What is it?
“Synergistic SWIR and IR retrievals of near-surface concentrations of CH4 and CO for Earth and Planetary atmospheres” was an ESA contract study (ESA contract number 4000107088) started in November 2012 and finished in December 2014. The consortium was led by NOVELTIS (management & engineering) and included BIRA (Belgium), Netherlands for space research – SRON (the Netherlands), ULB (Belgium), IAPS (Italy). The goal of the study was to review in a multidisciplinary context (Earth Observation and Planetary Science) the status of existing retrieval algorithms and to extend their capabilities when applied to remote sensing data collected (or to be collected in the near future) by instruments in orbit around the planets Earth and Mars, with atmospheric CO – Carbon monoxide and CH4 – Methane as specific targets. A special focus was placed on deriving the near-surface concentrations of these two species (i.e. getting information on the atmospheric region called the planetary boundary layer or PBL).
This specific objective is related to the detection and quantification of CH4 and CO sources which are located at the surface, but which impact (through atmospheric transport and photochemical processes) the composition of the global atmosphere of these two sister planets of our solar system.
There is a need for adequate knowledge of the sources of CO – Carbon monoxide and CH4 – Methane gases and their climate feedbacks. CO and CH4 concentrations are derived from remotely sensed measurements are then coupled with inverse modelling schemes;
Remote sensing of CO and CH4 from space can usually be performed in different spectral domains and with different geometries:
- Thermal infrared (TIR) nadir observations: total column and profile observations in the strong gas absorption wavelength regions (thermal emission from the surface and atmosphere)
- or short-wave infrared (SWIR) nadir observations: total column densities with equal sensitivity throughout the atmospheric column (solar reflected/backscattered radiation)
- Solar occultation or bright limb observations: vertical distributions.
The objectives were to develop and assess the performance of synergistic retrieval algorithms for near-surface concentrations of CH4 and CO from SWIR and IR passive remote sensing measurements for Earth and Mars atmospheres
The following key issues were addressed:
- Specification of the user requirements in terms of remote sensing Level 2 products
- Definition and implementation of synergistic retrieval methodologies
- Maximization of cross-fertilization between the Earth and Planetary scientific expertise
- Recommendations on the needs and specifications on co-localization of the SWIR and IR measurements
In a first task, the existing requirements, algorithms and data sets related to the retrieval of CO and CH4 in the atmospheres of the two planets have been reviewed. In a second task, specific algorithms have been selected as effectively running on the premises of the 4 academic institutes participating in the study. The satellite data sets (level 1 or L1 products i.e. spectra) available for testing the retrieval algorithms have been identified and collected (in some cases with a pre-processing as the spectral calibration of GOSAT spectra performed by NOVELTIS). The correlative measurements (other satellites, aircraft and ground-based stations) appropriate for validating the retrieved concentrations (level 2 or L2 products) have also been chosen and collected.
The third task was devoted to real retrievals and comparison exercises and have been separated by planet (Earth and Mars) and by type of retrieval (stand alone or non-synergistic and synergistic). Indeed for getting the best information on the full atmospheric profile of the target species (including their concentration in the PBL) it is expected that the combination (at the input of the inversion process) of L1 information originating from different sensors could be more efficient to generate reliable L2 products, than the separate generation of L2 products (total column, partial columns or sub-columns, profiles) with their strength and weaknesses. For example total column derived from nadir viewing instruments in the solar reflected shortwave infrared (SWIR) spectral region can be relatively precise since the light path through atmosphere is reaching the surface, but they do not provide vertical profile information.
Conversely nadir viewing instruments operating in the thermal infrared regions have a sensitivity to the vertical distribution of the species because of the dependence of the received signal to the temperature profile, but their sensitivity is reduced in the PBL when the thermal contrast is low (i.e. when the surface temperature is equal to the temperature of the first atmospheric layer). Many of the possible spectral L1/L1 or L1/L2 synergies have been studied: L1(TIR,GOSAT)/L1(SWIR,GOSAT) or L1(TIR,IASI-NG)/L1(SWIR,S5) for Earth, L1(TIR,PFS)/L2(TIR T(z),PFS) or L1(SWIR,NOMAD)/L1(SWIR,ACS) for Mars.
The following synergistic scenarios were therefore analysed:
- CH4 retrieval: GOSAT SWIR and TIR (nadir view) using B2 (1.6 µm) and B4 (7.7 µm), Sentinel-5-UVNS SWIR2 (1.6 µm) and IASI-NG TIR (7.7 µm) using synthetic data (nadir view), ACE-FTS (7.7 and 3.3 µm) solar occultation and IASI nadir (TIR, 7.7 µm)
- CO retrieval: Sentinel-5-UVNS SWIR2 (2.3 µm) and IASI-NG TIR (4.7 µm) synthetic data (nadir view), ACE-FTS solar occultation and IASI nadir (TIR, 4.7 µm) measurements
- CH4 retrieval: PFS-SW & PFS-LW (nadir view): 3.3 µm (CH4, MIR) & T(p) from CO2 (TIR), NOMAD (MIR) & ACS (TIR) solar occultation ® use L2/L1 synergy, NOMAD (MIR) & ACS (TIR) nadir view ® NOMAD alone at 3.3 µm is best
- CO retrieval: PFS-SW & PFS-LW (nadir view): CO at 4.7 µm (TIR) and 2.3 (SWIR) & T(p) from CO2 (TIR) L1/L2 synergies, NOMAD (MIR, 2.3 µm) & ACS (TIR, 4.7 µm) solar occultation, NOMAD (MIR, 2.3 µm) & ACS (TIR, 4.7 µm) nadir view ® L1/L1 synergy, NOMAD (MIR, 2.3 µm) nadir view & solar occultation geometric synergy.
The main obtained results are:
- Earth: TIR/SWIR spectral synergy clearly contributes to provide a better combined product for the vertical profiles and 0-2 km sub column of CO over land
- Earth: The efficiency and potential of TIR/SWIR synergistic retrievals has been demonstrated for CH4 on real GOSAT spectra. It allows retrieving meaningful separate sub-columns in the 0-5 km and 5 km-TOA altitude ranges. However, systematic biases in the TIR retrievals have been identified. Further work is needed to identify the causes of SWIR and TIR inconsistencies. For CH4, even if different SWIR windows can be used, progress in the methane spectroscopy is needed in the TIR band around 7.7 µm.
- Mars: The potential for CO profile retrievals using PFS TIR spectra at 4.7 µm (hence near-surface concentrations) has been demonstrated and should be implemented in further studies
- Mars: Synergistic retrievals (geometric or spectral) with ASIMUT have proved to be rather efficient. L2(nadir)/L1(occultation) synergy potentially interesting for low CH4 concentrations
In summary, many of the possible spectral L1/L1 or L1/L2 synergies have been studied: L1(TIR,GOSAT)/L1(SWIR,GOSAT) or L1(TIR,IASI-NG)/L1(SWIR,S5) for Earth, L1(TIR,PFS)/L2(TIR T(z),PFS) or L1(SWIR,NOMAD)/L1(SWIR,ACS) for Mars. Remaining difficulties have been identified for CH4 (with recommendations to solve them), but promising results have been obtained both for CO and CH4 and for both planets benefiting from cross-fertilisation.
- NOVELTIS (France): Carine Petit (project manager – from mid-term meeting to the end), Julien Chimot (proposal manager, project manager from kick-off to midterm meeting), Dr. Pascal Prunet, Dr. Claude Camy-Peyret, Dr. Olivier Lezeaux
- BIRA (Belgium): Dr. Ann Carine Vandaele, Dr. Martine de Maziere, Dr. Severine RObert, Dr. Evelyn de Wachter
- ULB (Belgium): Dr. Pierre-Francois Coheur, Sophie Bauduin, Rosa Astoreca
- SRON (the Netherlands): Dr. ochen Landgraf, IDr. lse Aben, Arno de Lange
- IAPS (Italy): Dr. Marco guiranna, Shoei Aoki, Alessandro Aronica
- ESA – ESTEC (the Netherlands): Dr. Anne Grete Straume-Lindner, Olivier Witasse
- Conference paper proceeding by Robert et al., 2014, at the Mars workshop here
- Description in the general studies programme (GSP) section of ESA + executive summary here
- CH4 retrieval from GOSAT TIR, in the frame of the SIROCCO project, by SRON here
- SIROCCO poster at the ESA ATMOS conference, in Crete 2015 here
- CH4 – Methane here
- CO – Carbon monoxide here