Verbesserung der spektroskopischen Parameter von Methan im mittleren Infrarotbereich für die atmosphärische Fernerkundung

Angesichts der messtechnischen Fortschritte der vergangenen Jahrzehnte und der neu geschaffenen Möglichkeiten, die sich mit den Fourier-Transform Spektrometern der neuesten Generation im Hinblick auf die Messgenauigkeit bieten, steigen auch die Anforderungen an die Auswertung atmosphärischer Messungen der bodengebundenen Fernerkundung. Um diesen Anforderungen gerecht zu werden, muss das Verständnis über die Wechselwirkung elektromagnetischer Strahlung mit den Spurengasen der Erdatmosphäre und deren Absorptionseigenschaften erweitert werden. Mit hochaufgelösten Labormessungen, die am Institut für Methodik der Fernerkundung (IMF) des Deutschen Zentrums für Luft- und Raumfahrt (DLR) in Oberpfaffenhofen aufgenommen wurden, konnten die Voraussetzungen für eine Verbesserung der spektroskopischen Parameter von Methan im mittleren Infrarotbereich geschaffen werden. Hierfür wurde ein Fourier-Transform Spektrometer der Firma Bruker Optics (Bruker IFS125HR) eingesetzt, das mit einer direkt am Spektrometer angebrachten Langweggaszelle betrieben wurde. Diese Multireflexionszelle wurde eigens für Zellenmessungen mit einstellbarem Absorptionsweg entwickelt und konnte zudem über einen geregelten Heiz- bzw. Kühlkreislauf auf eine zuvor definierte Temperatur gebracht werden. Aufgenommen wurden Hintergrundmessungen, Absorptionsmessungen mit reinem Methan sowie Messungen mit einer Methan-Luft-Mischung, die lediglich einen geringen Methananteil aufwiesen. Sowohl die Rein-Methan-Messungen als auch die Methan-Luft-Messungen wurden bei Raumtemperatur sowie bei hohen und tiefen Temperaturen mit unterschiedlichen Gesamtdrücken durchgeführt, die in groben Zügen den zu erwartenden Drücken, Temperaturen und Mischungsverhältnissen in der Atmosphäre entsprechen. Ausgehend von den daraus hervorgehenden Transmissionsspektren und den Absorptionsparametern für Methan, die der HITRAN-Datenbank für atmosphärische Spurengase entnommen wurden, konnte zunächst eine vorläufige Linienliste erstellt werden, deren Anfangswerte zum Teil mithilfe der Spektroskopiesoftware MIRS korrigiert werden mussten. Zudem musste die Parameterliste um zahlreiche Absorptionslinien ergänzt werden, die zwar in den Messspektren beobachtet aber nicht in der vorhandenen Linienliste zugeordnet werden konnten. Diese Parameterliste bildete dann den Ausgangspunkt dieser Arbeit, die eine verbesserte Methan-Linienliste für die bodengebundene atmosphärische Fernerkundung zum Ziel hat. Für die Auswertung der gemessenen Transmissionsspektren wurde eine Retrieval-Software entwickelt, die auf der Methode der kleinsten Quadrate basiert und eine über die Voigt-Funktion hinaus gehende Profilfunktion einsetzt. Diese Software erlaubt den simultanen Einsatz mehrerer Spektren bei gleichzeitiger Anpassung mehrerer Linien und Profilparameter in einem Durchlauf. Die implementierte Profilfunktion entspricht im Wesentlichen der Hartmann-Tran-Funktion, die allerdings auf die qSDR-Profilfunktion reduziert wurde und zudem um eine Line-Mixing-Näherung erster und zweiter Ordnung erweitert wurde. Nachdem in einem ersten Schritt neben den Linienpositionen und -intensitäten die Selbst-Parameter und das Line-Mixing erster Ordnung (Dispersionsanteil) mitsamt deren Temperaturabhängigkeiten an die Transmissionsspektren mit reinem Methan angepasst wurden, konnten anschließend die Fremd-Parameter sowie die entsprechenden Line-Mixing-Parameter erster und zweiter Ordnung bei Raumtemperatur bearbeitet werden. Abschließend wurde die Temperaturabhängigkeit der Fremd-Parameter bestimmt, die aus den Methan-Luft-Messungen bei hohen und tiefen Temperaturen gewonnen wurden. In einem Bereich zwischen 2400 und 3000 cm−1 konnten auf diese Weise die Intensitäten, Positionen und Selbst-Parameter von annähernd 30000 Spektrallinien ermittelt werden, deren Linienintensitäten größer als 2×10−26 cm/molecule waren. Zu diesen Linien zählten allerdings auch etwa 3500 meist schwach absorbierende Linien, die neu in die Methan-Linienliste mit aufgenommen wurden und höchstwahrscheinlich den fehlenden Linien der heißen Banden in der HITRAN-Linienliste zuzuordnen sind. Diese Annahme wird durch die unteren Energieniveaus bestärkt, die bei den neu hinzugefügten Linien mitgefittet und ansonsten bei den restlichen Linien konstant gehalten bzw. unverändert belassen wurden. Aufgrund der starken Druckverbreiterung in den Methan-Luft-Messungen konnte jedoch die überwiegende Mehrheit der Absorptionslinien in den Transmissionsspektren nicht mehr aufgelöst werden, sodass die Fremd-Parameter von lediglich 8700 Linien mit Intensitäten größer als 10−24 cm/molecule korrigiert wurden. Ein Vergleich der neuen Profilparameter mit den unveränderten HITRAN-Werten zeigt, dass die Linienpositionen und -intensitäten für bestimmte Vibrationsübergänge und Rotationsquantenzahlen deutliche Unterschiede aufweisen. Die Differenzen können mitunter mehrere Wellenzahlen in der Linienposition bzw. ein bis zwei Größenordnungen in der Linienintensität betragen. Zudem konnten die Parameter der druckinduzierte Selbst-Verbreiterung bestimmter Vibrationssymmetrien der heißen Banden insofern verbessert werden, als dass die neuen Parameterwerte nun dem Verlauf der Parameterwerte der kalten Banden folgen und mit steigender Rotationsquantenzahl vorwiegend abnehmen (und nicht wie zuvor zunehmen). Neben der Linienposition und -intensität wurden insgesamt 9 Selbst-Parameter und 13 Fremd-Parameter für die Anpassung der synthetischen Spektren an die gemessenen Transmissionsspektren eingesetzt. Zu den Selbst-Parametern zählten die druckinduzierte Verbreiterung, Verschiebung, Verjüngung, Verformung, der Dicke-Parameter sowie das Line-Mixing erster Ordnung mitsamt deren Temperaturabhängigkeiten. Bei den Fremd-Parametern wurden zudem die Parameter für die Line-Mixing-Näherung zweiter Ordnung sowie deren Temperaturabhängigkeiten berücksichtigt. Für alle Labormessungen gilt, dass die Residuen mit der erweiterten Profilfunktion und den zusätzlich zur Verfügung stehenden Parametern über den gesamten Spektralbereich mitunter deutlich gesenkt werden konnten. Eine Anwendung der neuen Methan-Linienliste auf Atmosphärenmessungen ausgewählter Messstationen der bodengebundenen Fernerkundung hat gezeigt, dass die Residuen innerhalb der eng bemessenen NDACC-Fenster sowie in ausgewählten breitbandigen Fenstern, ebenfalls deutlich reduziert werden. Zudem wurden die Gesamtsäulen, die vertikale Profile und Jahresgänge, die jeweils mit der neuen Methan-Linienliste und der unveränderten HITRAN-Linienliste sowie den NDACC-Fenstern ausgewertet wurden, miteinander verglichen. Die Unterschiede in der Gesamtsäule können hierbei bis zu ein Prozent betragen, während die vertikalen Profile in den bodennahen Regionen im Mittel einen kleineren Gradienten aufweisen als zuvor. Insgesamt kann festgestellt werden, dass die neu erstellte, verbesserte Linienliste für Methan den gestiegenen Anforderungen an die Auswertung bodengebundener Fernerkundung grundsätzlich gerecht wird

Retrievals of XCO2_{CO2}, XCH4_{CH4} and XCO_{CO} from portable, near-infrared Fourier transform spectrometer solar observations in Antarctica

The COllaborative Carbon Column Observing Network (COCCON) uses low-resolution, portable EM27/SUN Fourier transform spectrometers (FTSs) to make retrievals of column-averaged dry-air mole fractions (DMFs, represented as X$_{gas}$) of CO$_{2}$, CH$_{4}$, CO and H$_{2}$O from near-infrared solar absorption spectra. The COCCON has developed rapidly over recent years and complements the Total Carbon Column Observing Network (TCCON). In this work, we provide details of the first seasonal time series of near-infrared X$_{CO2}$, X$_{CH4}$ and X$_{CO}$ retrievals from measurements made in Antarctica during the deployment of an EM27/SUN to the Arrival Heights laboratory on Ross Island over the austral summer of 2019–2020 under the auspices of the COCCON. The DMFs of all three species were lower in Antarctica than at mid-latitude, and for X$_{CO2}$ and X$_{CO}$, the retrieved values were less variable. For X$_{CH4}$ however, the variability was significantly greater and it was found that this was strongly correlated to the proximity of the polar vortex. In order to ensure the stability of the instrument and the traceability of the retrievals, side-by-side comparisons to the TCCON station at Lauder, New Zealand and retrievals of the instrument line shape (ILS) were made before and after the measurements in Antarctica. These indicate that, over the course of the deployment, the instrument stability was such that the change in retrieved X$_{CO2}$ was well below 0.1%. The value of these data for satellite validation is demonstrated by making comparisons with the TROPOspheric Monitoring Instrument (TROPOMI) on the Sentinel-5 precursor (S5P) satellite. The dataset is available from the COCCON central facility hosted by the ESA Atmospheric Validation Data Centre (EVDC): https://doi.org/10.48477/coccon.pf10.arrivalheights.R02 (Pollard, 2021)

FTIR Measurements of Greenhouse Gases over Thessaloniki, Greece in the Framework of COCCON and Comparison with S5P/TROPOMI Observations

In this work, column-averaged dry-air mole fractions of carbon dioxide (XCO2), methane (XCH4) and carbon monoxide (XCO) are presented for the first time at a mid-latitude urban station, Thessaloniki, Greece, using the Bruker EM27/SUN ground-based low-resolution Fourier Transform spectrometer operated according to the requirements of the Collaborative Carbon Column Observing Network (COCCON). Two years of measurements are presented and examined for seasonal variability. The observed XCO2 levels show the expected seasonal cycle (spring maximum, late summer minimum) with a peak-to-peak amplitude of 12 ppm, with maximum values reported for winter 2021 exceeding 416 ppm. The XCH4 values are shown to increase in the second half of the year, with autumn showing the highest mean value of 1.878 ± 0.01 ppm. The XCO levels, following anthropogenic sources, show high winter and low summer values, exhibiting a rise again in August and September with a maximum value of 114 ± 3 ppb and a minimum in summer 2020 of 76 ± 3 ppb. Additionally, methane and carbon monoxide products obtained from the TROPOspheric Monitoring Instrument (TROPOMI), Sentinel-5P space borne sensor, are compared with the ground-based measurements. We report a good agreement between products. The relative mean bias for methane and carbon monoxide are −0.073 ± 0.647% and 3.064 ± 5.566%, respectively. Furthermore, a 15-day running average is subtracted from the original daily mean values to provide ΔXCO2, ΔXCO and ΔXCH4 residuals, so as to identify local sources at a synoptic scale. ΔXCO and ΔXCO2 show the best correlation in the winter (R2 = 0.898, slope = 0.007) season due to anthropogenic emissions in this period of the year (combustion of fossil fuels or industrial activities), while in summer no correlation is found. ΔXCO and ΔXCH4 variations are similar through both years of measurements and have a very good correlation in all seasons including winter (R2 = 0.804, slope = 1.209). The investigation of the X-gases comparison is of primary importance in order to identify local sources and quantify the impact of these trace gases to the deregulation of earth-climate system balance

Quantifying CH4_{4} emissions in hard coal mines from TROPOMI and IASI observations using the wind-assigned anomaly method

In this study, we use satellite-based total column-averaged dry-air mole fraction of CH4 (XCH4) from the TROPOspheric Monitoring Instrument (TROPOMI) and tropospheric XCH$_{4}$ (TXCH$_{4}$) from the Infrared Atmospheric Sounding Interferometer (IASI). In addition, the high-resolution model forecasts, XCH$_{4}$ and TXCH$_{4}$, from the Copernicus Atmosphere Monitoring Service (CAMS) are used to estimate the CH$_{4}$ emission rate averaged over 3 years (November 2017–December 2020) in the USCB region (49.3–50.8$^\circ$ N and 18–20$^\circ$ E). The wind-assigned anomaly method is first validated using the CAMS forecast data (XCH$_{4}$ and TXCH$_{4}$), showing a good agreement with the CAMS GLOBal ANThropogenic emission (CAMS-GLOB-ANT) inventory. It indicates that the wind-assigned method works well. This wind-assigned method is further applied to the TROPOMI XCH$_{4}$ and TROPOMI + IASI TXCH$_{4}$ by using the Carbon dioxide and Methane (CoMet) inventory derived for the year 2018. The calculated averaged total CH$_{4}$ emissions over the USCB region is about 496 kt yr$^{-1}$ (5.9×10$^{26}$ molec. s$^{-1}$) for TROPOMI XCH$_{4}$ and 437 kt yr$^{-1}$ (5.2×10$^{26}$ molec. s$^{-1}$) for TROPOMI + IASI TXCH$_{4}$. These values are very close to the ones given in the E-PRTR inventory (448 kt yr$^{-1}$) and the ones in the CoMet inventory (555 kt yr$^{-1}$), and are thus in agreement with these inventories. The similar estimates of XCH$_{4}$ and TXCH$_{4}$ also imply that for a strong source, the dynamically induced variations of the CH$_{4}$ mixing ratio in the upper troposphere and lower stratosphere region are of secondary importance. Uncertainties from different error sources (background removal and noise in the data, vertical wind shear, wind field segmentation, and angle of the emission cone) are approximately 14.8 % for TROPOMI XCH$_{4}$ and 11.4 % for TROPOMI + IASI TXCH$_{4}$. These results suggest that our wind-assigned method is quite robust and might also serve as a simple method to estimate CH$_{4}$ or CO$_{2}$ emissions for other regions

Observational constraints on methane emissions from Polish coal mines using a ground-based remote sensing network

Given its abundant coal mining activities, the Upper Silesian Coal Basin (USCB) in southern Poland is one of the largest sources of anthropogenic methane (CH$_{4}$) emissions in Europe. Here, we report on CH$_{4}$emission estimates for coal mine ventilation facilities in the USCB. Our estimates are driven by pairwise upwind–downwind observations of the column-average dry-air mole fractions of CH$_{4}$ (XCH$_{4}$) by a network of four portable, ground-based, sun-viewing Fourier transform spectrometers of the type EM27/SUN operated during the CoMet campaign in May–June 2018. The EM27/SUN instruments were deployed in the four cardinal directions around the USCB approximately 50 km from the center of the basin. We report on six case studies for which we inferred emissions by evaluating the mismatch between the observed downwind enhancements and simulations based on trajectory calculations releasing particles out of the ventilation shafts using the Lagrangian particle dispersion model FLEXPART. The latter was driven by wind fields calculated by WRF (Weather Research and Forecasting model) under assimilation of vertical wind profile measurements of three co-deployed wind lidars. For emission estimation, we use a Phillips–Tikhonov regularization scheme with the L-curve criterion. Diagnosed by the emissions averaging kernels, we find that, depending on the catchment area of the downwind measurements, our ad hoc network can resolve individual facilities or groups of ventilation facilities but that inspecting the emissions averaging kernels is essential to detect correlated estimates. Generally, our instantaneous emission estimates range between 80 and 133 kt CH$_{4}$ a$^{-1}$ for the southeastern part of the USCB and between 414 and 790 kt CH$_{4}$a$^{-1}$ for various larger parts of the basin, suggesting higher emissions than expected from the annual emissions reported by the E-PRTR (European Pollutant Release and Transfer Register). Uncertainties range between 23 % and 36 %, dominated by the error contribution from uncertain wind fields

Tropospheric water vapour isotopologue data (H₂¹⁶O, H₂¹⁸O, and HD¹⁶O) as obtained from NDACC/FTIR solar absorption spectra

We report on the ground-based FTIR (Fourier transform infrared) tropospheric water vapour isotopologue remote sensing data that have been recently made available via the database of NDACC (Network for the Detection of Atmospheric Composition Change; ftp://ftp.cpc.ncep.noaa.gov/ndacc/MUSICA/) and via doi:10.5281/zenodo.48902. Currently, data are available for 12 globally distributed stations. They have been centrally retrieved and quality-filtered in the framework of the MUSICA project (MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water). We explain particularities of retrieving the water vapour isotopologue state (vertical distribution of H216O, H218O, and HD16O) and reveal the need for a new metadata template for archiving FTIR isotopologue data. We describe the format of different data components and give recommendations for correct data usage. Data are provided as two data types. The first type is best-suited for tropospheric water vapour distribution studies disregarding different isotopologues (comparison with radiosonde data, analyses of water vapour variability and trends, etc.). The second type is needed for analysing moisture pathways by means of H2O, δD-pair distributions

Synergetic use of IASI and TROPOMI space borne sensors for generating a tropospheric methane profile product

The thermal infrared nadir spectra of IASI (Infrared Atmospheric Sounding Interferometer) are successfully used for retrievals of different atmospheric trace gas profiles. However, these retrievals offer generally reduced information about the lowermost tropospheric layer due to the lack of thermal contrast close to the surface. Spectra of scattered solar radiation observed in the near and/or short wave infrared, for instance by TROPOMI (TROPOspheric Monitoring Instrument) offer higher sensitivity near ground and are used for the retrieval of total column averaged mixing ratios of a variety of atmospheric trace gases. Here we present a method for the synergetic use of IASI profile and TROPOMI total column data. Our method uses the output of the individual retrievals and consists of linear algebra a posteriori calculations (i.e. calculation after the individual retrievals). We show that this approach is largely equivalent to applying the spectra of the different sensors together in a single retrieval procedure, but with the substantial advantage of being applicable to data generated with different individual retrieval processors, of being very time efficient, and of directly benefiting from the high quality and most recent improvements of the individual retrieval processors.This research has largely benefit from funds of the Deutsche Forschungsgemeinschaft (provided for the two projects MOTIV and TEDDY with IDs/Geschäftszeichen 290612604/GZ:SCHN1126/2-1 and 416767181/GZ:SCHN1126/5-1, respectively) and from support by the European Space Agency in the context the "Sentinel-5p+Innovation (S5p+I) - Water Vapour Isotopologues (H2O-ISO)" activities. Furthermore, we acknowledge funds from the Ministerio de Economía y Competividad from Spain for the project INMENSE (CGL2016-80688-P)

Quantification of CH4 emissions from waste disposal sites near the city of Madrid using ground- and space-based observations of COCCON, TROPOMI and IASI

We use different methane ground- and space-based remote sensing data sets for investigating the emission strength of three waste disposal sites close to Madrid. We present a method that uses wind-assigned anomalies for deriving emission strengths from satellite data and estimating their uncertainty to 9–14 %. The emission strengths estimated from the remote sensing data sets are significantly larger than the values published in the official register.ESA support through the COCCON-PROCEEDS and COCCON-PROCEEDS II projects. In addition, this research was funded by the Ministerio de Economía y Competitividad from Spain through the INMENSE project (CGL2016-80688-P). This research has largely benefit from funds of the Deutsche Forschungsgemeinschaft (provided for the two projects MOTIV and TEDDY with IDs/290612604 and 416767181, respectively)

Synergetic use of IASI profile and TROPOMI total-column level 2 methane retrieval products

The thermal infrared nadir spectra of IASI (Infrared Atmospheric Sounding Interferometer) are successfully used for retrievals of different atmospheric trace gas profiles. However, these retrievals offer generally reduced information about the lowermost tropospheric layer due to the lack of thermal contrast close to the surface. Spectra of scattered solar radiation observed in the near-infrared and/or shortwave infrared, for instance by TROPOMI (TROPOspheric Monitoring Instrument), offer higher sensitivity near the ground and are used for the retrieval of total-column-averaged mixing ratios of a variety of atmospheric trace gases. Here we present a method for the synergetic use of IASI profile and TROPOMI total-column level 2 retrieval products. Our method uses the output of the individual retrievals and consists of linear algebra a posteriori calculations (i.e. calculation after the individual retrievals). We show that this approach has strong theoretical similarities to applying the spectra of the different sensors together in a single retrieval procedure but with the substantial advantage of being applicable to data generated with different individual retrieval processors, of being very time efficient, and of directly benefiting from the high quality and most recent improvements of the individual retrieval processors. We demonstrate the method exemplarily for atmospheric methane (CH$_4$). We perform a theoretical evaluation and show that the a posteriori combination method yields a total-column-averaged CH$_4$ product (XCH$_4$) that conserves the good sensitivity of the corresponding TROPOMI product while merging it with the high-quality upper troposphere–lower stratosphere (UTLS) CH$_4$ partial-column information of the corresponding IASI product. As a consequence, the combined product offers additional sensitivity for the tropospheric CH$_4$ partial column, which is not provided by the individual TROPOMI nor the individual IASI product. The theoretically predicted synergetic effect is verified by comparisons to CH$_4$ reference data obtained from collocated XCH$_4$ measurements at 14 globally distributed TCCON (Total Carbon Column Observing Network) stations, CH$_4$ profile measurements made by 36 individual AirCore soundings, and tropospheric CH$_4$ data derived from continuous ground-based in situ observations made at two nearby Global Atmospheric Watch (GAW) mountain stations. The comparisons clearly demonstrate that the combined product can reliably detect the actual variations of atmospheric XCH$_4$, CH$_4$ in the UTLS, and CH$_4$ in the troposphere. A similar good reliability for the latter is not achievable by the individual TROPOMI and IASI products

Improved calibration procedures for the EM27/SUN spectrometers of the COllaborative Carbon Column Observing Network (COCCON)

In this study, an extension on the previously reported status of the COllaborative Carbon Column Observing Network\u27s (COCCON) calibration procedures incorporating refined methods is presented. COCCON is a global network of portable Bruker EM27/SUN FTIR spectrometers for deriving column-averaged atmospheric abundances of greenhouse gases. The original laboratory open-path lamp measurements for deriving the instrumental line shape (ILS) of the spectrometer from water vapour lines have been refined and extended to the secondary detector channel incorporated in the EM27/SUN spectrometer for detection of carbon monoxide (CO). The refinements encompass improved spectroscopic line lists for the relevant water lines and a revision of the laboratory pressure measurements used for the analysis of the spectra. The new results are found to be in good agreement with those reported by Frey et al. (2019) and discussed in detail. In addition, a new calibration cell for ILS measurements was designed, constructed and put into service. Spectrometers calibrated since January 2020 were tested using both methods for ILS characterization, open-path (OP) and cell measurements. We demonstrate that both methods can detect the small variations in ILS characteristics between different spectrometers, but the results of the cell method indicate a systematic bias of the OP method. Finally, a revision and extension of the COCCON network instrument-to-instrument calibration factors for XCO2, XCO and XCH4 is presented, incorporating 47 new spectrometers (of 83 in total by now). This calibration is based on the reference EM27/SUN spectrometer operated by the Karlsruhe Institute of Technology (KIT) and spectra collected by the collocated TCCON station Karlsruhe. Variations in the instrumental characteristics of the reference EM27/SUN from 2014 to 2017 were detected, probably arising from realignment and the dual-channel upgrade performed in early 2018. These variations are considered in the evaluation of the instrument-specific calibration factors in order to keep all tabulated calibration results consistent