The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an airborne imaging infrared limb sounder. It delivers temperature, trace gas mixing ratios, and information on clouds of the mid-upper troposphere and lower stratosphere (UTLS) with high spatial, temporal and spectral resolution. During the last ten years, GLORIA has participated in various measurement campaigns concerning atmospheric transport and composition all over the world. Its main carrier is the German research aircraft HALO (Gulfstream G550) but it has also been operated on the Russian M55 Geophysica stratospheric research aircraft and on a stratospheric balloon platform. Instrument development and operation are joint efforts of the federal German research centre Jülich and of the Karlsruhe Institute of Technology. The talk will present an overview of the instrument hardware, of processing scheme and instrument performance and of some selected scientific application cases.

The GLORIA spectrometer is a single-slide linear Michelson design with an adjustable maximum optical path difference of up to 8 cm. On research aircraft it is suspended by a three-axis gimballed frame system which allows compensation of aircraft movements and provides a panning ability for tomographic retrievals. On a balloon, elevation and azimuth are controlled by turning the spectrometer and gondola, respectively. The atmospheric scene is imaged on a HgCdTe-focal plane array providing spatial sampling with a few hundred metres sampling distance of the air mass at the tangent point. Spectral coverage is from 780 to 1400 cm-1, while large area blackbodies and a “deep space” view serve as targets for radiometric and spectral calibration. The instrument achieves a gain accuracy of 1% and a spectral accuracy of better than 5 ppm.

GLORIA science focusses on the composition and dynamic structure of the UTLS. The observation of small-scale variability and filamentation, caused by vertical and horizontal gradients of trace gases and by transport processes in this domain of the atmosphere, is an outstanding capability of the instrument. Three dimensional observations of temperature and trace gas distributions, provided by tomographic retrievals, are needed to better understand e.g. gravity wave excitation and the tropopause region. GLORIA was deployed in many field campaigns, addressing various scientific questions, e.g. the seasonality of composition and dynamics of air masses above the Atlantic, the influence of climate change and biomass burning on polar regions, the transport pathways in the Asian monsoon, wave-driven isentropic exchange above the Atlantic, and atmospheric transport, composition and dynamics in the Southern hemisphere.

The intense scientific use of GLORIA in the last ten years led to many improvements in the instrument and algorithms. GLORIA demonstrates that the technique of limb imaging Fourier transform spectroscopy is powerful and robust. The high spatial and spectral resolution of the acquired data offers self-validation opportunities that allow deducing instrument properties like e.g. non-linearity from flight data. The wide spectral coverage and good sensitivity permitted unexpected scientific insights, like e.g. the detection of ammonium nitrate in the Asian Tropopause Aerosol Layer.
GLORIA is versatile, complex and operates reliably under difficult environmental conditions. It demonstrates that this technique is mature for application in space. The CAIRT mission will bring atmospheric limb-IR imaging to a new level by its global deployment and strongly enhanced vertical coverage.

Anthropogenic atmospheric emissions of the reactive nitrogen components nitrogen dioxide (NO2) and ammonia (NH3) have majorly altered the global nitrogen cycle in the past 100 years, with devastating consequences to biodiversity, soil, water and air quality. Thanks to effective legislation, NO2 emissions are declining worldwide. Unfortunately, this is not the case for NH3 whose concentrations are on the rise in Europe and most other parts of the developed world, underlining the need for stricter legislation supported by effective monitoring means.
Both species are currently actively monitored with several satellite sounders, which provide daily global measurements. However, the spatial resolution of current sounders is inadequate for resolving the highly heterogonous spatial distributions of these species. This is particularly the case for point source emitters, for which satellites are currently only able to quantify the largest and most isolated ones. To fill the important gap in the monitoring landscape, a satellite called Nitrosat has been proposed in answer to ESA’s Earth Explorer call. The satellite, which recently entered Phase 0 studies, would allow making simultaneous measurements of NO2 and NH3 at a spatial resolution of 500 meter or below. In support to the Nitrosat proposal, ESA is funding the Nitrocam project (Nitrogen cycle airborne measurements), with at its core more than ten airborne campaigns in Germany. Each campaign surveyed a gapless area of at least 20 by 10 km at high spatial resolution, and each time spectroscopic measurements were performed in the visible and infrared spectral domain.
Here we present the results of the 2020 and 2021 campaigns performed in the surroundings of Berlin, Germany, with a focus on the flights that surveyed industrial point sources of NH3. In particular, we present the measurements performed over an industrial fertilizer production plant and a plant producing soda ash. We present detailed emission flux calculations, including uncertainty budget calculations and explore several independent retrieval techniques. We show that the measurements, downsampled to the Nitrosat spatial resolution, allow deriving accurately the emission fluxes, successfully demonstrating the Nitrosat measurement concept and scientific potential.

The IFTS instrument is an imaging Fourier transform spectrometer proposed as part of the larger Canadian AIM-North/Arctic Observing Mission (AOM) project currently under consideration. The IFTS is designed to provide high spatial and temporal measurements of CO2 and CH4 column averages over land at northern latitudes. The combination of an imaging technique and a highly elliptical orbit provides a unique opportunity to both monitor emissions and quantify natural and anthropogenic sources and sinks. A version of the instrument developed for sub-orbital testing measures high spectral resolution radiances in the nadir-viewing geometry in the oxygen A band and CO2 and CH4 absorption bands near 1.6 micron. The suborbital demonstration of the instrument from a high-altitude stratospheric balloon platform is planned for the second half of 2022. Here we provide an overview of the retrieval algorithms being developed for the project, as well as preliminary results from simulation sensitivity studies.

In recent years, the availability of a large amount of data has stimulated the use of synergistic approaches to gain the largest amount of information from measurements provided by a great number of satellite missions. Two classes of strategies are widely used to combine multiple sets of independent retrievals of the same atmospheric target: the synergistic retrieval (SR) and the a posteriori combination of the retrieved products. The Complete Data Fusion (CDF) algorithm was presented as a new a posteriori method to combine independent measurements of the same vertical profile of an atmospheric parameter into a single estimate for a concise and complete characterization of the atmospheric state. The CDF is able to reduce the complexity of managing high volumes of data and to improve their quality with respect to the operational outcome of individual instruments.

FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment has been selected to be the ninth Earth Explorer mission of the European Space Agency (ESA) and it is scheduled for launch on a polar orbiting satellite in 2027. The core instrument will be a Far-InfraRed (FIR) spectrometer measuring, at nadir, the upwelling spectral radiance emitted by the Earth across the most relevant infrared part of the electromagnetic spectrum, from 100 to 1600 cm−1 (from 100 to 6.25 microns in wavelength), with a spectral resolution of 0.5 cm-1.

FORUM will fly in loose formation with the MetOp-SG satellite, hosting the Infrared Atmospheric Sounding Interferometer – New Generation (IASI-NG). Since IASI-NG will measure the Middle-InfraRed (MIR) part of the upwelling atmospheric spectrum (from 645 to 2760 cm-1), the matching FORUM FIR spectra can be effectively used to synergistically complement the IASI-NG measurements. The Level 2 products of FORUM include the water vapour profile, surface spectral emissivity in dry regions, cloud parameters and spectral fluxes.

In this work, we present the results obtained from the application of two synergistic approaches, the SR and the CDF, to IASI-NG and FORUM synthetic measurements. We carried out two sets of test retrievals, based on synthetic clear-sky measurements, emulating an idealized situation in which both FORUM and IASI-NG measure, either with perfect matching or with a realistic mismatch, for 900 times, the same area over the Antarctic Plateau surface covered by coarse snow. The retrieval products considered in this study include the profiles of water vapour and temperature, surface temperature and spectral emissivity from 100 to 2300 cm-1. To characterize the quality of synergistic and fused products we evaluate their number of degrees of freedom as well as their total errors from the statistics of the differences between retrieved and true state parameters. Finally we compare the results obtained by the two methods by considering the statistics of the differences between synergistic and fused products.

We find that in case of perfectly matching measurements, SR and CDF provide results that differ by less than 1/10 of their associated noise retrieval error. If a realistic mismatch (both in time and space) between the measurements is considered, the two methods provide more different results but with differences that are still within the retrieval error. The results obtained from the two methods are compared and, for both of them, the effect of the mismatch on the synergistic products is quantified.

In the frame of the study “FORUM-Scienza” (FORUM-Science), funded by the Italian Space Agency (ASI), a fast code to simulate top of the atmosphere Far InfraRed (FIR) and Thermal InfraRed (TIR) spectral radiances has been developed. The code, named σ-FORUM, is an upgraded version of the fast radiative transfer model developed for IASI (σ-IASI [1,2]). It enables the simulation of both clear and cloudy sky monochromatic radiances, simultaneously computing their derivatives with respect to the injected atmospheric and surface state. σ-FORUM has been embedded in a Bayesian retrieval code (named FAst Retrieval Model - FARM) that enables to invert measured radiances to obtain surface emissivity, surface temperature, and vertical distributions of temperature, trace gases and cloud properties. FARM can be applied to any broadband, spectrally resolved ground-based, airborne or satellite measurement in the FIR and TIR frequency regions (from 50 to 2600 cm-1). Optionally, the code can also operate the synergistic retrieval from co-located measurements of two different sounders, like the ones anticipated for FORUM and IASI-NG.
In this work, we report the structure of FARM, its performance on FORUM simulated measurements, and some results obtained from the analysis of FORUM-like measurements acquired with two FORUM demonstrators, specifically, the REFIR-PAD measurements collected in a stratospheric balloon flight campaign at Teresina (Nord-East Brazil)
References
[1] Amato, U.; Masiello, G.; Serio, C.; Viggiano, M. (2002). The σ-IASI code for the calculation of infrared atmospheric radiance and its derivatives. ENVIRONMENTAL MODELLING & SOFTWARE, vol. 17/7; p. 651-667, doi:10.1016/S1364-8152(02)00027
[2] Martinazzo, M.; Magurno, D.; Cossich, W.; Serio, C.; Masiello, G.; Maestri, T. (2021) Assessment of the accuracy of scaling methods for radiance simulations at far and mid infrared wavelengths. JOURNAL OF QUANTITATIVE SPECTROSCOPY & RADIATIVE TRANSFER http://dx.doi.org/10.1016/j.jqsrt.2021.107739
[3]L.Palchetti, C.Belotti, G.Bianchini, F.Castagnoli, B.Carli, U.Cortesi, M.Pellegrini, C.Camy-Peyret, P.Jeseck, and Y.Té, (2006) Technical note: First spectral measurement of the Earth s upwelling emission using an uncooled wideband Fourier transform spectrometer, Atmos. Chem. Phys. 6, pp.5025-5030.

The IASI family of instruments has been sounding the atmosphere since 2006 onboard the Metop satellite series. IASI is a versatile mission, allowing the measurement of both meteorological parameters (such as temperature) and atmospheric composition for infrared absorbing species, using a Fourier transform spectrometer. With its long observation record and frequent overpasses, IASI is able to follow changes at different spatial scales.

Using the radiance data recorded in the thermal infrared spectral range, concentrations for atmospheric pollutants such as carbon monoxide (CO), ozone (O3), sulfur dioxide (SO2) and ammonia (NH3) can be derived. IASI CO and O3 fields are currently assimilated in regional and global models in order to predict air quality over Europe; SO2 alerts are delivered when exceptional levels are encountered, and NH3 source hotspots are tracked throughout the globe.

Near-real time observation of these atmospheric pollutants allow to follow them at city, country and continent scales. This talk will present the recent findings, with a focus on fire events, pollution episodes, and other unexpected features.