New algorithms for atmospheric correction and retrieval of biophysical parameters in earth observation : application to ENVISAT/MERIS data

An algorithm for the derivation of atmospheric and surface biophysical products from the MEdium Resolution Imaging Spectrometer Instrument (MERIS) on board ENVIronmental SATellite (ENVISAT/MERIS) Level 1b data over land has been developed. Georectified aerosol optical thickness (AOT), columnar water vapor (CWV), spectral surface reflectance and chlorophyll fluorescence (CF) maps are generated. Emphasis has been put on implementing a robust software able to provide those products on an operational manner, making no use of ancillary parameters apart from those attached to MERIS images. For this reason, it has been named Self-Contained Atmospheric Parameters Estimation from MERIS data (SCAPE-M). The fundamentals of the algorithm and the validation of the derived products are presented in this thesis. Errors of ±0.03, ±4% and ±8% have been estimated for AOT, CWV and surface reflectance retrievals, respectively, by means of a sensitivity analysis. More than 200 MERIS images have been processed in order to assess the method performance under a range of atmospheric and geographical conditions. A good comparison is found between SCAPE-M AOT retrievals and ground-based measurements taken during the SPectra bARrax Campaigns (SPARC) 2003 and 2004, except for a date when an episode of Saharan dust intrusion was detected. Comparison of SCAPE-M retrievals with data from AErosol RObotic NETwork (AERONET) stations showed a square Pearson's correlation coefficient R2 of about 0.7-0.8. Those values grow up to more than 0.9 in the case of CWV after comparison with the same stations. A good correlation is also found with the ESA Level 2 official CWV product, although slight different performances with varying surface elevation are detected. Retrieved surface reflectance maps have been intercompared with reflectance data derived from MERIS images by the Bremen AErosol Retrieval (BAER) method in the first place

Satellites Detect Abatable Super-Emissions in One of the World¿s Largest Methane Hotspot Regions

[EN] Reduction of fossil fuel-related methane emissions has been identified as an essential means for climate change mitigation, but emission source identification remains elusive for most oil and gas production basins in the world. We combine three complementary satellite data sets to survey single methane emission sources on the west coast of Turkmenistan, one of the largest methane hotspots in the world. We found 29 different emitters, with emission rates >1800 kg/h, active in the 2017¿2020 time period, although older satellite data show that this type of emission has been occurring for decades. We find that all sources are linked to extraction fields mainly dedicated to crude oil production, where 24 of them are inactive flares venting gas. The analysis of time series suggests a causal relationship between the decrease in flaring and the increase in venting. At the regional level, 2020 shows a substantial increase in the number of methane plume detections concerning previous years. Our results suggest that these large venting point sources represent a key mitigation opportunity as they emanate from human-controlled facilities, and that new satellite methods promise a revolution in the detection and monitoring of methane point emissions worldwide.The authors thank the team that realized the TROPOMI instrument and its data products, consisting of the partnership between Airbus Defense and Space Netherlands, KNMI, SRON, and TNO, commissioned by NSO and ESA. Sentinel-5 Precursor is part of the EU Copernicus program, Copernicus (modified) Sentinel-5P data (2018-2020) have been used. We thank the Sentinel Hub service for providing the EO Browser service. Thanks to the Environmental Defense Fund (EDF) for providing data about the O&G fields of the study area, and the Carbon Limits group for contributing to the verification of the emission sources. We thank the Italian Space Agency for the PRISMA data used in this work. Dr. Yongguang Zhang from the University of Nanjing is also thanked for his support to get access to ZY1 AHSI data, and Dr. Javier Gorrono from Universitat Politecnica de Valencia for his assistance in the uncertainty estimations. Authors Itziar Irakulis-Loitxate and Luis Guanter received funding from ESA Contract 4000134929.Irakulis-Loitxate, I.; Guanter-Palomar, LM.; Joannes D. Maasakkers; Daniel Zavala-Araiza; Ilse Aben (2022). Satellites Detect Abatable Super-Emissions in One of the World¿s Largest Methane Hotspot Regions. Environmental Science & Technology (Online). 56(4):2143-2152. https://doi.org/10.1021/acs.est.1c048732143215256

A unified vegetation index for quantifying the terrestrial biosphere

[EN] Empirical vegetation indices derived from spectral reflectance data are widely used in remote sensing of the biosphere, as they represent robust proxies for canopy structure, leaf pigment content, and, subsequently, plant photosynthetic potential. Here, we generalize the broad family of commonly used vegetation indices by exploiting all higher-order relations between the spectral channels involved. This results in a higher sensitivity to vegetation biophysical and physiological parameters. The presented nonlinear generalization of the celebrated normalized difference vegetation index (NDVI) consistently improves accuracy in monitoring key parameters, such as leaf area index, gross primary productivity, and sun-induced chlorophyll fluorescence. Results suggest that the statistical approach maximally exploits the spectral information and addresses long-standing problems in satellite Earth Observation of the terrestrial biosphere. The nonlinear NDVI will allow more accurate measures of terrestrial carbon source/sink dynamics and potentials for stabilizing atmospheric CO2 and mitigating global climate change.G.C.-V. was supported by the European Research Council (ERC) under the ERC Consolidator Grant 2014 project SEDAL (647423). M.C.-T. and F.J.G.-H. were supported by the EUMETSAT Satellite Application Facility on Land Surface Analysis (LSA-SAF). SR research was financially supported by the NASA Earth Observing System MODIS project (grant NNX08AG87A). J.A.G. acknowledges the support of NASA ABoVE award number NNX15AT78A. S.W. acknowledges funding from the Emmy Noether Programme (GlobFluo project) of the German Research Foundation (GU 1276/1-1) as well as funding from the European Union's Horizon 2020 research and innovation program under grant agreement 776186 (CHE project) and agreement 776810 (VERIFY project).Camps-Valls, G.; Campos-Taberner, M.; Moreno-Martínez, Á.; Walther, S.; Duveiller, G.; Cescatti, A.; Mahecha, MD.... (2021). A unified vegetation index for quantifying the terrestrial biosphere. Science Advances. 7(9):1-11. https://doi.org/10.1126/sciadv.abc74471117

Quantifying methane emissions from the global scale down to point sources using satellite observations of atmospheric methane

[EN] We review the capability of current and scheduled satellite observations of atmospheric methane in the shortwave infrared (SWIR) to quantify methane emissions from the global scale down to point sources. We cover retrieval methods, precision and accuracy requirements, inverse and mass balance methods for inferring emissions, source detection thresholds, and observing system completeness. We classify satellite instruments as area flux mappers and point source imagers, with complementary attributes. Area flux mappers are high-precision (< 1 %) instruments with 0.1-10 km pixel size designed to quantify total methane emissions on regional to global scales. Point source imagers are fine-pixel (< 60 m) instruments designed to quantify individual point sources by imaging of the plumes. Current area flux mappers include GOSAT (2009-present), which provides a high-quality record for interpretation of long-term methane trends, and TROPOMI (2018-present), which provides global continuous daily mapping to quantify emissions on regional scales. These instruments already provide a powerful resource to quantify national methane emissions in support of the Paris Agreement. Current point source imagers include the GHGSat constellation and several hyperspectral and multispectral land imaging sensors (PRISMA, Sentinel-2, Landsat-8/9, WorldView-3), with detection thresholds in the 100-10 000 kg h(-1) range that enable monitoring of large point sources. Future area flux mappers, including MethaneSAT, GOSAT-GW, Sentinel-5, GeoCarb, and CO2M, will increase the capability to quantify emissions at high resolution, and the MERLIN lidar will improve observation of the Arctic. The averaging times required by area flux mappers to quantify regional emissions depend on pixel size, retrieval precision, observation density, fraction of successful retrievals, and return times in a way that varies with the spatial resolution desired. A similar interplay applies to point source imagers between detection threshold, spatial coverage, and return time, defining an observing system completeness. Expanding constellations of point source imagers including GHGSat and Carbon Mapper over the coming years will greatly improve observing system completeness for point sources through dense spatial coverage and frequent return times.This research has been supported by the Collaboratory to Advance Methane Science (CAMS) and the National Aeronautics and Space Administration, Earth Sciences Division (grant no. NNH20ZDA001N-CMS).Jacob, DJ.; Varon, DJ.; Cusworth, DH.; Dennision, PE.; Frankenberg, C.; Gautam, R.; Guanter-Palomar, LM.... (2022). Quantifying methane emissions from the global scale down to point sources using satellite observations of atmospheric methane. ATMOSPHERIC CHEMISTRY AND PHYSICS. 14:9617-9646. https://doi.org/10.5194/acp-22-9617-2022961796461

Satellites Detect a Methane Ultra-emission Event from an Offshore Platform in the Gulf of Mexico

[EN] Mitigation of methane emissions from fossil fuel extraction, processing, and transport is one of the most effective ways to slow global warming. Satellite-based methods are instrumental for the detection, characterization, and quantification of this type of emissions. However, despite the rapid development of satellite-based methane plume detection methods for terrestrial surfaces, there is still an important observational gap with respect to offshore oil and gas infrastructure, which accounts for roughly 30% of global production. In this work, we have used observations from the WorldView-3 and Landsat 8 satellite missions in a particular observation-illumination geometry to image offshore methane plumes from space. The study site is an offshore oil and gas production platform in the Gulf of Mexico, near the coast of Campeche, in one of Mexico¿s major oil producing fields. Our data suggest that the platform vented high volumes of methane during a 17-day ultra-emission event, amounting to 0.04 ± 0.01 Tg of methane (equivalent to 3.36 million tons of carbon dioxide) released to the atmosphere if integrated over time. Our results illustrate how satellites can detect methane plumes from offshore infrastructure, which represents a significant breakthrough in the monitoring of industrial methane emissions from space.The authors thank the European Space Agency and European Space Imaging for access to WV3 data through the third-party mission plan. Javier Gorrono is funded by an ESA Living Planet Fellowship (ESA Contract No. 4000130980/20/I-NS). Authors Itziar Irakulis-Loitxate, Javier Gorron~o, and Luis Guanter received funding from ESA contract 4000134929. Elena Sanchez-Garcia is thanked for her support for the selection of the study site, and Maxar Technologies, Inc., for the acquisition of WV3 SWIR data for this study.Irakulis-Loitxate, I.; Gorroño-Viñegla, J.; Zavala-Araiza, D.; Guanter-Palomar, LM. (2022). Satellites Detect a Methane Ultra-emission Event from an Offshore Platform in the Gulf of Mexico. Environmental Science & Technology Letters. 9(6):520-525. https://doi.org/10.1021/acs.estlett.2c002255205259

Coupled retrieval of the three phases of water from spaceborne imaging spectroscopy measurements

[EN] Measurements of reflected solar radiation by imaging spectrometers can quantify water in different states (solid, liquid, gas) thanks to the discriminative absorption shapes. We developed a retrieval method to quantify the amount of water in each of the three states from spaceborne imaging spectroscopy data, such as those from the German EnMAP mission. The retrieval couples atmospheric radiative transfer simulations from the MODTRAN5 radiative transfer code to a surface reflectance model based on the Beer-Lambert law. The model is inverted on a per-pixel basis using a maximum likelihood estimation formalism. Based on a unique coupling of the canopy reflectance model HySimCaR and the EnMAP end-to-end simulation tool EeteS, we performed a sensitivity analysis by comparing the retrieved values with the simulation input leading to an R-2 of 0.991 for water vapor and 0.965 for liquid water. Furthermore, we applied the algorithm to airborne AVIRIS-C data to demonstrate the ability to map snow/ice extent as well as to a CHRIS-PROBA dataset for which concurrent field measurements of canopy water content were available. The comparison between the retrievals and the ground measurements showed an overall R-2 of 0.80 for multiple crop types and a remarkable clustering in the regression analysis indicating a dependency of the retrieved water content from the physical structure of the vegetation. In addition, the algorithm is able to produce smoother and more physically-plausible water vapor maps than the ones from the band ratio approaches used for multispectral data, since biases due to background reflectance are reduced. The demonstrated potential of imaging spectroscopy to provide accurate quantitative measures of water from space will be further exploited using upcoming spaceborne imaging spectroscopy missions like PRISMA or EnMAP.This study is funded within the EnMAP scientific preparation program under the DLR Space Administration with resources from the German Federal Ministry for Economic Affairs and Energy, Berlin, Germany (grant ID 59EE1923) and the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences.Bohn, N.; Guanter-Palomar, LM.; Kuester, T.; Preusker, R.; Segl, K. (2020). Coupled retrieval of the three phases of water from spaceborne imaging spectroscopy measurements. Remote Sensing of Environment. 242:1-16. https://doi.org/10.1016/j.rse.2020.11170811624

Glacier Ice Surface Properties in South-West Greenland Ice Sheet: First Estimates From PRISMA Imaging Spectroscopy Data

[EN] Snow and ice melt processes on the Greenland Ice Sheet are a key in Earth's energy balance and are acutely sensitive to climate change. Melting dynamics are directly related to a decrease in surface albedo, amongst others caused by the accumulation of light-absorbing particles (LAPs). Featuring unique spectral patterns, these accumulations can be mapped and quantified by imaging spectroscopy. We present first results for the retrieval of glacier ice properties from the spaceborne PRISMA imaging spectrometer by applying a recently developed simultaneous inversion of atmospheric and surface state using optimal estimation. The image analyzed in this study was acquired over the South-West margin of the Greenland Ice Sheet in late August 2020. The area is characterized by patterns of both clean and dark ice associated with a high amount of LAPs deposited on the surface. We present retrieval maps and uncertainties for grain size, liquid water, and algae concentration, as well as estimated reflectance spectra for different surface properties. We then show the feasibility of using imaging spectroscopy to interpret multiband sensor data to achieve high accuracy, frequently repeated observations of changing snow and ice conditions. For example, the impurity index calculated from multiband Sentinel-3 Ocean and Land Colour Instrument measurements is dependent on dust particles, but we show that algae concentration alone can be predicted from this data with less than 20% uncertainty. Our study evidences that present and upcoming orbital imaging spectroscopy missions such as PRISMA, Environmental Mapping and Analysis Program, Copernicus Hyperspectral Imaging Mission, and the Surface Biology and Geology designated observable, can significantly support research of melting ice sheets.This work has been done in the frame of EnMAP, which is funded under the DLR Space Administration with resources from the German Federal Ministry of Economic Affairs and Energy (Grant No. 50 EE 0850) and contributions from DLR, GFZ, and OHB System AG. A portion of this research took place at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). US Government Support acknowledged. Open access funding enabled and organized by Projekt DEAL.Bohn, N.; Di Mauro, B.; Colombo, R.; Thompson, DR.; Susiluoto, J.; Carmon, N.; Turmon, MJ.... (2022). Glacier Ice Surface Properties in South-West Greenland Ice Sheet: First Estimates From PRISMA Imaging Spectroscopy Data. Journal of Geophysical Research: Biogeosciences. 127(3):1-21. https://doi.org/10.1029/2021JG006718121127

Mapping methane plumes at very high spatial resolution with the WorldView-3 satellite

[EN] The detection of methane emissions from industrial activities can help enable effective climate change mitigation strategies. These industrial emissions, such as from oil and gas (O&G) extraction and coal mining, typically occur as large plumes of highly concentrated gas. Different satellite missions have recently shown the potential to map such methane plumes from space. In this work, we report on the promising potential of the WorldView-3 (WV-3) satellite mission for methane mapping. This relies on its unique very high spatial resolution (up to 3.7¿m) data in the shortwave infrared part of the spectrum, which is complemented by a good spectral sampling of the methane absorption feature at 2300¿nm and a high signal to noise ratio. The proposed retrieval methodology is based on the calculation of methane concentration enhancements from pixel-wise estimates of methane transmittance at WV-3 SWIR band 7 (2235¿2285¿nm), which is positioned at a highly-sensitive methane absorption region. A sensitivity analysis based on end-to-end simulations has helped to understand retrieval errors and detection limits. The results have shown the good performance of WV-3 for methane mapping, especially over bright and homogeneous areas. The potential of WV-3 for methane mapping has been further tested with real data, which has led to the detection of 26 independent point emissions over different methane hotspot regions, such as O&G extraction fields in Algeria and Turkmenistan, and the Shanxi coal mining region in China. In particular, the detection of very small leaks (<¿100¿kg¿h¿1) from oil pipelines in Turkmenistan shows the unique capability of WV-3 for mapping industrial methane emissions from space. The mission includes pointing capabilities that allow for a daily revisit over these oil pipelines or other critical infrastructure.This research has been supported by the ESA (contract no. 4000134929) and the ESA Living Planet Fellowship (ESA contract no. 4000130980/20/I-NS).Sánchez-García, E.; Gorroño-Viñegla, J.; Irakulis-Loitxate, I.; Varon, DJ.; Guanter-Palomar, LM. (2022). Mapping methane plumes at very high spatial resolution with the WorldView-3 satellite. Atmospheric Measurement Techniques. 15(6):1657-1674. https://doi.org/10.5194/amt-15-1657-20221657167415

Mapping methane point emissions with the PRISMA spaceborne imaging spectrometer

The authors would like to thank the Italian Space Agency, and inparticular Ettore Lopinto (Mission Director) , for the PRISMA data used in this work. Markus Foote (University of Utah) is also thanked for making the MAG1C code available to the scientific community. Javier Gorrono is funded by the ESA Living Planet Fellowship. PRISMA data can be accessed through the Data Access tab of PRISMA's website.1Guanter-Palomar, LM.; Irakulis-Loitxate, I.; Gorroño-Viñegla, J.; Sánchez-García, E.; Cusworth, DH.; Varon, DJ.; Cogliati, S.... (2021). Mapping methane point emissions with the PRISMA spaceborne imaging spectrometer. Remote Sensing of Environment. 265:1-14. https://doi.org/10.1016/j.rse.2021.112671S11426

Assessing bi-directional effects on the diurnal cycle of measured solar-induced chlorophyll fluorescence in crop canopies

[EN] Solar-induced chlorophyll fluorescence (SIF) has shown to be a good proxy of gross primary production (GPP) across multiple spatial and temporal scales. The dependence of top-of-canopy measurements of SIF on the illumination and observation directions has been recognized as an important factor in the relationship between SIF and GPP across multiple spatiotemporal scales. In this study, we investigated angular effects in diurnal observed SIF (SIFobs) measurements taken with a multi-angle SIF measuring system in a wheat-corn rotational field during the growing season of 2018. Our results reveal strong angular dependencies in diurnal measurements of top-of-canopy SIF for both red SIF obs (RSIFobs) and far-red SIFobs (FRSIFobs). We also observe that the correlation between SIF obs (RSIF obs and FRSIFobs) and GPP depends on both the illumination and viewing directions. To mitigate angular effects in SIFobs, an approach based on spectral reflectance measurements was used. The derived canopy total SIF emission (SIFtotal) at both red and far-red bands (RSIFtotal and FRSIFtotal) showed less angular dependencies and stronger relationships to APAR and GPP than RSIFobs and FRSIFobs. These results highlight the important role of angular effects in SIF when interpreting directional SIF obs retrievals from space, which normally include large variations in sun-target-viewing geometries. This study contributes to our understanding of angular effects on SIF-GPP relationships and subsequently help improve the estimation of GPP from SIF data.This research was supported by the National Key R&D Program of China (2019YFA0606601), International Cooperation and Exchange Programs between NSFC and DFG (41761134082), and the General Program of NSFC (41671421). Z.Y. Z was supported by the Innovative and Practical Program of Graduate Student in Jiangsu Province (SJKY190037) and the China Scholarship Council (CSC) (201906190110). A. P-C was support by a Fellowship from the Academy of Finland (288039). We would thank Mr. Zhaohui Li from Nanjing University and Mr. Pengju Wu and other researchers from ShangQiu station of Farmland Irrigation Research Institute of Chinese Academy of Agricultural Sciences for the field measurements. We also would like to thank Prof. Christiaan van der Tol for making SCOPE model (v1.73) publicly available (https://github.com/Christiaanvandertol/SCOPE).We greatly appreciate the anonymous reviewers for their insightful and constructive comments that helped us to improve our manuscript.Zhang, Z.; Zhang, Y.; Zhang, Q.; Chen, JM.; Porcar-Castell, A.; Guanter-Palomar, LM.; Wu, Y.... (2020). Assessing bi-directional effects on the diurnal cycle of measured solar-induced chlorophyll fluorescence in crop canopies. Agricultural and Forest Meteorology. 295:1-12. https://doi.org/10.1016/j.agrformet.2020.10814711229