Geostationary Emission Explorer for Europe (G3E): mission concept and initial performance assessment

Atmos. Meas. Tech., 8, 4719–4734, 2015 www.atmos-meas-tech.net/8/4719/2015/ doi:10.5194/amt-8-4719-2015 © Author(s) 2015. CC Attribution 3.0 License. Geostationary Emission Explorer for Europe (G3E): mission concept and initial performance assessment A. Butz1 , J. Orphal1 , R. Checa-Garcia1 , F. Friedl-Vallon1 , T. von Clarmann1 , H. Bovensmann2 , O. Hasekamp3 , J. Landgraf3 , T. Knigge4 , D. Weise4 , O. Sqalli-Houssini4 , and D. Kemper4 1 IMK-ASF, Karlsruhe Institute of Technology (KIT), Leopoldshafen, Germany 2 Institute for Environmental Physics, University of Bremen, Bremen, Germany 3 Netherlands Institute for Space Research (SRON), Utrecht, the Netherlands 4 Airbus Defence and Space, Friedrichshafen, Germany Correspondence to: A. Butz () Received: 25 May 2015 – Published in Atmos. Meas. Tech. Discuss.: 8 July 2015 Revised: 23 October 2015 – Accepted: 2 November 2015 – Published: 10 November 2015 Abstract. The Geostationary Emission Explorer for Europe (G3E) is a concept for a geostationary satellite sounder that aims to constrain the sources and sinks of greenhouse gases carbon dioxide (CO2 ) and methane (CH4 ) for continental-scale regions. Its primary focus is on central Europe. G3E carries a spectrometer system that collects sunlight backscattered from the Earth’s surface and atmosphere in the near-infrared (NIR) and shortwave-infrared (SWIR) spectral range. Solar absorption spectra allow for spatiotemporally dense observations of the column-average concentrations of carbon dioxide (XCO2 ), methane (XCH4 ), and carbon monoxide (XCO). The mission concept in particular facilitates sampling of the diurnal variation with several measurements per day during summer. Here, we present the mission concept and carry out an initial performance assessment of the retrieval capabilities. The radiometric performance of the 4 grating spectrometers is tuned to reconcile small ground-pixel sizes (∼ 2 km × 3 km at 50◦ latitude) with short single-shot exposures (∼ 2.9 s) that allow for sampling continental regions such as central Europe within 2 h while providing a sufficient signal-to-noise ratio. The noise errors to be expected for XCO2 , XCH4 , and XCO are assessed through retrieval simulations for a European trial ensemble. Generally, single-shot precision for the targeted XCO2 and XCH4 is better than 0.5 % with some exception for scenes with low infrared surface albedo observed under low sun conditions in winter. For XCO, precision is generally better than 10 %. Performance for aerosol and cirrus loaded atmospheres is assessed by mimicking G3E’s slant view on Europe for an ensemble of atmospheric scattering properties used previously for evaluating nadir-viewing lowEarth-orbit (LEO) satellites. While retrieval concepts developed for LEO configurations generally succeed in mitigating aerosol- and cirrus-induced retrieval errors for G3E’s setup, residual errors are somewhat greater in geostationary orbit (GEO) than in LEO. G3E’s deployment in the vicinity of the Meteosat Third Generation (MTG) satellites has the potential to make synergistic use of MTG’s sounding capabilities e.g. with respect to characterization of aerosol and cloud properties or with respect to enhancing carbon monoxide retrievals by combining G3E’s solar and MTG’s thermal infrared spectra. 1 Introduction Satellite remote sensing of man-made greenhouse gases has been suggested as a key enabling technology to facilitate policy-relevant monitoring of anthropogenic emissions and their interaction with the biogeochemical environment (e.g. Rayner and O’Brien, 2001; Ciais et al., 2014). Nadir-viewing satellite instruments such as the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) (Burrows et al., 1995; Bovensmann et al., 1999; Gottwald and Bovensmann, 2011), the Greenhouse Gases observing SATellite (GOSAT) (Kuze et al., 2009), and the Orbiting Carbon Observatory (OCO-2) (Crisp et al., 2004) demonstrate that the employed solar backscatter technique Published by Copernicus Publications on behalf of the European Geosciences Union. 4720 A. Butz et al.: Geostationary Emission Explorer for Europe (G3E) is able to deliver column-average concentrations of carbon dioxide (XCO2 ) and methane (XCH4 ) (Frankenberg et al., 2005; Butz et al., 2011; Reuter et al., 2011; O’Dell et al., 2012) approaching the accuracy goal on the sub-percent level (e.g. Miller et al., 2007; Chevallier et al., 2007; Bergamaschi et al., 2007). The inferred concentration fields allow for detecting anthropogenic emissions over source regions such as urban centers and major fossil fuel production sites (e.g. Schneising et al., 2013; Kort et al., 2012; Reuter et al., 2014a; Kort et al., 2014). Likewise, SCIAMACHY- and GOSATderived XCO2 and XCH4 have been shown to successfully constrain patterns of biogeochemical sources and sinks either by feeding the satellite soundings into inverse models or by correlating observed concentration variability with climate variables (e.g. Bergamaschi et al., 2009; Guerlet et al., 2013a; Parazoo et al., 2013; Schneising et al., 2013; Ross et al., 2013; Basu et al., 2014; Reuter et al., 2014b). The spatiotemporal resolution of current and upcoming satellite missions, however, is insufficient to reliably monitor point source emissions and to budget diffuse biogeochemical sources and sinks on regional scales (100 km × 100 km) (Hungershoefer et al., 2010). Overcoming this limitation, Bovensmann et al. (2010) and Buchwitz et al. (2013) suggest the dedicated greenhouse gas mission CarbonSat that employs imaging capabilities to map the ground scene over a swath of a few hundred kilometers with about 2 km × 2 km horizontal resolution. Together, the imaging capabilities and the high spatial resolution permit contrasting the foreground emission plumes to background concentrations by exploiting the spatiotemporal context of the scene. The Sentinel-5 Precursor (S5P) (Veefkind et al., 2012), due for launch in 2016, and the post-2020 Sentinel-5 (S5) (Ingmann et al., 2012) will target XCH4 only (e.g. Butz et al., 2012) with a viewing swath exceeding 1000 km but only moderate horizontal resolution of several tens km2 . Velazco et al. (2011) examine a satellite constellation concept with five CarbonSatlike satellites. Such a constellation simultaneously allows for daily coverage and high horizontal resolution and thus, delivers improved capabilities to constrain anthropogenic emissions. Common to the current and next-generation greenhouse gas sounders is their deployment in low-Earth orbit (LEO) which favors global coverage. However, depending on the exact orbit altitude and on the instruments’ imaging capabilities, LEO restricts the number of revisits to a few per month per location for instruments with high spatial resolution such as CarbonSat. Daily revisits either require a constellation of satellites or come at the expense of only moderate spatial resolution such as for S5P and S5. In con (...truncated)