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The GHG-CCI project is one of several projects of the European Space Agency's (ESA) Climate Change Initiative (CCI). The goal of the CCI is to generate and deliver data sets of various satellite-derived Essential Climate Variables (ECVs) in line with GCOS (Global Climate Observing System) requirements. The “ECV Greenhouse Gases” (ECV GHG) is the global distribution of important climate relevant gases – atmospheric CO 2 and CH 4 – with a quality sufficient to obtain information on regional CO 2 and CH 4 sources and sinks. Two satellite instruments deliver the main input data for GHG-CCI: SCIAMACHY/ENVISAT and TANSO-FTS/GOSAT. The first order priority goal of GHG-CCI is the further development of retrieval algorithms for near-surface-sensitive column-averaged dry air mole fractions of CO 2 and CH 4 , denoted XCO 2 and XCH 4 , to meet the demanding user requirements. GHG-CCI focuses on four core data products: XCO 2 from SCIAMACHY and TANSO and XCH 4 from the same two sensors. For each of the four core data products at least two candidate retrieval algorithms have been independently further developed and the corresponding data products have been quality-assessed and inter-compared. This activity is referred to as “Round Robin” (RR) activity within the CCI. The main goal of the RR was to identify for each of the four core products which algorithms should be used to generate the Climate Research Data Package (CRDP). The CRDP will essentially be the first version of the ECV GHG. This manuscript gives an overview of the GHG-CCI RR and related activities. This comprises the establishment of the user requirements, the improvement of the candidate retrieval algorithms and comparisons with ground-based observations and models. The manuscript summarizes the final RR algorithm selection decision and its justification. Comparison with ground-based Total Carbon Column Observing Network (TCCON) data indicates that the “breakthrough” single measurement precision requirement has been met for SCIAMACHY and TANSO XCO 2 (< 3 ppm) and TANSO XCH 4 (< 17 ppb). The achieved relative accuracy for XCH 4 is 3–15 ppb for SCIAMACHY and 2–8 ppb for TANSO depending on algorithm and time period. Meeting the 0.5 ppm systematic error requirement for XCO 2 remains a challenge: approximately 1 ppm has been achieved at the validation sites but also larger differences have been found in regions remote from TCCON. More research is needed to identify the causes for the observed differences. In this context GHG-CCI suggests taking advantage of the ensemble of existing data products, for example, via the EnseMble Median Algorithm (EMMA). [ABSTRACT FROM AUTHOR]

The European environmental satellite ENVISAT was successfully launched on 1st of March 2002. The UV/visible/near-infrared grating spectrometer SCIAMACHY is part of ENVISAT''s atmospheric science payload. SCIAMACHY observes the atmosphere in nadir, limb, and solar and lunar occultation viewing geometries with moderate spectral resolution (0.2–1.5 nm). At the University of Bremen a modified DOAS algorithm (WFM-DOAS) is being developed primarily for the retrieval of CH4, CO, CO2, H2O, N2O, and O2 total column amounts from ratios of SCIAMACHY nadir radiance and solar irradiance spectra in the near-infrared and visible spectral regions. First preliminary results concerning this activity are presented. SCIAMACHY is currently (September 2002) in its commissioning phase and only preliminary, i.e., not yet fully calibrated, Level 1 data products are available, generated mainly for initial Level 0 to 1 processing verification purposes. A method aimed at improving the retrieval in case of systematic artifacts resulting from, e.g., residual calibration errors, is presented. This study focuses on methane vertical column retrieval using channel 8 (2260–2385 nm). One of the major scientific objectives of the SCIAMACHY methane measurements is to derive information on methane (surface) sources and sinks. Such an application requires a relative radiometric accuracy close to the signal-to-noise performance of the instrument (S/N ∼50–100 in channel 8 for albedo 0.1 and solar zenith angle 60°) and an accurate and fast retrieval algorithm. This study presents first steps undertaken to reach this ambitious goal, focusing on the retrieval algorithm. [Copyright &y& Elsevier]

The SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY) is part of the European satellite ENVISAT launched at 1st March 2002. It is a follow-on instrument of the Global Ozone Monitoring Experiment (GOME) flying on ERS-2 and, compared to GOME, has extended capabilities. Using measurements of the direct extra-terrestrial solar spectrum and sun-light reflected and scattered by the earth atmosphere or surface SCIAMACHY detects atmospheric absorption of several trace species absorbing in spectral regions from the ultraviolet to the near infrared (240–2380 nm). Vertical columns of H2O, N2O, CO, and CH4 can be retrieved using SCIAMACHY Channel 8 near-infrared nadir measurements. In this study, selected atmospheric and instrument specific errors relevant for the retrieval are analysed. Spectral windows of Channel 8 are considered, which are currently used for the operational near-real-time processing. For this purpose, spectral error patterns have been simulated as well as sun-normalised model radiances for nadir scanning mode, the latter using the radiative transfer model SCIATRAN. Focus are the polarisation sensitivity and dark signals of the instrument. Further on, accuracy estimates for a number of different atmospheric scenarios are presented. [Copyright &y& Elsevier]

A radiative transfer model, SCIATRAN, has been developed mainly for the retrieval of atmospheric constituents from global nadir radiance measurements of the SCIAMACHY satellite spectrometer. This is a further development of the successful GOMETRAN. SCIATRAN solves the radiative transfer equation using the Finite Difference Method for a plane-parallel vertically inhomogeneous atmosphere taking into account multiple scattering. The present program version utilizes the pseudo-spherical approach, including refraction, appropriate for solar zenith angles up to about 92°. SCIATRAN comprises, amongst others, the following features: (i) quasi-analytical calculation of weighting functions of atmospheric and surface parameters, (ii) DOAS airmass factor calculations, (iii) height resolved radiation fluxes, including actinic fluxes for photolysis rate calculations, (iv) inelastic rotational Raman scattering by N2 and O2 molecules, (v) parameterization schemes for aerosols and clouds, (vi) a line-by-line and a correlated-k distribution mode for line absorption, and (vii) thermal emission. This paper presents an overview of the SCIATRAN model, focusing on the pseudo-spherical approach developed to consider the sphericity of the Earth. SCIATRAN will be made available via the world wide web for non-commercial scientific applications. [Copyright &y& Elsevier]

A method for the retrieval of vertical column amounts of atmospheric water vapour from measurements of the Global Ozone Monitoring Experiment (GOME) in the visible spectral region is presented. Originally developed for the determination of global ozone concentrations, the GOME instrument is operating successfully on ERS-2 since 1995, providing information also on several other atmospheric constituents like NO2, BrO, OClO, H2CO, SO2, and recently also H2O. The method for the derivation of water vapour amounts is based on the Differential Optical Absorption Spectroscopy (DOAS) approach, which has been extended to take into account effects arising from a strongly wavelength dependent absorption. Further improvements of the algorithm now allow to retrieve water vapour column densities under all cloud conditions. First results show an acceptable agreement between total water vapour columns derived from GOME measurements and H2O columns obtained from the Special Sensor Microwave Imager SSM/I data, although the scatter of the GOME data is high, which needs to be further investigated. [Copyright &y& Elsevier]

Abstract: Pressing open questions about the carbon cycle can be addressed with precise measurements of the three most abundant CO2 isotopologues 16O12C16O, 16O13C16O, and . Such measurements can, e.g., help to further constrain oceanic and biospheric net fluxes or to differentiate between the gross biospheric fluxes photosynthesis and respiration. The (about ) spectral region contains separated absorption lines of the three most abundant CO2 isotopologues. Their spectral properties make this spectral region well suited for the use of a light path proxy method for the retrieval of and (the ratio of heavier to lighter isotopologues relative to a standard). An optimal estimation based light path proxy retrieval for and has been set up, applicable to GOSAT (Greenhouse gases Observing Satellite) and ground-based FTS (Fourier transform spectrometer) measurements. Initial results show that it is possible to retrieve and from ground-based FTS instruments with a precision of and from GOSAT with a precision of about . Comparison of the achievable precision with the expected atmospheric signals shows that ground-based FTS remote sensing measurements have the potential to gain valuable information on and if averaging a sufficient number of measurements. It seems unlikely that this applies also to GOSAT because of the lower precision and a conceptual larger sensitivity to scattering related errors in satellite viewing geometry. [Copyright &y& Elsevier]

Abstract: A successor version of the SCIATRAN radiative transfer model (RTM) has been developed to perform radiative transfer modeling in any observation geometry appropriate to measurements of the scattered solar radiation in the Earth’s atmosphere. The model is designed to be used as a forward model in the retrieval of atmospheric constituents from measurements of scattered solar light by satellite, ground-based, or airborne instruments in UV–Vis–NIR spectral region. Furthermore, it can be used to calculate air mass factors or fluxes. The new generation of the SCIATRAN model comprises all features of the latest SCIATRAN 1.2 RTM supporting additionally radiative transfer calculations in a spherical atmosphere. The program is written in FORTRAN 95 and suitable for parallel execution using the OpenMP standard. The wavelength range covered by the radiative transfer model is extended to 175–2380nm including Schuman-Runge and Herzberg absorption bands of oxygen. The SCIATRAN 2.0 model exhibits the following new capabilities: (i) modeling of the scattered solar radiation in limb viewing geometry as well as any kind of measurements of the scattered radiation within the atmosphere, (ii) corresponding quasi-analytical calculation of weighting functions of atmospheric parameters, (iii) airmass factor calculations for ground-based, space and airborne measurements including off-axis geometry, (v) accounting for photochemically active species, i.e., radiative transfer calculations can be performed using solar zenith angle dependent vertical distributions of atmospheric species, (iv) height resolved radiation fluxes, including actinic fluxes for photolysis rate calculations, (vi) inelastic rotational Raman scattering in any supported viewing geometry, (vii) new effective approximations for radiative transfer modeling in presence of clouds. The SCIATRAN model is freely available via the world wide web for non-commercial scientific applications. [Copyright &y& Elsevier]

Since 1995, the Global Ozone Monitoring Experiment (GOME) on board the second European Remote Sensing satellite provides information about spatial distribution and temporal variation of total ozone. Current operational data products are generated using the Differential Optical Absorption Spectroscopy (DOAS) technique. Various validation exercises indicate shortcomings in the current retrieval using the standard DOAS approach. Especially under ozone hole conditions at high polar latitudes GOME tends to overestimate total ozone as compared to ground-based measurements. In this study, we introduce a modified DOAS method for ozone retrieval from GOME which fits directly the vertical columns rather than slant columns. We present preliminary results which demonstrate the advantage of the new approach. It will be shown that not only the differences to ground-based data are reduced but also the fit residuals are improved in comparison with the operational retrieval. [Copyright &y& Elsevier]

We present two novel earth observation products derived from the BESD and EMMA XCO 2 products which were respectively retrieved from SCIAMACHY and GOSAT observations within the GreenHouse Gas project of ESA's Climate Change Initiative (GHG-CCI). These products are inferred by a Carbon Cycle Data Assimilation System (CCDAS) and consist of net and gross biosphere-atmosphere fluxes of carbon dioxide on a global 0.5° grid. As a further dataset provided by the CCI, the burnt area product developed by its Fire忌i project was used in the CCDAS to prescribe the emission component from biomass burning. The new flux products are provided with per-pixel uncertainty ranges. Fluxes with uncertainty ranges can also be provided aggregated in space and time, e.g. over given regions or as annual means. For both, posterior flux fields inferred from BESD and EMMA products, transport model simulations show reasonable agreement with the atmospheric carbon dioxide concentration observed at flask sampling stations. This means that the information provided by the terrestrial and transport models, the respective GHG ECV product, the burnt area ECV product, a product of the Fraction of Absorbed Photosynthetically Active Radiation used to drive the model, and the atmospheric flask samples is largely consistent. The most prominent feature in the posterior net flux is the tropical source of CO 2 inferred from both products. But for the EMMA product this release, especially over South America, is with 300 gC/m 2 /year much more pronounced than for BESD. This confirms findings by a recent intercomparison of transport inversions using GOSAT data by Houweling et al. (2015) . The reason for the larger net flux is increased heterotrophic respiration. For both products the posterior 2010 sink over Europe (without Russia) is in the range of a recent compilation of European flux estimates by Reuter et al. (2016b) . The posterior 2010 uptake of Australia (including Oceania) inferred from the EMMA product is 1.3 ± 0.2 PgC/year and appears to confirm the high sink also derived from GOSAT by Detmers et al. (2015) over a slightly different period and area. While for some regions (USA, Canada, Europe, Russia, Asia) the one standard deviation uncertainty ranges derived from BESD and EMMA do overlap, for some other regions (Brazil, Africa, Australia) this is not the case. It is not clear yet whether this is due to the uncertainty specifications in the respective products or the handling of uncertainty in the assimilation chain. Assumptions on correlation of observational uncertainty in space and time have a considerable impact on the inferred flux fields (≈ 60 gC/m 2 /year). The effect of adding an uncertainty that approximates the error in the retrieval system is of similar size. [ABSTRACT FROM AUTHOR]