon. « Evidence of systematic errors in SCIAMACHY-observed CO2 ; due to aerosols ». Atmospheric Chemistry and Physics Discussions 5, n^o 3 (25 mai 2005) : 3313–40. http://dx.doi.org/10.5194/acpd-5-3313-2005.

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Abstract. SCIAMACHY CO2 measurements show a large variability in total column CO2 over the Sahara desert of up to 10% that is not anticipated from in situ measurements and cannot be explained by results of atmospheric models. Comparisons with colocated aerosol measurements by TOMS and MISR over the Sahara indicate that the seasonal variation of SCIAMACHY-observed CO2 strongly resembles seasonal variations of windblown dust. Correlation coefficients of monthly datasets of colocated MISR aerosol optical depth and SCIAMACHY CO2 vary between 0.6 and 0.8, indicating that about half of the CO2 variance is explained by aerosol optical depth. Radiative transfer model calculations confirm the role of dust and can explain the size of the errors. Sensitivity tests suggest that the remaining variance may largely be explained by variations in the vertical distribution of dust. Further calculations for a few typical aerosol classes and a broad range of atmospheric conditions show that the impact of aerosols on SCIAMACHY retrieved CO2 is by far the largest over the Sahara, but may also reach significant levels elsewhere. Inverse modelling calculations indicate that continental scale source and sink estimation on the basis of SCIAMACHY CO2 data without aerosol correction leads to significant errors. To improve terrestrial CO2 flux estimates by inverse modelling using SCIAMACHY measurements at 1.6μm, aerosol correction will be needed. Methods for correcting aerosol-induced errors exist, but so far mainly on the basis of theoretical considerations. As demonstrated by this study, SCIAMACHY may contribute to a verification of such methods using real data.

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Barkley, M. P., P. S. Monks, A. J. Hewitt, T. Machida, A. Desai, N. Vinnichenko, T. Nakazawa, M. Yu Arshinov, N. Fedoseev et T. Watai. « Assessing the near surface sensitivity of SCIAMACHY atmospheric CO2 ; retrieved using (FSI) WFM-DOAS ». Atmospheric Chemistry and Physics Discussions 7, n^o 1 (21 février 2007) : 2477–530. http://dx.doi.org/10.5194/acpd-7-2477-2007.

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Abstract. Satellite observations of atmospheric CO2 offer the potential to identify regional carbon surface sources and sinks and to investigate carbon cycle processes. The extent to which satellite measurements are useful however, depends on the near surface sensitivity of the chosen sensor. In this paper, the capability of the SCIAMACHY instrument on board ENVISAT, to observe lower tropospheric and surface CO2 variability is examined. To achieve this, atmospheric CO2 retrieved from SCIAMACHY near infrared (NIR) spectral measurements, using the Full Spectral Initiation (FSI) WFM-DOAS algorithm, is compared to in situ aircraft observations over Siberia and additionally to tower and surface CO2 data over Mongolia, Europe and North America. Preliminary validation of daily averaged SCIAMACHY/FSI CO2 against ground based Fourier Transform Spectrometer (FTS) column measurements made at Park Falls, reveal a negative bias of about −2.0% for collocated measurements within ±1.0\\degree of the site. However, at this spatial threshold SCIAMACHY can only capture the variability of the FTS observations at monthly timescales. To observe day to day variability of the FTS observations, the collocation limits must be increased. Furthermore, comparisons to in-situ CO2 observations demonstrate that SCIAMACHY is capable of observing lower tropospheric variability on (at least) monthly timescales. Out of seventeen time series comparisons, eleven have correlation coefficients of 0.7 or more, and have similar seasonal cycle amplitudes. Additional evidence of the near surface sensitivity of SCIAMACHY, is provided through the significant correlation of FSI derived CO2 with MODIS vegetation indices at over twenty selected locations in the United States. The SCIAMACHY/MODIS comparison reveals that at many of the sites, the amount of CO2 variability is coincident with the amount of vegetation activity. It is evident, from this analysis, that SCIAMACHY therefore has the potential to detect CO2 variability within the lowermost troposphere arising from the activity of the terrestrial biosphere.

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Barkley, M. P., P. S. Monks, A. J. Hewitt, T. Machida, A. Desai, N. Vinnichenko, T. Nakazawa, M. Yu Arshinov, N. Fedoseev et T. Watai. « Assessing the near surface sensitivity of SCIAMACHY atmospheric CO2 ; retrieved using (FSI) WFM-DOAS ». Atmospheric Chemistry and Physics 7, n^o 13 (9 juillet 2007) : 3597–619. http://dx.doi.org/10.5194/acp-7-3597-2007.

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Abstract. Satellite observations of atmospheric CO2 offer the potential to identify regional carbon surface sources and sinks and to investigate carbon cycle processes. The extent to which satellite measurements are useful however, depends on the near surface sensitivity of the chosen sensor. In this paper, the capability of the SCIAMACHY instrument on board ENVISAT, to observe lower tropospheric and surface CO2 variability is examined. To achieve this, atmospheric CO2 retrieved from SCIAMACHY near infrared (NIR) spectral measurements, using the Full Spectral Initiation (FSI) WFM-DOAS algorithm, is compared to in-situ aircraft observations over Siberia and additionally to tower and surface CO2 data over Mongolia, Europe and North America. Preliminary validation of daily averaged SCIAMACHY/FSI CO2 against ground based Fourier Transform Spectrometer (FTS) column measurements made at Park Falls, reveal a negative bias of about −2.0% for collocated measurements within ±1.0° of the site. However, at this spatial threshold SCIAMACHY can only capture the variability of the FTS observations at monthly timescales. To observe day to day variability of the FTS observations, the collocation limits must be increased. Furthermore, comparisons to in-situ CO2 observations demonstrate that SCIAMACHY is capable of observing a seasonal signal that is representative of lower tropospheric variability on (at least) monthly timescales. Out of seventeen time series comparisons, eleven have correlation coefficients of 0.7 or more, and have similar seasonal cycle amplitudes. Additional evidence of the near surface sensitivity of SCIAMACHY, is provided through the significant correlation of FSI derived CO2 with MODIS vegetation indices at over twenty selected locations in the United States. The SCIAMACHY/MODIS comparison reveals that at many of the sites, the amount of CO2 variability is coincident with the amount of vegetation activity. The presented analysis suggests that SCIAMACHY has the potential to detect CO2 variability within the lowermost troposphere arising from the activity of the terrestrial biosphere.

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du Piesanie, A., A. J. M. Piters, I. Aben, H. Schrijver, P. Wang et S. Noël. « Validation of two independent retrievals of SCIAMACHY water vapour columns using radiosonde data ». Atmospheric Measurement Techniques Discussions 6, n^o 1 (21 janvier 2013) : 665–702. http://dx.doi.org/10.5194/amtd-6-665-2013.

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Abstract. Two independently derived SCIAMACHY total water vapour column (WVC) products are compared with integrated water vapour data calculated from radiosonde measurements, and with each other. The two SCIAMACHY WVC products are retrieved with two different retrieval algorithms applied in the visible and short wave infrared wavelength regions respectively. The first SCIAMACHY WVC product used in the comparison is ESA's level 2 version 5.01 WVC product derived with the Air Mass Corrected Differential Absorption Spectroscopy (AMC-DOAS) retrieval algorithm (SCIAMACHY-ESA). The second SCIAMACHY WVC product is derived using the Iterative Maximum Likelihood Method (IMLM) developed by Netherlands Institute for Space Research (SCIAMACHY-IMLM). Both SCIAMACHY WVC products are compared with collocated water vapour amounts determined from daily relative humidity radiosonde measurements obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) radiosonde network, over an 18 month and 2 yr period respectively. Results indicate a good agreement between the WVC amounts of SCIAMACHY-ESA and the radiosonde, and a mean difference of 0.03 g cm−2 is found for cloud free conditions. Overall the SCIAMACHY-ESA WVC amounts are smaller than the radiosonde WVC amounts, especially over oceans. For cloudy conditions the WVC bias has a clear dependence on the cloud top height and increases with increasing cloud top heights larger than approximately 2 km. A likely cause for this could be the different vertical profile shapes of water vapour and O2 leading to different relative changes in their optical thickness, which makes the AMF correction method used in the algorithm less suitable for high clouds. The SCIAMACHY-IMLM WVC amounts compare well to the radiosonde WVC amounts during cloud free conditions over land. A mean difference of 0.08 g cm−2 is found which is consistent with previous results when comparing daily averaged SCIAMACHY-IMLM WVC amounts with ECMWF model data globally. Furthermore, we show that the measurements for cloudy conditions (cloud fraction ≥ 0.5) with low clouds (cloud pressure ≥ 930 hPa) above the ocean and land compare quite well with radiosonde data.

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Bracher, A., M. Sinnhuber, A. Rozanov et J. P. Burrows. « Using a photochemical model for the validation of NO2 ; satellite measurements at different solar zenith angles ». Atmospheric Chemistry and Physics 5, n^o 2 (10 février 2005) : 393–408. http://dx.doi.org/10.5194/acp-5-393-2005.

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Abstract. SCIAMACHY (Scanning Imaging Spectrometer for Atmospheric Chartography) aboard the recently launched Environmental Satellite (ENVISAT) of ESA is measuring solar radiance upwelling from the atmosphere and the extraterrestrial irradiance. Appropriate inversion of the ultraviolet and visible radiance measurements, observed from the atmospheric limb, yields profiles of nitrogen dioxide, NO2, in the stratosphere (SCIAMACHY-IUP NO2 profiles V1). In order to assess their accuracy, the resulting NO2 profiles have been compared with those retrieved from the space borne occultation instruments Halogen Occultation Experiment (HALOE, data version v19) and Stratospheric Aerosol and Gas Experiment II (SAGE II, data version 6.2). As the HALOE and SAGE II measurements are performed during local sunrise or sunset and because NO2 has a significant diurnal variability, the NO2 profiles derived from HALOE and SAGE II have been transformed to those predicted for the solar zenith angles of the SCIAMACHY measurement by using a 1-dimensional photochemical model. The model used to facilitate the comparison of the NO2 profiles from the different satellite sensors is described and a sensitivity ananlysis provided. Comparisons between NO2 profiles from SCIAMACHY and those from HALOE NO2 but transformed to the SCIAMACHY solar zenith angle, for collocations from July to October 2002, show good agreement (within +/-12%) between the altitude range from 22 to 33km. The results from the comparison of all collocated NO2 profiles from SCIAMACHY and those from SAGE II transformed to the SCIAMACHY solar zenith angle show a systematic negative bias of 10 to 35% between 20km to 38km with a small standard deviation between 5 to 14%. These results agree with those of Newchurch and Ayoub (2004), implying that above 20km NO2 profiles from SAGE II sunset are probably somewhat high.

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de Laat, A. T. J., R. Dijkstra, H. Schrijver, P. Nédélec et I. Aben. « Validation of six years of SCIAMACHY carbon monoxide observations using MOZAIC CO profile measurements ». Atmospheric Measurement Techniques Discussions 5, n^o 1 (29 février 2012) : 1985–2010. http://dx.doi.org/10.5194/amtd-5-1985-2012.

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Abstract. This paper presents a validation study of SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) carbon monoxide (CO) total column measurements from the Iterative Maximum Likelihood Method (IMLM) algorithm using vertically integrated profile aircraft measurements obtained within the MOZAIC project for the six year time period of 2003–2008. Overall we find a good agreement between SCIAMACHY and airborne measurements for both mean values – also on a year-to-year basis – as well as seasonal variations. Several locations show large biases that are attributed to local effects like orography and proximity of large emission sources. Differences were detected for individual years: 2003, 2004 and 2006 have larger biases than 2005, 2007 and 2008, which appear to be related to SCIAMACHY instrumental issues but require more research. Results from this study are consistent with, and complementary to, findings from a previous validation study using ground-based measurements (de Laat et al., 2010). Despite the presence of some biases, this study provides additional confidence that SCIAMACHY, if individual measurements are of sufficient quality – good signal-to-noise – can be used to determine the spatial distribution and seasonal cycles of CO total columns.

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Houweling, S., W. Hartmann, I. Aben, H. Schrijver, J. Skidmore, G. J. Roelofs et F. M. Breon. « Evidence of systematic errors in SCIAMACHY-observed CO2 ; due to aerosols ». Atmospheric Chemistry and Physics 5, n^o 11 (8 novembre 2005) : 3003–13. http://dx.doi.org/10.5194/acp-5-3003-2005.

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Abstract. SCIAMACHY CO2 measurements show a large variability in total column CO2 over the Sahara desert of up to 10%, which is not anticipated from in situ measurements and cannot be explained by results of atmospheric models. Comparisons with colocated aerosol measurements by TOMS and MISR over the Sahara indicate that the seasonal variation of SCIAMACHY-observed CO2 strongly resembles seasonal variations of windblown dust. Correlation coefficients of monthly datasets of colocated MISR aerosol optical depth and SCIAMACHY CO2 vary between 0.6 and 0.8, indicating that about half of the CO2 variance is explained by aerosol optical depth. Radiative transfer model calculations confirm the role of dust and can explain the size of the errors. Sensitivity tests suggest that the remaining variance may largely be explained by variations in the vertical distribution of dust. Further calculations for a few typical aerosol classes and a broad range of atmospheric conditions show that the impact of aerosols on SCIAMACHY retrieved CO2 is by far the largest over the Sahara, but may also reach significant levels elsewhere. Over the continents, aerosols lead mostly to overestimated CO2 columns with the exception of biomass burning plumes and dark coniferous forests. Inverse modelling calculations confirm that aerosol correction of SCIAMACHY CO2 measurements is needed to derive meaningful source and sink estimates. Methods for correcting aerosol-induced errors exist, but so far mainly on the basis of theoretical considerations. As demonstrated by this study, SCIAMACHY may contribute to a verification of such methods using real data.

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de Laat, A. T. J., R. Dijkstra, H. Schrijver, P. Nédélec et I. Aben. « Validation of six years of SCIAMACHY carbon monoxide observations using MOZAIC CO profile measurements ». Atmospheric Measurement Techniques 5, n^o 9 (5 septembre 2012) : 2133–42. http://dx.doi.org/10.5194/amt-5-2133-2012.

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Abstract. This paper presents a validation study of SCanning Imaging Absorption SpectroMeter for Atmospheric CartograpHY (SCIAMACHY) carbon monoxide (CO) total column measurements from the Iterative Maximum Likelihood Method (IMLM) algorithm using vertically integrated profile aircraft measurements obtained within the MOZAIC project for the six year time period of 2003–2008. Overall we find a good agreement between SCIAMACHY and airborne measurements for both mean values – also on a year-to-year basis – as well as seasonal variations. Several locations show large biases that are attributed to local effects like orography and proximity of large emission sources. Differences were detected for individual years: 2003, 2004 and 2006 have larger biases than 2005, 2007 and 2008, which appear to be related to SCIAMACHY instrumental issues but require more research. Results from this study are consistent with, and complementary to, findings from a previous validation study using ground-based measurements (de Laat et al., 2010b). According to this study, the SCIAMACHY data, if individual measurements are of sufficient quality – good signal-to-noise, can be used to determine the spatial distribution and seasonal cycles of CO total columns over clean areas. Biases found over areas with strong emissions (Africa, China) could be explained by low sensitivity of the instrument in the boundary layer and users are recommended to avoid using the SCIAMACHY data while trying to quantify CO burden and/or retrieve CO emissions in such areas.

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Lerot, C., M. Van Roozendael, J. van Geffen, J. van Gent, C. Fayt, R. Spurr, G. Lichtenberg et A. von Bargen. « Six years of total ozone column measurements from SCIAMACHY nadir observations ». Atmospheric Measurement Techniques Discussions 1, n^o 1 (27 novembre 2008) : 249–79. http://dx.doi.org/10.5194/amtd-1-249-2008.

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Abstract. Total O3 columns have been retrieved from six years of SCIAMACHY nadir UV radiance measurements using SDOAS, an adaptation of the GDOAS algorithm previously developed at BIRA-IASB for the GOME instrument. GDOAS and SDOAS have been implemented by the German Aerospace Center (DLR) in the version 4 of the GOME Data Processor (GDP) and in version 3 of the SCIAMACHY Ground Processor (SGP), respectively. The processors are being run at the DLR processing centre on behalf of the European Space Agency (ESA). We first focus on the description of the SDOAS algorithm with particular attention to the impact of uncertainties on the reference O3 absorption cross-sections. Second, the resulting SCIAMACHY total ozone data set is globally evaluated through large-scale comparisons with results from GOME and OMI as well as with ground-based correlative measurements. The various total ozone data sets are found to agree within 2% on average. However, a negative trend of 0.2–0.4%/year has been identified in the SCIAMACHY O3 columns; this probably originates from instrumental degradation effects that have not yet been fully characterized.

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Lerot, C., M. Van Roozendael, J. van Geffen, J. van Gent, C. Fayt, R. Spurr, G. Lichtenberg et A. von Bargen. « Six years of total ozone column measurements from SCIAMACHY nadir observations ». Atmospheric Measurement Techniques 2, n^o 1 (7 avril 2009) : 87–98. http://dx.doi.org/10.5194/amt-2-87-2009.

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Abstract. Total O3 columns have been retrieved from six years of SCIAMACHY nadir UV radiance measurements using SDOAS, an adaptation of the GDOAS algorithm previously developed at BIRA-IASB for the GOME instrument. GDOAS and SDOAS have been implemented by the German Aerospace Center (DLR) in the version 4 of the GOME Data Processor (GDP) and in version 3 of the SCIAMACHY Ground Processor (SGP), respectively. The processors are being run at the DLR processing centre on behalf of the European Space Agency (ESA). We first focus on the description of the SDOAS algorithm with particular attention to the impact of uncertainties on the reference O3 absorption cross-sections. Second, the resulting SCIAMACHY total ozone data set is globally evaluated through large-scale comparisons with results from GOME and OMI as well as with ground-based correlative measurements. The various total ozone data sets are found to agree within 2% on average. However, a negative trend of 0.2–0.4%/year has been identified in the SCIAMACHY O3 columns; this probably originates from instrumental degradation effects that have not yet been fully characterized.

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Noël, S., H. Bovensmann, M. W. Wuttke, J. P. Burrows, M. Gottwald, E. Krieg, A. P. H. Goede et C. Muller. « Nadir, limb, and occultation measurements with SCIAMACHY ». Advances in Space Research 29, n^o 11 (juin 2002) : 1819–24. http://dx.doi.org/10.1016/s0273-1177(02)00102-3.

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Azam, F., K. Bramstedt, A. Rozanov, K. Weigel, H. Bovensmann, G. P. Stiller et J. P. Burrows. « SCIAMACHY lunar occultation water vapor measurements : retrieval and validation results ». Atmospheric Measurement Techniques Discussions 5, n^o 1 (3 février 2012) : 1029–73. http://dx.doi.org/10.5194/amtd-5-1029-2012.

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Abstract. SCIAMACHY lunar occultation measurements have been used to derive vertical profiles of stratospheric water vapor for the Southern Hemisphere in the near infrared (NIR) spectral range of 1350–1420 nm. The focus of this study is to present the retrieval methodology including the sensitivity studies and optimizations for the implementation of the radiative transfer model on SCIAMACHY lunar occultation measurements. The study also includes the validation of the data product with the collocated measurements from two satellite occultation instruments and two instruments measuring in limb geometry. The SCIAMACHY lunar occultation water vapor measurements comparisons with the ACE-FTS instrument have shown an agreement of 5% on the average that is well within the reported biases of ACE in the stratosphere. The comparisons with HALOE have also shown good results where the agreement between the instruments is within 5%. The validations of the lunar occultation water vapor measurements with MLS instrument are exceptionally good varying between 1.5 to around 4%. The validations with MIPAS are in the range of 10%. A validated dataset of water vapor vertical distributions from SCIAMACHY lunar occultation measurements is expected to facilitate the understanding of physical and chemical processes in the southern mid-latitudes and the dynamical processes related to polar vortex.

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van Diedenhoven, B., O. P. Hasekamp et I. Aben. « Surface pressure retrieval from SCIAMACHY measurements in the O2A Band : validation of the measurements and sensitivity on aerosols ». Atmospheric Chemistry and Physics Discussions 5, n^o 2 (14 mars 2005) : 1469–99. http://dx.doi.org/10.5194/acpd-5-1469-2005.

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Abstract. We perform surface pressure retrievals from cloud-free Oxygen A band measurements of SCIAMACHY. These retrievals can be well validated because surface pressure is a quantity that is, in general, accurately known from meteorological models. Therefore, surface 5 pressure retrievals and their validation provide important insight into the quality of the instrument calibration. Furthermore, they can provide insight into retrievals which are affected by similar radiation transport processes, for example the retrieval of total columns of H2O, CO, CO2 and CH4. In our retrieval aerosols are neglected. Using synthetic measurements, we show that for low to moderate or high surface albedos this 10 leads to an under- or overestimation of the retrieved surface pressures, respectively. The surface pressures retrieved from the SCIAMACHY measurements indeed show this dependence on surface albedo, when compared to the corresponding pressures from a meteorological database. However, an offset of about 30 hPa was found, which can not be caused by neglecting aerosols in the retrieval. The same offset was found 15 when comparing the retrieved surface pressures to those retrieved from co-located GOME Oxygen A band measurements. This implies a calibration error in the SCIAMACHY measurements. By adding an offset of 1% of the continuum reflectance at 756nm to the SCIAMACHY reflectance measurements, this systematic bias vanishes.

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Taha, G., D. F. Rault, R. P. Loughman, A. E. Bourassa et C. von Savigny. « SCIAMACHY stratospheric aerosol extinction profile retrieval ». Atmospheric Measurement Techniques Discussions 3, n^o 6 (24 novembre 2010) : 5343–74. http://dx.doi.org/10.5194/amtd-3-5343-2010.

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Abstract. The Ozone Mapper and Profiler Suite Limp Profiler (OMPS/LP) algorithm is used to retrieve ozone and aerosol profiles using a series of 120 SCIAMACHY limb measurements collocated with SAGE II solar occultation events. The primary goal of the study is to ascertain the capability of the OMPS/LP retrieval algorithm to accurately retrieve the vertical distribution of stratospheric aerosol extinction coefficient so as to better account for aerosol effects in the ozone profiling retrieval process. Using simulated radiances, we show that the aerosol extinction coefficient can be retrieved from limb scatter measurements within 5% and a standard deviation better than 15%, which is more than sufficient to improve the OMPS/LP ozone products to be used as Environmental Data Records. We also illustrate the ability of SCIAMACHY limb measurements to retrieve stratospheric aerosol profiles with accuracy comparable to other instruments. The retrieved aerosol profiles agree with collocated SAGE II measurements on average to within 25%, with a standard deviation of 35%.

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Bramstedt, K., S. Noël, H. Bovensmann, M. Gottwald et J. P. Burrows. « Precise pointing knowledge for SCIAMACHY solar occultation measurements ». Atmospheric Measurement Techniques 5, n^o 11 (27 novembre 2012) : 2867–80. http://dx.doi.org/10.5194/amt-5-2867-2012.

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Abstract. We present a method to precisely determine the viewing direction for solar occultation instruments from scans over the solar disk. Basic idea is the fit of the maximum intensity during the scan, which corresponds to the center of the solar disk in the scanning direction. We apply this method to the solar occultation measurements of the satellite instrument SCIAMACHY, which scans the Sun in elevation direction. The achieved mean precision is 0.46 mdeg, which corresponds to an tangent height error of about 26 m for individual occultation sequences. The deviation of the derived elevation angle from the geolocation information given along with the product has a seasonal cycle with an amplitude of 2.26 mdeg, which is in tangent height an amplitude of about 127 m. The mean elevation angle offset is −4.41 mdeg (249 m). SCIAMACHY's sun follower device controls the azimuth viewing direction during the occultation measurements. The derived mean azimuth direction has an standard error of 0.65 mdeg, which is about 36 m in horizontal direction at the tangent point. We observe also a seasonal cycle of the azimuth mispointing with an amplitude of 2.3 mdeg, which is slightly increasing with time. The almost constant mean offset is 88 mdeg, which is about 5.0 km horizontal offset at the tangent point.

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du Piesanie, A., A. J. M. Piters, I. Aben, H. Schrijver, P. Wang et S. Noël. « Validation of two independent retrievals of SCIAMACHY water vapour columns using radiosonde data ». Atmospheric Measurement Techniques 6, n^o 10 (31 octobre 2013) : 2925–40. http://dx.doi.org/10.5194/amt-6-2925-2013.

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Abstract. Two independently derived SCIAMACHY total water vapour column (WVC) products are compared with integrated water vapour data calculated from radiosonde measurements, and with each other. The two SCIAMACHY WVC products are retrieved with two different retrieval algorithms applied in the visible and short-wave infrared wavelength regions respectively. The first SCIAMACHY WVC product used in the comparison is ESA's level 2 version 5.01 WVC product derived with the Air Mass Corrected Differential Optical Absorption Spectroscopy (AMC-DOAS) retrieval algorithm applied in the visible wavelength range (SCIAMACHY-ESA). The second SCIAMACHY WVC product is derived using the iterative maximum likelihood method (IMLM) in the short-wave infrared wavelength range and developed by Netherlands Institute for Space Research (SCIAMACHY-IMLM). Both SCIAMACHY WVC products are compared with collocated water vapour amounts determined from daily relative humidity radiosonde measurements obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) radiosonde network. The SCIAMACHY-ESA WVC product is compared with radiosonde-derived WVC amounts for an 18-month period from February 2010 to mid-August 2011, and the SCIAMACHY-IMLM WVC amounts are compared with radiosonde WVC amounts for the two individual years of 2004 and 2009. In addition the WVC amounts from SCIAMACHY-ESA and SCIAMACHY-IMLM are also compared with each other for a 1-month period for June 2009. The AMC-DOAS method used to retrieve SCIAMACHY-ESA WVC is able to correct for water vapour present below the clouds and can be used during cloudy conditions over both land and ocean surfaces. Results indicate a good agreement between the WVC amounts of SCIAMACHY-ESA and that of radiosondes, with a mean difference of −0.32 g cm−2 for all collocated cases. Overall the SCIAMACHY-ESA WVC amounts are smaller than the radiosonde WVC amounts, especially over oceans. For cloudy conditions the WVC bias has a clear dependence on the cloud top height and increases with increasing cloud top heights larger than approximately 2 km. A likely cause for this could be the different vertical profile shapes of water vapour and O2 leading to different relative changes in their optical thickness, which makes the air mass factor (AMF) correction method used in the algorithm less suitable for high clouds. The SCIAMACHY-IMLM product's water vapour measurements are best used over land surfaces during cloud-free conditions, and in these cases a good agreement is found when compared to radiosonde WVC amounts, with a mean difference of 0.08 g cm−2. It is shown that over ocean surfaces during cloudy conditions the partial SCIAMACHY-IMLM water vapour column above the cloud can be well estimated by using the simultaneously retrieved methane column to calculate the cloud top height. Comparing the two satellite WVC products with each other indicates that SCIAMACHY-ESA consistently measures higher WVC amounts than those of SCIAMACHY-IMLM. Furthermore, the importance of the choice of cloud product is highlighted, as intercomparisons between the two SCIAMACHY WVC products indicate that using different cloud products to screen water vapour data for cloud-free conditions influences the data selection and may ultimately lead to a variation in results. In the last section of the paper, various options for filtering the two SCIAMACHY WVC data sets are discussed and best selection criteria suggested.

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von Savigny, C., C. E. Robert, G. Baumgarten, H. Bovensmann et J. P. Burrows. « Comparison of NLC particle sizes derived from SCIAMACHY/Envisat observations with ground-based LIDAR measurements at ALOMAR (69° ; N) ». Atmospheric Measurement Techniques Discussions 2, n^o 2 (27 avril 2009) : 1161–84. http://dx.doi.org/10.5194/amtd-2-1161-2009.

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Abstract. SCIAMACHY, the Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY provides measurements of limb-scattered solar radiation in the 220 nm to 2380 nm wavelength range since summer 2002. Measurements in the UV spectral range are well suited for the retrieval of particle sizes of noctilucent clouds (NLCs) and have been used to compile the largest existing satellite data base of NLC particle sizes. This paper presents a comparison of SCIAMACHY NLC size retrievals with the extensive NLC particle size data set based on ground-based LIDAR measurements at the Arctic LIDAR Observatory for Middle Atmosphere Research (ALOMAR, 69° N, 16° E) for the Northern Hemisphere NLC seasons 2003 to 2007. Most of the presented SCIAMACHY NLC particle size retrievals are based on cylindrical particles and a Gaussian particle size distribution with a fixed width. If the differences in spatial as well as vertical resolution between SCIAMACHY and the ALOMAR LIDAR are taken into account, very good agreement is found. The mean particle size derived from SCIAMACHY limb observations for the ALOMAR overpasses in 2003 to 2007 is 56.2 nm with a standard deviation of 12.5 nm, and the LIDAR observations yield a value of 54.2 nm with a standard deviation of 17.4 nm.

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von Savigny, C., C. E. Robert, G. Baumgarten, H. Bovensmann et J. P. Burrows. « Comparison of NLC particle sizes derived from SCIAMACHY/Envisat observations with ground-based LIDAR measurements at ALOMAR (69° N) ». Atmospheric Measurement Techniques 2, n^o 2 (18 septembre 2009) : 523–31. http://dx.doi.org/10.5194/amt-2-523-2009.

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Abstract. SCIAMACHY, the Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY has provided measurements of limb-scattered solar radiation in the 220 nm to 2380 nm wavelength range since summer of 2002. Measurements in the UV spectral range are well suited for the retrieval of particle sizes of noctilucent clouds (NLCs) and have been used to compile the largest existing satellite data base of NLC particle sizes. This paper presents a comparison of SCIAMACHY NLC size retrievals with the extensive NLC particle size data set based on ground-based LIDAR measurements at the Arctic LIDAR Observatory for Middle Atmosphere Research (ALOMAR, 69° N, 16° E) for the Northern Hemisphere NLC seasons 2003 to 2007. Most of the presented SCIAMACHY NLC particle size retrievals are based on cylindrical particles and a Gaussian particle size distribution with a fixed width of 24 nm. If the differences in spatial as well as vertical resolution between SCIAMACHY and the ALOMAR LIDAR are taken into account, very good agreement is found. The mean particle size derived from SCIAMACHY limb observations for the ALOMAR overpasses in 2003 to 2007 is 56.2 nm with a standard deviation of 12.5 nm, and the LIDAR observations yield a value of 54.2 nm with a standard deviation of 17.4 nm.

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Noël, S., K. Bramstedt, A. Rozanov, H. Bovensmann et J. P. Burrows. « Stratospheric methane profiles from SCIAMACHY solar occultation measurements derived with onion peeling DOAS ». Atmospheric Measurement Techniques Discussions 4, n^o 4 (29 juillet 2011) : 4801–23. http://dx.doi.org/10.5194/amtd-4-4801-2011.

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Abstract. Stratospheric methane (CH4) profiles have been derived from solar occultation measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on ENVISAT with an updated version of the Onion Peeling DOAS (ONPD) method. The SCIAMACHY solar occultation measurements cover the latitudinal range between about 50° N and 70° N. Currently, reasonable results are obtained between 20 and 40 km altitude. Comparisons with correlative ACE-FTS measurements show an average agreement within the expected accuracy of the ACE-FTS data of about 10 %. To demonstrate the capability of SCIAMACHY solar occultation measurements in the context of greenhouse gas monitoring, time series of stratospheric CH4 profiles covering the period from 2003 to 2010 have been generated. The SCIAMACHY CH4 profile solar occultation temporal series shows a strong seasonal cycle. This is attributed to the variations in both time and space of the retrieved data set. At lower altitudes, the observed temporal variations are explained by variations of the tropopause height. The temporal data set is also impacted by variations of the size and duration of the polar vortex in the northern hemisphere. The data set provides unique information about CH4 changes in the stratosphere at mid to high latitudes.

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Noël, S., K. Bramstedt, A. Rozanov, H. Bovensmann et J. P. Burrows. « Stratospheric methane profiles from SCIAMACHY solar occultation measurements derived with onion peeling DOAS ». Atmospheric Measurement Techniques 4, n^o 11 (29 novembre 2011) : 2567–77. http://dx.doi.org/10.5194/amt-4-2567-2011.

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Abstract. Stratospheric methane (CH4) profiles have been derived from solar occultation measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on ENVISAT with an updated version of the Onion Peeling DOAS (ONPD) method. The SCIAMACHY solar occultation measurements cover the latitudinal range between about 50° N and 70° N. Currently, reasonable results are obtained between 20 and 40 km altitude. Comparisons with correlative ACE-FTS measurements show an average agreement within the expected accuracy of the ACE-FTS data of about 10%. To demonstrate the capability of SCIAMACHY solar occultation measurements in the context of greenhouse gas monitoring, time series of stratospheric CH4 profiles covering the period from 2003 to 2010 have been generated. The SCIAMACHY CH4 profile solar occultation temporal series shows a strong seasonal cycle. This is attributed to the variations in both time and space of the retrieved data set. At lower altitudes, the observed temporal variations are explained by variations of the tropopause height. The temporal data set is also impacted by variations of the size and duration of the polar vortex in the northern hemisphere. The data set provides unique information about CH4 changes in the stratosphere at mid to high latitudes.

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Hilboll, A., A. Richter, A. Rozanov, Ø. Hodnebrog, A. Heckel, S. Solberg, F. Stordal et J. P. Burrows. « Improvements to the retrieval of tropospheric NO2 ; from satellite – stratospheric correction using SCIAMACHY limb/nadir matching and comparison to Oslo CTM2 simulations ». Atmospheric Measurement Techniques 6, n^o 3 (1 mars 2013) : 565–84. http://dx.doi.org/10.5194/amt-6-565-2013.

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Abstract. Satellite measurements of atmospheric trace gases have proved to be an invaluable tool for monitoring the Earth system. When these measurements are to be used for assessing tropospheric emissions and pollution, as for example in the case of nadir measurements of nitrogen dioxide (NO2), it is necessary to separate the stratospheric from the tropospheric signal. The SCIAMACHY instrument offers the unique opportunity to combine its measurements in limb- and nadir-viewing geometries into a tropospheric data product, using the limb measurements of the stratospheric NO2 abundances to correct the nadir measurements' total columns. In this manuscript, we present a novel approach to limb/nadir matching, calculating one stratospheric NO2 value from limb measurements for every single nadir measurement, abandoning global coverage for the sake of spatial accuracy. For comparison, modelled stratospheric NO2 columns from the Oslo CTM2 are also evaluated for stratospheric correction. Our study shows that stratospheric NO2 columns from SCIAMACHY limb measurements very well reflect stratospheric conditions. The zonal variability of the stratospheric NO2 field is captured by our matching algorithm, and the quality of the resulting tropospheric NO2 columns improves considerably. Both stratospheric datasets need to be adjusted to the level of the nadir measurements, because a time- and latitude-dependent bias to the measured nadir columns can be observed over clean regions. After this offset is removed, the two datasets agree remarkably well, and both stratospheric correction methods provide a significant improvement to the retrieval of tropospheric NO2 columns from the SCIAMACHY instrument.

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Gebhardt, C., A. Rozanov, R. Hommel, M. Weber, H. Bovensmann, J. P. Burrows, D. Degenstein, L. Froidevaux et A. M. Thompson. « Stratospheric ozone trends and variability as seen by SCIAMACHY from 2002 to 2012 ». Atmospheric Chemistry and Physics 14, n^o 2 (24 janvier 2014) : 831–46. http://dx.doi.org/10.5194/acp-14-831-2014.

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Abstract. Vertical profiles of the rate of linear change (trend) in the altitude range 15–50 km are determined from decadal O3 time series obtained from SCIAMACHY1/ENVISAT2 measurements in limb-viewing geometry. The trends are calculated by using a multivariate linear regression. Seasonal variations, the quasi-biennial oscillation, signatures of the solar cycle and the El Niño–Southern Oscillation are accounted for in the regression. The time range of trend calculation is August 2002–April 2012. A focus for analysis are the zonal bands of 20° N–20° S (tropics), 60–50° N, and 50–60° S (midlatitudes). In the tropics, positive trends of up to 5% per decade between 20 and 30 km and negative trends of up to 10% per decade between 30 and 38 km are identified. Positive O3 trends of around 5% per decade are found in the upper stratosphere in the tropics and at midlatitudes. Comparisons between SCIAMACHY and EOS MLS3 show reasonable agreement both in the tropics and at midlatitudes for most altitudes. In the tropics, measurements from OSIRIS4/Odin and SHADOZ5 are also analysed. These yield rates of linear change of O3 similar to those from SCIAMACHY. However, the trends from SCIAMACHY near 34 km in the tropics are larger than MLS and OSIRIS by a factor of around two. 1 SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY 2 European environmental research satellite 3 Earth Observing System (EOS) Microwave Limb Sounder (MLS) 4 Optical Spectrograph and InfraRed Imager System 5 Southern Hemisphere ADditional OZonesondes

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Bracher, A., M. Sinnhuber, A. Rozanov et J. P. Burrows. « Using photochemical models for the validation of NO2 ; satellite measurements at different solar zenith angles ». Atmospheric Chemistry and Physics Discussions 4, n^o 5 (21 septembre 2004) : 5515–48. http://dx.doi.org/10.5194/acpd-4-5515-2004.

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Abstract. SCIAMACHY (Scanning Imaging Spectrometer for Atmospheric Chartography) aboard the recently launched Environmental Satellite (ENVISAT) of ESA is measuring solar radiance upwelling from the atmosphere and the extraterrestrial irradiance. Appropriate inversion of the ultraviolet and visible radiance measurements, observed from the atmospheric limb, yields profiles of nitrogen dioxide, NO2, in the stratosphere. In order to assess their accuracy, the resulting NO2 profiles have been compared with those retrieved from the space borne occultation instruments Halogen Occultation Experiment (HALOE, data version v19) and Stratospheric Aerosol and Gas Experiment II (SAGE II, data version 6.20). As the HALOE and SAGE II measurements are performed during local sunrise or sunset and because NO2 has a significant diurnal variability, the NO2 profiles derived from HALOE and SAGE II have been transformed to those predicted for the solar zenith angles of the SCIAMACHY measurement by using a 1-D photochemical model. The model used to facilitate the comparison of the NO2 profiles from the different satellite sensors is described and an error assessment provided. Comparisons between NO2 profiles from SCIAMACHY and those from HALOE NO2 but transformed to the SCIAMACHY solar zenith angle, for collocations from July to October 2002, show good agreement (within +/−15%) between the altitude range from 22 to 33 km. The results from the comparison of all collocated NO2 profiles from SCIAMACHY and those from SAGE II transformed to the SCIAMACHY solar zenith angle show a systematic negative bias of 10 to 35% between 20 km to 38 km with a small standard deviation between 5 to 14%. These results agree with those of Newchurch and Ayoub (2004), implying that above 20 km NO2 profiles from SAGE II sunset are probably somewhat high.

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Palm, M., C. v. Savigny, T. Warneke, V. Velazco, J. Notholt, K. Künzi, J. Burrows et O. Schrems. « Intercomparison of O3 ; profiles observed by SCIAMACHY, ground based microwave and FTIR instruments ». Atmospheric Chemistry and Physics Discussions 5, n^o 1 (18 février 2005) : 911–36. http://dx.doi.org/10.5194/acpd-5-911-2005.

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Abstract. Ozone profiles retrieved from limb scattering measurements of the SCIAMACHY instrument based on the satellite ENVISAT are compared to ground based low altitude resolution remote sensors. All profiles are retrieved using optimal estimation. Following the work of 5 Rodgers and Connor (2003) the retrievals of the ground based instruments are simulated using the SCIAMACHY retrieval. The SCIAMACHY results and the results of the ground based microwave radiometer in Bremen and Ny Alesund agree within the expected covariance of the intercomparison. There are not enough coincident measurements of the FTIR instrument in order to allow for a conlusive statistical 10 treatment. However, preliminary intercomparison results are presented.

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van Diedenhoven, B., O. P. Hasekamp et I. Aben. « Surface pressure retrieval from SCIAMACHY measurements in the O2 ; A Band : validation of the measurements and sensitivity on aerosols ». Atmospheric Chemistry and Physics 5, n^o 8 (11 août 2005) : 2109–20. http://dx.doi.org/10.5194/acp-5-2109-2005.

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Abstract. We perform surface pressure retrievals from cloud-free Oxygen A band measurements of SCIAMACHY. These retrievals can be well validated because surface pressure is a quantity that is, in general, accurately known from meteorological models. Therefore, surface pressure retrievals and their validation provide important insight into the quality of the instrument calibration. Furthermore, they can provide insight into retrievals which are affected by similar radiation transport processes, for example the retrieval of total columns of H2O, CO, CO2 and CH4. In our retrieval aerosols are neglected. Using synthetic measurements, it is shown that for low to moderate surface albedos this leads to an underestimation of the retrieved surface pressures. For high surface albedos this generally leads to an overestimation of the retrieved surface pressures. The surface pressures retrieved from the SCIAMACHY measurements indeed show this dependence on surface albedo, when compared to the corresponding pressures from a meteorological database. However, an offset of about 20 hPa was found, which can not be caused by neglecting aerosols in the retrieval. The same offset was found when comparing the retrieved surface pressures to those retrieved from co-located GOME Oxygen A band measurements. This implies a calibration error in the SCIAMACHY measurements. By adding an offset of 0.86% of the continuum reflectance at 756 nm to the SCIAMACHY reflectance measurements, this systematic bias vanishes.

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Borsdorff, Tobias, Joost aan de Brugh, Haili Hu, Philippe Nédélec, Ilse Aben et Jochen Landgraf. « Carbon monoxide column retrieval for clear-sky and cloudy atmospheres : a full-mission data set from SCIAMACHY 2.3 µm reflectance measurements ». Atmospheric Measurement Techniques 10, n^o 5 (11 mai 2017) : 1769–82. http://dx.doi.org/10.5194/amt-10-1769-2017.

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Abstract. We discuss the retrieval of carbon monoxide (CO) vertical column densities from clear-sky and cloud contaminated 2311–2338 nm reflectance spectra measured by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) from January 2003 until the end of the mission in April 2012. These data were processed with the Shortwave Infrared CO Retrieval algorithm (SICOR) that we developed for the operational data processing of the Tropospheric Monitoring Instrument (TROPOMI) that will be launched on ESA's Sentinel-5 Precursor (S5P) mission. This study complements previous work that was limited to clear-sky observations over land. Over the oceans, CO is estimated from cloudy-sky measurements only, which is an important addition to the SCIAMACHY clear-sky CO data set as shown by NDACC and TCCON measurements at coastal sites. For Ny-Ålesund, Lauder, Mauna Loa and Reunion, a validation of SCIAMACHY clear-sky retrievals is not meaningful because of the high retrieval noise and the few collocations at these sites. The situation improves significantly when considering cloudy-sky observations, where we find a low mean bias b = ±6. 0 ppb and a strong correlation between the validation and the SCIAMACHY results with a mean Pearson correlation coefficient r = 0. 7. Also for land observations, cloudy-sky CO retrievals present an interesting complement to the clear-sky data set. For example, at the cities Tehran and Beijing the agreement of SCIAMACHY clear-sky CO observations with MOZAIC/IAGOS airborne measurements is poor with a mean bias of b = 171. 2 ppb and 57.9 ppb because of local CO pollution, which cannot be captured by SCIAMACHY. For cloudy-sky retrievals, the validation improves significantly. Here the retrieved column is mainly sensitive to CO above the cloud and so not affected by the strong local surface emissions. Adjusting the MOZAIC/IAGOS measurements to the vertical sensitivity of the retrieval, the mean bias adds up to b = 52. 3 ppb and 5.0 ppb for Tehran and Beijing. At the less urbanised region around the airport Windhoek, local CO pollution is less prominent and so MOZAIC/IAGOS measurements agree well with SCIAMACHY clear-sky retrievals with a mean bias of b = 15. 5 ppb, but can be even further improved for cloudy SCIAMACHY observations with a mean bias of b = 0. 2 ppb. Overall the cloudy-sky CO retrievals from SCIAMACHY short-wave infrared measurements present a major extension of the clear-sky-only data set, which more than triples the amount of data and adds unique observations over the oceans. Moreover, the study represents the first application of the S5P algorithm for operational CO data processing on cloudy observations prior to the launch of the S5P mission.

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Langowski, M., C. von Savigny, J. P. Burrows, W. Feng, J. M. C. Plane, D. R. Marsh, D. Janches, M. Sinnhuber et A. C. Aikin. « Global investigation of the Mg atom and ion layers using SCIAMACHY/Envisat observations between 70 km and 150 km altitude and WACCM-Mg model results ». Atmospheric Chemistry and Physics Discussions 14, n^o 2 (22 janvier 2014) : 1971–2019. http://dx.doi.org/10.5194/acpd-14-1971-2014.

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Abstract. Mg and Mg+ concentration fields in the upper mesosphere/lower thermosphere (UMLT) region are retrieved from SCIAMACHY/Envisat limb measurements of Mg and Mg+ dayglow emissions using a 2-D tomographic retrieval approach. The time series of monthly means of Mg and Mg+ for number density as well as vertical column density in different latitudinal regions are shown. Data from the limb mesosphere-thermosphere mode of SCIAMACHY/Envisat are used, which covers the 50 km to 150 km altitude region with a vertical sampling of 3.3 km and a highest latitude of 82°. The high latitudes are not covered in the winter months, because there is no dayglow emission during polar night. The measurements were performed every 14 days from mid-2008 until April 2012. Mg profiles show a peak at around 90 km altitude with a density between 750 cm−3 and 2000 cm−3. Mg does not show strong seasonal variation at mid-latitudes. The Mg+ peak occurs 5–15 km above the neutral Mg peak at 95–105 km. Furthermore, the ions show a significant seasonal cycle with a summer maximum in both hemispheres at mid- and high-latitudes. The strongest seasonal variations of the ions are observed at mid-latitudes between 20–40° and densities at the peak altitude range from 500 cm−3 to 6000 cm−3. The peak altitude of the ions shows a latitudinal dependence with a maximum at mid-latitudes that is up to 10 km higher than the peak altitude at the equator. The SCIAMACHY measurements are compared to other measurements and WACCM model results. In contrast to the SCIAMACHY results, the WACCM results show a strong seasonal variability for Mg with a winter maximum, which is not observable by SCIAMACHY, and globally higher peak densities. Although the peak densities do not agree the vertical column densities agree, since SCIAMACHY results show a wider vertical profile. The agreement of SCIAMACHY and WACCM results is much better for Mg+, showing the same seasonality and similar peak densities. However, there are the following minor differences: there is no latitudinal dependence of the peak altitude for WACCM and the density maximum, passing the equatorial region during equinox conditions, is not reduced as for SCIAMACHY.

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Noël, S., K. Bramstedt, A. Rozanov, H. Bovensmann et J. P. Burrows. « Water vapour profiles from SCIAMACHY solar occultation measurements derived with an onion peeling approach ». Atmospheric Measurement Techniques Discussions 3, n^o 1 (15 janvier 2010) : 203–35. http://dx.doi.org/10.5194/amtd-3-203-2010.

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Abstract. A new retrieval method has been developed to derive water vapour number density profiles from solar occultation measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY). This method is intentionally kept simple and based on a combination of an onion peeling approach with a modified DOAS (Differential Optical Absorption Spectroscopy) fit in the wavelength region around 940 nm. Reasonable resulting water vapour profiles are currently obtained in the altitude range 15–45 km. Comparisons of the SCIAMACHY profiles with water vapour data provided by the Atmospheric Chemistry Explorer Fourier Transform Spectrometer (ACE-FTS) show an average agreement within about 5% between 20 and 45 km. SCIAMACHY water vapour data tend to be systematically higher than ACE-FTS. These results are in principal confirmed by comparisons with water vapour profiles derived from model data of the European Centre for Medium Range Weather Forecasts (ECMWF), although ECMWF concentrations are systematicly lower than both corresponding SCIAMACHY and ACE-FTS data at all altitudes.

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