Abstract. The GESat GEN1 mission is a 16U microsatellite developed and operated by Absolut Sensing to demonstrate high-resolution methane monitoring using a compact SWIR Fizeau interferometer. GEN1 provides a 50m ground sampling distance to monitor facility-scale methane plumes. It is rated for an average detection rate of 500 kg.h−1, with the capacity to resolve leaks as small as 100 kg.h−1 in pristine environmental conditions. The system is designed to achieve a column retrieval precision of 120 ppbv, defined as the mean standard deviation of the retrieved column-averaged dry-air mole fraction of methane over operational conditions. This corresponds to a random error of approximately 6.5% relative to a backgroundconcentration of 1850 ppbv.The paper first presents the GEN1 mission and its interferometric payload, designed to enable selective methane sensing from a highly compact platform. It then describes the retrieval architecture, based on a physics-guided parametric algorithm that occupies an intermediate complexity between matched-filter methods, as used for missions such as Tanager-1 and EMIT (Carbon Mapper, 2024), and full physics schemes using high accuracy radiative transfer code such as libRadtran or 4A/OP (Mayer and Kylling, 2005; Scott and Chédin, 1981; Chéruy et al., 1995). The proposed model generates top-of-atmosphere radiances in the SWIR [1550, 1700nm] region with no systematic bias relative to reference full-physics model and a random error below 0.2 ppbv in methane column retrieval. Moreover, on identical computing hardware, the physics-guided parametric approach achieves a processing speed approximately 20×103 times faster than the full-physics model. This strategy therefore provides a balanced trade-off between robustness, accuracy, and computational cost, enabling large-scale processing compatible with constellation-level operations while preserving radiometric fidelity required for quantitative methane retrievals. Supported by this instrument and retrieval design, representative in-orbit results are reported, including methane plume detections over industrial facilities, together with an assessment of current limitations and planned evolutions of the processing chain. Finally, the paper concludes by evaluating the mission’s performance against its design specifications.With more than 160 acquisitions processed, the system achieves an average precision of 111 ppbv, successfully meeting the mission’s primary target, with an average detection sensitivity of 450 kg.h−1. Notably, several measurements achieved precision better than 80 ppbv, which enables an ultimate methane plume detection limit corresponding to emission rates below 100 kg.h−1 under ideal meteorological conditions. This on-orbit demonstration confirms the viability of the Fizeau interferometric approach for high-resolution monitoring and paves the way for future constellation deployment.

Ammonia (NH3) is a short-lived atmospheric pollutant with significant environmental and health impacts. Monitoring NH3 remains challenging, as diurnal variability at local scales is still poorly documented. In this study, we analyze two years (July 2022-June 2024) of NH3 total columns from the Geostationary Interferometric Infrared Sounder onboard China's FengYun-4B (GIIRS-B) over East Asia. After applying quality and uncertainty filters, we find good agreement with Infrared Atmospheric Sounding Interferometer (IASI) morning observations (weighted Pearson R = 0.64) and identify relationships between NH3 and skin/land surface temperature over five major NH3 hotspots, suggesting contributions from agriculture (urea fertilizer use), livestock, and secondary urban sources. GIIRS-B's high temporal resolution reveals a clear bimodal diurnal pattern, with NH3 enhancements in the early morning and mid-afternoon in four regions. A dedicated analysis of GIIRS-B NH3 retrieval uncertainties provides a realistic physical benchmark for geostationary infrared observations. Using radiative transfer simulations (4A/OP) driven by atmospheric chemistry model outputs (CHIMERE), we evaluate the potential of the European InfraRed Sounder (IRS) onboard MTG-S to retrieve NH3 at sub-daily resolution. IRS uncertainties are generally larger and more variable than those of IASI, but under favorable thermal-contrast conditions they can become comparable. GIIRS-B and IRS exhibit consistent diurnal uncertainty patterns with nighttime maxima and daytime minima, confirming the realism of the IRS performance assessment. These results highlight the added value of geostationary sounders for improving NH3 emission monitoring, source attribution, and diurnal process understanding in support of future European air-quality regulations.

MERLIN (MEthane Remote LIdar missioN) is a space mission developed by France and Germany to monitor the atmospheric methane using an IPDA (Integrated Path Differential Absorption) lidar. The mission is scheduled for launch at the end of the decade. Currently, the LMD (Laboratoire de Météorologie Dynamique) is working on improving spectroscopy data, end-to-end modelling of the instrument to study the sensitivity of measurements to different sources of uncertainty, developing an inversion system to study the impact of certain data processing parameters, setting up measurement campaigns to better understand methane variability and fluxes, as well as developing a ground-based differential absorption lidar for CH 4 profiling.

The post-launch spectral and radiometric performances of MAJIS, the VISNIR imaging spectrometer of the ESA Jupiter Icy Moon Explorer (JUICE), have been evaluated using observations performed during the Lunar-Earth Gravitational Assist (LEGA) of 19–20 August 2024 and observations of the Internal Calibration Unit (ICU). Observations of the Earth provided a comprehensive check of the spectral performances taking advantage of narrow atmospheric absorption bands over the full wavelength range of MAJIS (0.5–5.56 µm). This was of particular interest for wavelengths larger than 3.5 µm due to limitations of the ground calibration setup in this wavelength range. The radiometric performance of MAJIS has been reassessed considering the updated spectral calibration and the comparison of ICU observations before and after launch. On this basis, the observations of the Earth and Moon by MAJIS were compared to that of other instruments. The very good agreement with atmospheric spectral features observed by Earth observation instruments validate the updated spectral calibration of MAJIS. Comparing radiances for the Earth is not straightforward due to the very specific photometric angles for MAJIS observations (phase ∼ 90°, “glint spot”) and the high time variability of cloud patterns. A good agreement has been obtained within these limitations and the MAJIS radiance evaluations for the Moon are also consistent with that obtained by instruments on lunar orbiters, which indicates that the post-launch absolute radiometric calibration of MAJIS is close to the mark. These comparisons benefited from the high quality of the MAJIS data obtained during the LEGA with a very high operability and a signal to noise ratio (SNR) up to 400 (more with stacking). Extrapolating the operating conditions at 1 AU to those at 5 AU confirm that MAJIS will obtain high quality data during the science operations phase around Jupiter.

MERLIN (MEthane Remote LIdar missioN) is a space mission developed by France and Germany to monitor the atmospheric methane using an IPDA (Integrated Path Differential Absorption) lidar. The mission is scheduled for launch at the end of the decade. Currently, the LMD (Laboratoire de Météorologie Dynamique) is working on improving spectroscopy data, end-to-end modelling of the instrument to study the sensitivity of measurements to different sources of uncertainty, developing an inversion system to study the impact of certain data processing parameters, setting up measurement campaigns to better understand methane variability and fluxes, as well as developing a ground-based differential absorption lidar for CH4 profiling.

Considering scattering in radiative transfer calculations often leads to extensively long computation times that can be prohibitive, especially in the operational context of satellite missions. Over time, numerous methods were developed to accelerate scattering calculations. In this paper, based and further built upon the work of O'Dell [1], we describe the implementation of the Low-Stream Interpolation acceleration technique in the 4A/OP radiative transfer model. The method's acceleration relies on the execution of computationally expensive calculations only for representative points, which represent the regions in the gas absorption optical depth space called "bins". For all other points the calculation is performed at a low accuracy and by subsequent interpolation. We have considered a number of method's aspects, and introduced various modifications in order to optimise its accuracy and computation time. This includes: a) an extension of the method to Jacobians, b) modifications of bin and sub-bin divisions, c) implementation of automatic binning and its comparison to fixed bins, d) improvement of the computation of representative points, e) improvement of the definition of "significant scattering" used by the method, f) avoiding the redundancy of high-accuracy computation at large gas absorption optical depths, g) optimisations of the method computation domain size and h) evaluations of various possible accelerations of low-accuracy calculations. We applied the method on the MicroCarb O2 and CO2 bands in nearinfrared and shortwave-infrared over a large variety of geophysical cases and discussed the impacts of the modifications and the overall performance. We confirmed that the Low-Stream Interpolation is a powerful technique to accelerate the scattering calculations and, in our model, it provided relative accuracies on polarised and unpolarised radiances and Jacobians lower than 0.05 % with acceleration of 10-50 times.

Abstract. The JUpiter ICy moons Explorer spacecraft (JUICE) successfully performed a Lunar and Earth gravity assist maneuver on 19 and 20th August 2024, which provided an excellent opportunity to test its instruments before its arrival at Jupiter in 2031. Here we focus on the evaluation of the performances of the infrared channel of the Moon and Jupiter Imaging Spectrometer (MAJIS) based on Earth observations acquired over part of the Pacific Ocean at tropical latitudes. We specifically compare MAJIS observations with co-located ones from the Infrared Atmospheric Sounding Interferometer (IASI). The two instruments overlap in the spectral range 3.6 and 5.56 µm. Having removed spectra contaminated by clouds or ocean glint, we find an excellent match between the absolute radiance of the two instruments. We argue that most of the differences can be explained by natural variability of water vapour content. Once this effect is taken into account, our results suggest that the radiometric calibration of MAJIS is better than 10 %, rising to ~15 % in the range 5.25–5.56 µm. We then compare MAJIS to synthetic spectra generated with a radiative transfer algorithm, the OPerational version of the Automatized Atmospheric Absorption Atlas (4A/OP). Both the comparison of MAJIS to IASI and to synthetic spectra reveal a small spectral shift of MAJIS spectra beyond 4 µm, of the order of 4 nm. The use of 4A/OP allows us to highlight the detection and sensitivity of MAJIS spectra to several trace species: methane (CH4), nitrous oxide (N2O), ozone (O3) and carbone monoxide (CO). Based on the residuals between MAJIS and synthetic spectra, we estimate the signal-to-noise ratio to lie in the range 200–300, meeting nominal expectations. These excellent performances point to promising future jovian atmospheric observations.

Thanks to its continuous spectral coverage of the whole thermal infrared domain, the IASI sounder offers the possibility to monitor on the long term several essential climate variables, including mid-tropospheric columns of the 3 major greenhouse gases influenced by human activitie: carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). To tackle the very small seasonal variability of these gases compared to their background values, combined to the strong dependence of IR radiances to atmospheric temperature and the simultaneous sensitivity of the channels to several gases, a non-linear inference scheme has been developed at LMD. Since 2007, mid-tropospheric columns of methane have been derived for both day and night conditions, over land and over sea. The retrieval scheme strongly relies on careful validation of level1c spectra, characterization of systematic radiative biases and severe cloud and aerosol screening. CH4 fields are delivered on ‘near real time’ (D-1) basis to the Copernicus Atmosphere Monitoring Service (CAMS) and are assimilated in ECMWF C-IFS system, along with total columns from GOSAT, to produce forecast of vertical profiles of atmospheric concentration. Owing to its 20 year-program, IASI also participates to the establishment of long time series in the Copernicus Climate Change Service (C3S). The retrievals are thus used for a variety of purpose: assimilation to produce CH4/CO2 profile forecasts; estimation of surface fluxes using “top-down” atmospheric inversions; characterization of specific emissions such as biomass burnings. In this talk we will present the latest development of the retrieval and application of methane. In particular, we will present the extension and validation of the retrieval to the high latitude regions achieved during the ESA MethaneCAMP project. By using AirCore 0-30 km profiles of methane concentration acquired at Sodankylä and Kiruna and several stations of the French AirCore network, we will also highlight the crucial need to better understand the variation of stratospheric methane in order to combine satellite-derived methane columns with simulations from atmospheric transport models. Finally, we will present long-term and interannual variability of methane as seen by IASI, with a focus of 2020-2021 methane anomaly and the characterization of specific emissions such as biomass burnings or NordStream leakage.

Ammonia (NH3) is an atmospheric pollutant mainly emitted by the agricultural sector, which has an effect on public health since it is a precursor of fine particles (PM2.5). The diurnal variability of NH3 in the atmosphere and its transformation into particles are poorly constrained and strongly depend on meteorological parameters, in particular temperature. This strongly influences our ability to correctly simulate NH3 emissions and associated particulate pollution events in atmospheric models The IRS (InfraRed Sounder) instrument which will be launched on the MTG (Meteosat Third Generation) satellite into geostationary orbit in late 2024, will offer the ability to evaluate NH3 diurnal variabilities and its dependence on atmospheric temperature with frequent measurements (every 30-45 minutes over Europe and Africa) and fine spatial resolution (4 km x 4 km at the Equator and Greenwich meridian). This work shows the potential of the European geostationary IRS-MTG mission to capture the spatio-temporal variability of ammonia and temperature focusing on a case study over the Brittany region in France. Synthetic spectra are simulated from the 4A/OP radiative transfer model using atmospheric states derived from the CHIMERE chemistry-transport model. The IRS NH3 observations are compared to the current IASI observations in terms of vertical sensitivity and error budget. The uncertainty analysis over the Brittany region is calculated using NH3 Jacobians computed from the 4A/OP radiative code and the noise covariance matrix provided by each satellite.

L’étude de l’atmosphère terrestre par télédétection spatiale dans l’infrarouge constitue une activité importante du Laboratoire de Météorologie Dynamique depuis sa création. L’équipe ABC(t) qui étudie l’Atmosphère, de la Biosphère et le Climat par télédétection est reconnue internationalement pour la conception, l’exploitation et la validation de nombreuses missions spatiales basés sur l’analyse du rayonnement atmosphérique émis et diffusé par l’atmosphère et la surface terrestre dans l’infrarouge. Les applications scientifiques sont innombrables et se concentrent sur l’étude et le suivi des variables climatiques essentielles pour l’atmosphère et du bilan radiatif de la planète. Forte de cette expertise, l’équipe participe activement aux diverses missions spatiales qui ont jalonné l’étude de l’atmosphère ces dernières décennies, en lien très fort avec les agences spatiales nationale et européennes.

La cohérence et la pluridisciplinarité des activités (physique moléculaire, transfert radiatif, analyse statistique, traitement et classification de grands flux de données) et les coopérations et échanges internes et externes noués tout au long d’un parcours de plusieurs décennies auront peut-être contribué à établir au LMD/IPSL un ensemble d’activités et de compétences de tout premier plan.

The Earth's radiative cooling is a key driver of climate. Determining how it is affected by greenhouse gas concentration is a core question in climate-change sciences. Due to the complexity of radiative transfer processes, current practices to estimate this cooling require the development and use of a suite of radiative transfer models whose accuracy diminishes as we move from local, instantaneous estimates to global estimates over the whole globe and over long periods of time (decades). Here, we show that recent advances in nonlinear Monte Carlo methods allow a paradigm shift: a completely unbiased estimate of the Earth's infrared cooling to space can be produced using a single model, integrating the most refined spectroscopic models of molecular gas energy transitions over a global scale and over years, all at a very low computational cost (a few seconds).

Observation from space, being global and quasi-continuous, is a first importance tool for aerosol studies. Remote sensing in the visible domain has been widely used to obtain better characterization of these particles and their effect on solar radiation. On the opposite, remote sensing of aerosols in the thermal infrared domain still remains marginal. However, knowledge of the effect of aerosols on terrestrial radiation is needed for the evaluation of their total radiative forcing. Infrared remote sensing provides a way to retrieve other aerosol characteristics, including their mean altitude. Moreover, observations are possible at night and day, over ocean and over land. In this context, six years (2003-2008) of the 2nd generation vertical sounder AIRS observations have been processed over the tropical belt (30°N-30°S).

Fits to a solar occultation spectrum measured at 21.7 km tangent altitude from balloon above Esrange, Sweden in Dec. 1999 (Black points). The residual trace represents the difference of the measured and calculated transmittances. Colored lines represent the contributions of individual gases (Red: HNO3; Yellow: CO2; Green: H2O; Blue: Other). The HITRAN 2020 line list was used to represent the non-HNO3 absorptions -- mainly CO2 and H2O. For the HNO3 line list we used: (a) HITRAN (i.e., none), (b) ExoMol computed for 220K, (c) the JPL empirical pseudo-line-list, and (d) the HNO3 line list from this work. Each panel shows improved spectral fits compared with the previous panel, from 8.84% RMS in panel (a) to 1.77% RMS in panel (d).

Observation from space, being global and quasi-continuous, is a first importance tool for aerosol studies. Remote sensing in the visible domain has been widely used to obtain better characterization of these particles and their effect on solar radiation. On the opposite, remote sensing of aerosols in the thermal infrared domain still remains marginal. However, knowledge of the effect of aerosols on terrestrial radiation is needed for the evaluation of their total radiative forcing. Infrared remote sensing provides a way to retrieve other aerosol characteristics, including their mean altitude. Moreover, observations are possible at night and day, over ocean and over land.

Ammonia (NH3) is an atmospheric pollutant mainly emitted by the agricultural sector. It is a precursor of fine particles (PM2.5) and therefore has a major effect on public health, and climate change. The volatilization process of NH3 and its lifetime in the atmosphere, as well as its transformation into particles, are poorly constrained and strongly depend on meteorological parameters, in particular temperature.Although current satellite measurements have evaluated NH3 spatio-temporal variabilities at various scales (global, regional, and local), observations of NH3 diurnal variability and their diurnal variability and dependence to temperature are poorly constrained. This strongly influences our ability to correctly simulate NH3 emissions and associated particulate pollution events in atmospheric models.The IRS (InfraRed Sounder) instrument which will be launched on the MTG (Meteosat Third Generation) satellite into geostationary orbit in late 2024, will offer the ability to deepen this analysis with more frequent measurements (every 30-45 minutes over Europe and Africa) and better spatially resolved observations (4 km x 4 km at the Equator).In this presentation, we show the potential of the new geostationary IRS-MTG mission to assess spatio-temporal variabilities of ammonia and temperature focusing on a case study over the high NH3 emitted region of Brittany (France). Using atmospheric states simulated using the CHIMERE chemistry-transport model at the effective spatial resolution of IRS over Brittany, synthetic spectra are computed using the 4A/OP radiative transfer model. NH3 measurement-sensitivity of the future IRS-MTG mission is discussed with regards to the presently available IASI observations.

This is the second of two back-to-back works, whose goal is to generate line lists for the v(1) band (3551.766 cm(-1)) and for the v(1)+v(9)-v(9) and v(1)+v(7)-v(7) hot bands for HNO3 in the 2.8 mu m region. Also, we computed line lists for the weaker v(1)-v(9), v(1)+v(9), and v(1)+v(7) bands, centered at 3093.53 cm(-1), 4006.97 cm(-1) and 4127.78 cm(-1), respectively. While the first paper (Ref.[1], the preceding article in this issue), here labeled as "Part-1", was devoted to the description of the determination of the needed spectroscopic parameters in term of line positions and intensities, this paper describes the resulting line lists and the validation process. For this task, we use laboratory spectra recorded at high resolution and described in Part-1, together with the cross-sections established by the Pacific Northwest Laboratory at low resolution. Also, we use high resolution stratospheric solar occultation spectra recorded by the MkIV balloon-borne instrument of the NASA Jet Propulsion Laboratory (JPL). Similar inter-comparisons with laboratory or atmospheric spectra are also performed using the pseudo-line-list generated at JPL in 2005 and with the ab initio ExoMol list calculated by the University College of London. It proves that the line list generated during this work provides significant better agreements between observed and computed spectra.

•First high resolution analysis of the ν1 band of HNO3 at 3551.7 cm−1.•Fourier transform spectroscopy.•Evidence of large amplitude torsional splittings in the 11 vibrational state of HNO3.•First analysis of ν1-ν9, ν1 + ν9 and ν1 + ν7 bands for nitric acid.•SOLEIL synchrotron.

Fifty nine high sensitivity spectra of the R(6) manifold of the 2 nu 3 band of methane in air, near 1.64 mu m, have been recorded in support of the MERLIN mission. For this purpose, a cavity ring down spectrometer (CRDS) with a spectrally narrowed and stable (sub-kHz) laser source was coupled to a temperature regulated high-finesse optical cavity. The frequency scale of each spectrum was accurately determined from measurements of the beat note between a part of the laser light and the closest tooth of a frequency comb referenced to a rubidium clock. Series of spectra were recorded between 243 and 313 K with a 10 K temperature step. For each series, total pressure values of 50, 100, 250, 500 and 750 Torr were adopted. A multi-spectrum fitting procedure with the Hartmann-Tran (HT) line profile, including the first-order line-mixing parameter, has been used to derive the spectroscopic parameters for each of the six R(6) components, along with the temperature dependence of the line-shape parameters. The results show that the fitted effective model is able to reproduce the experimental spectra with a relative precision better than 0.2% for the entire R(6) manifold spectral region and better than 0.05% at the ON-line position of the MERLIN mission for the 250, 500 and 750 Torr spectra. The relative precision increases to 0.3% and the residuals at the ON-line position to 0.1% when including the 50 and 100 Torr spectra. Comparisons with ground-based atmospheric measurements show that these data significantly improve the modeling of methane absorption in this spectral region. The complete line list of the methane spectrum in the region of the R(6) manifold allowing notable improvement of the modeling of the absorption cross-section at the ON-line position of the MERLIN mission is provided as Supplementary Material. (c) 2023 Elsevier Ltd. All rights reserved.