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

L'hydratation de la surface de Mars vue par l'imageur spectral OMEGA

Water is currently present on Mars as ice, vapor and surface hydration. Hydration is known to be adsorbed water on minerals or prisoned in their structure. It can influence the Martian water cycle as well as enable mineral alteration or exobiology. This PhD thesis studies the global and seasonal aspects of hydration using the data from the visible and near infrared imaging spectrometer OMEGA. Our work is based on the 3 µm hydration absorption feature, which has required to develop an efficient scheme to get the albedo spectra in the OMEGA long wavelength channel (L channel). We had in particular to assess the surface thermal emission from the spectra. The L channel photometric efficiency undergoes strong variations with time; we have therefore derived an adapted new calibration using an innovating method to get a representative spatial and seasonal coverage. Our study reveals that hydration is everywhere on Mars with water contents between 3 and 7 weight %, which is evidence of overall adsorbed water as well as global alteration. The hydration dependency with temperature and pressure is consistent with laboratory measurements, and composition may influence the water bond strength. Brightest surfaces are likely to be more hydrated. The influence on the water cycle is proved by the observation of seasonal variations as well as an increase over icy sub-surfaces. Finally, carbonates are expected as an alteration product in hydrated surfaces, but our systematic research on the 3.4 µm proves their missing in large amounts on Mars. This result also puts constrains on atmospheric history scenarios.L'eau est présente aujourd'hui sur Mars sous forme de glace, de vapeur et d'hydratation du sol. L'hydratation, constituée d'eau adsorbée sur les minéraux ou emprisonnée dans leur structure, peut influencer le cycle de l'eau et favoriser des processus d'exobiologie ou d'altération. Cette thèse porte sur l'étude globale et saisonnière de cette hydratation grâce aux données de l'imageur spectral visible et proche-infrarouge OMEGA, en utilisant une forte bande d'absorption à 3 µm. Première étude systématique sur la voie L (grandes longueurs d'onde) d'OMEGA, cette thèse a développé un algorithme rapide pour en calculer les spectres de réflectance, nécessitant d'estimer l'émission thermique sur les données. La réponse photométrique de la voie L étant fortement variable, nous avons proposé un nouvel étalonnage adapté pour obtenir une couverture spatiale et saisonnière représentative. Notre étude révèle que l'hydratation est présente partout sur Mars, pour des teneurs massiques en eau entre 3 et 12%, preuve de l'omniprésence de l'eau adsorbée mais aussi d'une altération générale. La dépendance de l'hydratation avec la pression et la température est conforme aux mesures de laboratoire, et la composition influe sur la force d'adsorption. Les sols clairs semblent plus hydratés. L'influence sur le cycle de l'eau est prouvée par l'observation de variations saisonnières et d'une teneur en eau élevée là où le sous-sol est glacé. Enfin, les carbonates sont un produit d'altération attendu en présence d'eau adsorbée, mais notre recherche systématique sur leur bande à 3.4 µm prouve leur absence globale. Ce résultat est primordial pour les scénarios de l'histoire de l'atmosphère

The SSHR Solar Reference Spectrum

International audienceThe determination of many high-resolution solar reference spectra at high accuracy is crucial and represents a fundamental input for solar physics (Sun modeling), terrestrial atmospheric photochemistry and Earth's climate (climate's modeling). Thus, we present a new solar irradiance reference spectrum at high resolution representative of a solar minimum. The SOLAR Spectrum at High Resolution (SSHR) is developed by normalizing high spectral resolution solar line data to the absolute irradiance scale of the SOLAR-ISS reference spectrum. The resulting disk-integrated solar spectrum has at least 0.01 nm spectral resolution and spans 300-4400 nm. Below 1000 nm, the spectral resolution is less than 0.001 nm. One of our motivations is to develop a new radiometrically well calibrated solar spectrum with high spectral resolution for disk-integrated, but also for the first time for disk-center or intermediate cases. These spectra must meet the needs of the MicroCarb mission and the 4AOP radiative transfer software

Radiometric and Spectral Inter-comparison of IASI: IASI-A / IASI-B, IASI / AIRS, IASI / CrIS

IASI-A has been in operation on board MetOp-A since June 2007 and is now considered as a reference sensor for the radiometric calibration at high spectral resolution. IASI-B on board MetOp-B was launched on September 17th, 2012, and finished its commissioning phase in April 2013. CNES is in charge of the performance monitoring of the two IASI instruments. One of the associated tasks is to compare the calibration of IASI-A with IASI-B, and to compare each one with other similar sensors (AIRS and CrIS). Our goal is to check the IASI data quality with an external reference and to establish the high radiometric accuracy required for climate data records. After more than one year of IASI-B data, we are now in long-term routine monitoring. Our inter-comparisons are now valid on every season and observation condition. We first present the radiometric inter-calibration between IASI-A and IASI-B. An original method has been developed based on the observations of common regions with a 50 minutes temporal gap and in different viewing conditions due to the orbital configuration. The input dataset filtering has been tuned to minimize the geophysical biases. The sounding pixels are spatially averaged on a 300x300km² area. Radiometry is compared at full spectral resolution. We also present the radiometric inter-calibration of IASI (A and B) with Aqua/AIRS and NPP/CrIS. Our method is based on simultaneous nadir overpasses, occurring at high altitudes. The difference in spectral sampling and resolution is handled through either the use of spectrally broad pseudo-channels or the reconvolution of IASI spectra. Results for with more than one year of data for IASI-A / AIRS, IASI-B / AIRS, IASI-A / CRIS, IASI-B / CRIS and IASI-A / IASI-B show small biases of the order of 0 to 0.2K. The five sounders show very stable inter-calibration. Several diagnosis tools are presented to understand the small residuals of inter-calibration. An indirect inter-comparison IASI-B / IASI-A is also presented using double differences with AIRS and CRIS. We also present a long-term monitoring of the spectral cross-calibration, based on the cross-correlation of spectra on several spectrally broad pseudo-channels

Use of the 1.27 μm O2 absorption band for CO2 and methane mixing ratio estimates in nadir viewing from space: Potential and application to Microcarb

International audienceMonitoring CO2 from space is essential to characterize the spatio/temporal distribution of this major greenhouse gas, and quantify its sources and sinks. The mixing ratio of CO2 to dry air can be derived from the CO2/O2 column ratio. The O2 column is usually derived form its absorption signature on the solar reflected spectra over the O2 A-band (OCO-2, Tanso/Gosat). As a result of atmospheric scattering, the atmospheric path length varies with the aerosols load, their vertical distribution, and their optical properties. The spectral distance between the O2 A-band (0.76 μm) and the CO2 absorption band (1.6 μm) results in significant uncertainties due to the varying spectral properties of the aerosols over the globe. There is another O2 absorption band at 1.27 μm with weaker lines than in the A-band. As the wavelength is much nearer to the CO2 and CH4 bands, there is less uncertainty when using it as a proxy of the atmospheric path length to the CO2 and CH4 bands. This O2 band is used by the TCCON network implemented for the validation of space-based GHG observations. However, this absorption band is contaminated by the spontaneous emission of the excited molecule O2*, which is produced by the photo-dissociation of O3 molecules in the stratosphere and mesosphere. From a satellite looking nadir, this emission has a similar magnitude as the absorption signal that is used. In the frame of the CNES Microcarb project, scientific studies have been performed in 2016-2017 to explore the problems associated to this airglow emission and methods to correct it. The intensities observed by SCIAMACHY/ENVISAT in limb viewing have been compared to a model of the emission based on the chemical-transport model Reprobus. The airglow intensities depend mostly on the Solar Zenith Angle and the agreement data/model is quite good. It was shown that, provided the spectra is acquired with a sufficient spectral resolution and SNR, the contribution of the O2* emission at 1.27 μm to the observed spectral radiance may be disentangled from the lower atmosphere/ground absorption signature. The CO2 mixing ratio may be retrieved with the accuracy required for quantifying the CO2 sources (pressure level error < 1 hPa, mixing ratio error < 0.4 ppmv). As a result of these studies, it was decided to include such a band in the Microcarb design, although keeping the O2 A band for reference. Some detailed results of these O2* studies and their 2018 update will be presented. We advocate for the inclusion of such a band in other GHG monitoring future space missions, such as GOSAT-2 and EU/ESA CO2M missions, for a better GHG retrieval

Use of the 1.27 μm O2 absorption band for CO2 and methane mixing ratio estimates in nadir viewing from space: Potential and application to Microcarb

International audienceMonitoring CO2 from space is essential to characterize the spatio/temporal distribution of this major greenhouse gas, and quantify its sources and sinks. The mixing ratio of CO2 to dry air can be derived from the CO2/O2 column ratio. The O2 column is usually derived form its absorption signature on the solar reflected spectra over the O2 A-band (OCO-2, Tanso/Gosat). As a result of atmospheric scattering, the atmospheric path length varies with the aerosols load, their vertical distribution, and their optical properties. The spectral distance between the O2 A-band (0.76 μm) and the CO2 absorption band (1.6 μm) results in significant uncertainties due to the varying spectral properties of the aerosols over the globe. There is another O2 absorption band at 1.27 μm with weaker lines than in the A-band. As the wavelength is much nearer to the CO2 and CH4 bands, there is less uncertainty when using it as a proxy of the atmospheric path length to the CO2 and CH4 bands. This O2 band is used by the TCCON network implemented for the validation of space-based GHG observations. However, this absorption band is contaminated by the spontaneous emission of the excited molecule O2*, which is produced by the photo-dissociation of O3 molecules in the stratosphere and mesosphere. From a satellite looking nadir, this emission has a similar magnitude as the absorption signal that is used. In the frame of the CNES Microcarb project, scientific studies have been performed in 2016-2017 to explore the problems associated to this airglow emission and methods to correct it. The intensities observed by SCIAMACHY/ENVISAT in limb viewing have been compared to a model of the emission based on the chemical-transport model Reprobus. The airglow intensities depend mostly on the Solar Zenith Angle and the agreement data/model is quite good. It was shown that, provided the spectra is acquired with a sufficient spectral resolution and SNR, the contribution of the O2* emission at 1.27 μm to the observed spectral radiance may be disentangled from the lower atmosphere/ground absorption signature. The CO2 mixing ratio may be retrieved with the accuracy required for quantifying the CO2 sources (pressure level error < 1 hPa, mixing ratio error < 0.4 ppmv). As a result of these studies, it was decided to include such a band in the Microcarb design, although keeping the O2 A band for reference. Some detailed results of these O2* studies and their 2018 update will be presented. We advocate for the inclusion of such a band in other GHG monitoring future space missions, such as GOSAT-2 and EU/ESA CO2M missions, for a better GHG retrieval

Use of the 1.27 μm O2 absorption band for CO2 and methane mixing ratio estimates in nadir viewing from space: Potential and application to Microcarb

International audienceMonitoring CO2 from space is essential to characterize the spatio/temporal distribution of this major greenhouse gas, and quantify its sources and sinks. The mixing ratio of CO2 to dry air can be derived from the CO2/O2 column ratio. The O2 column is usually derived form its absorption signature on the solar reflected spectra over the O2 A-band (OCO-2, Tanso/Gosat). As a result of atmospheric scattering, the atmospheric path length varies with the aerosols load, their vertical distribution, and their optical properties. The spectral distance between the O2 A-band (0.76 μm) and the CO2 absorption band (1.6 μm) results in significant uncertainties due to the varying spectral properties of the aerosols over the globe. There is another O2 absorption band at 1.27 μm with weaker lines than in the A-band. As the wavelength is much nearer to the CO2 and CH4 bands, there is less uncertainty when using it as a proxy of the atmospheric path length to the CO2 and CH4 bands. This O2 band is used by the TCCON network implemented for the validation of space-based GHG observations. However, this absorption band is contaminated by the spontaneous emission of the excited molecule O2*, which is produced by the photo-dissociation of O3 molecules in the stratosphere and mesosphere. From a satellite looking nadir, this emission has a similar magnitude as the absorption signal that is used. In the frame of the CNES Microcarb project, scientific studies have been performed in 2016-2017 to explore the problems associated to this airglow emission and methods to correct it. The intensities observed by SCIAMACHY/ENVISAT in limb viewing have been compared to a model of the emission based on the chemical-transport model Reprobus. The airglow intensities depend mostly on the Solar Zenith Angle and the agreement data/model is quite good. It was shown that, provided the spectra is acquired with a sufficient spectral resolution and SNR, the contribution of the O2* emission at 1.27 μm to the observed spectral radiance may be disentangled from the lower atmosphere/ground absorption signature. The CO2 mixing ratio may be retrieved with the accuracy required for quantifying the CO2 sources (pressure level error < 1 hPa, mixing ratio error < 0.4 ppmv). As a result of these studies, it was decided to include such a band in the Microcarb design, although keeping the O2 A band for reference. Some detailed results of these O2* studies and their 2018 update will be presented. We advocate for the inclusion of such a band in other GHG monitoring future space missions, such as GOSAT-2 and EU/ESA CO2M missions, for a better GHG retrieval

Time Variations of Low Albedo Regions of Mars in the OMEGA/MEx Dataset

International audienc

Use of the 1.27 µm O2 absorption band for CO2 and methane estimates in nadir viewing from space: Potential and application to Microcarb.

International audienceMonitoring CO2 from space is essential to characterize the spatio/temporal distribution of this major greenhouse gas, and quantify its sources and sinks. The mixing ratio of CO2 to dry air can be derived from the CO2/O2 column ratio. The O2 column is usually derived form its absorption signature on the solar reflected spectra over the O2 A-band (OCO-2, Tanso/Gosat). As a result of atmospheric scattering, the atmospheric path length varies with the aerosols load, their vertical distribution, and their optical properties. The spectral distance between the O2 A-band (0.76 μm) and the CO2 absorption band (1.6 μm) results in significant uncertainties due to the varying spectral properties of the aerosols over the globe. There is another O2 absorption band at 1.27 μm with weaker lines than in the A-band. As the wavelength is much nearer to the CO2 and CH4 bands, there is less uncertainty when using it as a proxy of the atmospheric path length to the CO2 and CH4 bands. This O2 band is used by the TCCON network implemented for the validation of space-based GHG observations. However, this absorption band is contaminated by the spontaneous emission of the excited molecule O2*, which is produced by the photo-dissociation of O3 molecules in the stratosphere and mesosphere. From a satellite looking nadir, this emission has a similar magnitude as the absorption signal that is used. In the frame of the CNES Microcarb project, scientific studies have been performed in 2016-2017 to explore the problems associated to this airglow emission and methods to correct it. The intensities observed by SCIAMACHY/ENVISAT in limb viewing have been compared to a model of the emission based on the chemical-transport model Reprobus. The airglow intensities depend mostly on the Solar Zenith Angle and the agreement data/model is quite good. It was shown that, provided the spectra is acquired with a sufficient spectral resolution and SNR, the contribution of the O2* emission at 1.27 μm to the observed spectral radiance may be disentangled from the lower atmosphere/ground absorption signature. The CO2 mixing ratio may be retrieved with the accuracy required for quantifying the CO2 sources (pressure level error < 1 hPa, mixing ratio error < 0.4 ppmv). As a result of these studies, it was decided to include such a band in the Microcarb design, although keeping the O2 A band for reference. Some detailed results of these O2* studies and their 2018 update will be presented. We advocate for the inclusion of such a band in other GHG monitoring future space missions, such as GOSAT-2 and EU/ESA CO2M missions, for a better GHG retrieval