Estimating the impact of the 1991 Pinatubo eruption on mesospheric temperature by analyzing HALOE (UARS) temperature data

The Mt. Pinatubo eruption in 1991 had a severe impact on the Earth system with a well-documented warming of the tropical lower stratosphere and a general cooling of the surface. This study focuses on the impact of this event on the mesosphere by analyzing solar occultation temperature data from the Halogen Occultation Experiment (HALOE) instrument on the Upper Atmosphere Research Satellite (UARS). Previous analysis of lidar temperature data found positive temperature anomalies of up to 12.9 K in the upper mesosphere that peaked in 1993 and were attributed to the Pinatubo eruption. Fitting the HALOE data according to a previously published method indicates a maximum warming of the mesosphere region of 3.3 K and does not confirm significantly higher values reported for that lidar time series. An alternative fit is proposed that assumes a more rapid response of the mesosphere to the volcanic event and approximates the signature of the Pinatubo with an exponential decay function having an e-folding time of 6 months. It suggests a maximum warming of 5.5 K if the mesospheric perturbation is assumed to reach its peak 4 month after the eruption. We conclude that the HALOE time series probably captures the decay of a Pinatubo-induced mesospheric warming at the beginning of its measurement period.

Nadir retrieval of ice clouds and dust from NOMAD/UVIS on board Exomars TGO

<p>The NOMAD (“Nadir and Occultation for MArs Discovery”) spectrometer suite on board the ExoMars Trace Gas Orbiter (TGO) has been designed to investigate the composition of Mars' atmosphere using a suite of three spectrometers operating in the UV-visible and infrared. NOMAD is a spectrometer operating in ultraviolet (UV), visible and infrared (IR) wavelengths covering large parts of the 0.2-4.3 µm spectral range [1].</p> <p>The UV-visible “UVIS” instrument covers the spectral range from 200 to 650 nm and can perform solar occultation, nadir and limb observations [2]. The main purpose of UVIS is dedicated to the analysis and monitoring of ozone and aerosols such as dust and ice clouds.  In the present work we will present preliminary results of the aerosol retrieval in the UV recorded in nadir geometry: spatial and seasonal distribution of ice clouds and dust.</p> <div> <p> </p> <p>References<br />[1] Vandaele et al. 2018. Space Sci. Rev.<br />[2] Patel et al., 2017. Applied Optics.</p> </div> <p> </p>

Update on CO2 and temperature profiles retrievals from NOMAD-SO on board ExoMars TGO

<ul> <li>The SO channel of the NOMAD instrument</li> </ul> <p>The NOMAD-SO channel [1] is an infrared spectrometer working in the 2.2 to 4.3 µm spectral range (2200-4500 cm<sup>-1</sup>) and started to perform solar occultation measurement on April 21, 2018. The instrument is composed of an echelle grating coupled to an Acousto-Optical Tunable Filter for the diffraction order selection. As TGO is on a quasi-circular orbit at around 400 km of altitude, it performs one orbit every two hours. During a solar occultation measurement, SO scans six diffraction orders each second. These diffraction orders are recorded on four bins leading to a vertical sampling lower than one km. The calibration of the SO channel is described in [2] and is still being fine-tuned.</p> <ul> <li>CO<sub>2</sub> density and temperature profiles retrievals</li> </ul> <p>Several diffraction orders probe different altitude ranges as they contain CO<sub>2</sub> lines with different intensities that appear and saturate at different altitudes. Correct temperature profiles are necessary for the retrieval of several species and the profiles have to be carefully retrieved as their inversion is very sensitive to noise. We use the following retrieval scheme:</p> <p>For each solar occultation measurement, we derive a slant column profile of CO<sub>2</sub> using ASIMUT-ALVL [3]. ASIMUT is a radiative transfer program developed at BIRA-IASB and uses the Optimal Estimation Method for regularization [4]. The GEM-Mars GCM provides the <em>a priori</em> profiles of CO<sub>2</sub> local density, pressure and temperature. We then apply an iterated Tikhonov regularization to derive a regularized local density profile using an improved version of the algorithm described in [5]. This method requires the selection of a regularization parameter to reduce as much as possible the presence of noise in the profile while keeping the real variations. This retrieval scheme allows a fine-tuning of the reguralization parameter. We finally apply the hydrostatic equilibrium equation to derive the temperature profiles [6]. We derived the NOMAD-SO CO<sub>2</sub> and temperature profiles for MY34 and 35.</p>

The Martian environment observed by NOMAD on ExoMars Trace Gas Orbiter

<p>The NOMAD (“Nadir and Occultation for MArs Discovery”) spectrometer suite on board the ExoMars Trace Gas Orbiter has been designed to investigate the composition of Mars' atmosphere, with a particular focus on trace gases, clouds and dust. The instrument probes the ultraviolet and infrared regions covering large parts of the 0.2-4.3 µm spectral range [1,2], with 3 spectral channels: a solar occultation channel (SO – Solar Occultation; 2.3–4.3 μm), a second infrared channel capable of nadir, solar occultation, and limb sounding (LNO – Limb Nadir and solar Occultation; 2.3–3.8 μm), and an ultraviolet/visible channel (UVIS – Ultraviolet and Visible Spectrometer, 200–650 nm). Since its arrival at Mars in April 2018, NOMAD performed solar occultation, nadir and limb observations dedicated to the determination of the composition and structure of the atmosphere.</p><p>NOMAD has been accumulating data about the Martian atmosphere and its surface since its insertion. We will present some results covering the atmosphere composition including clouds and dust, climatologies of water, carbon monoxide and ozone. We also report on the different discoveries highlighted by the instrument by pointing to a series of contributions to this conference that will present in detail several specific studies, like recent progress in the instrument calibration, the latest CO2 and temperature vertical profiles, studies of aerosol nature and distribution, water vapor profiles and variability, carbon monoxide vertical distribution, dayglow observations; detection of HCl, its vertical profiles and in general advances in the analysis of the spectra recorded by the three channels of NOMAD.</p><p>References</p><p>[1] Vandaele, A.C., et al., 2015. Planet. Space Sci. 119, 233-249.</p><p>[2] Vandaele et al., 2018. Space Sci. Rev., 214:80, doi.org/10.1007/s11214-11018-10517-11212.</p><p> </p>

Variation of CO/CO2 profiles in the Marian mesosphere and lower thermosphere retrieved from TGO/NOMAD

<p>CO is produced by the photodissociation of CO<sub>2</sub> and recycled to CO<sub>2</sub> by the catalytic cycle involving HOx in the Martian atmosphere [e.g., McElroy & Donahue, 1972]. In the mesosphere and lower thermosphere (MLT) region of Mars, the number density of CO is determined by photodissociation, diffusion, and atmospheric circulation. The increase of the CO mixing ratio in the MLT region and further enhancement in the polar region due to the transport of CO-enriched air via meridional circulation are predicted in the 3D models [Daerden et al., 2018; Holmes et al., 2019]. On the other hand, the decrease in the CO mixing ratio in the MLT region during a global dust storm is detected by TGO/ACS, which suggests that the increase in the hygropause altitude leads to the increase in the vertical range over which OH becomes available to convert into CO<sub>2</sub> [Olsen et al., 2021]. Additionally, a substantial variation of the homopause altitude has been investigated [Slipski et al., 2018; Jakosky et al., 2017; Yoshida et al., 2020], which suggests that the order of magnitude changes in the eddy diffusion coefficient at the homopause [Slipski et al., 2018], and then variations in the profile of CO mixing ratio in the MLT region. However, the effects of change in the eddy diffusion coefficient on the profile of CO mixing ratio have not been investigated. The variability of the CO mixing ratio profiles can be a clue for understanding the dynamical coupling between the lower and the upper atmospheres.</p> <p>To clarify the contributions of photochemistry, diffusion, and atmospheric circulation to the CO/CO<sub>2</sub> profiles in the MLT region, we use the Nadir and Occultation for MArs Discovery (NOMAD) instrument aboard Trace Gas Orbiter (TGO). NOMAD solar occultation is designed as the combination of the Acousto Optical Turnable Filter and echelle grating [Neefs et al., 2015; Thomas et al., 2016]. NOMAD solar occultation operates in the wavelength range of 2.2 - 4.3 μm (2320 to 4350 cm<sup>-1</sup>) with a high spectral resolution (λ/dλ = 20000) [Vandaele et al., 2018]. It provides us CO and CO<sub>2</sub> spectra below 100 km and 180 km altitudes, respectively.</p> <p>In this study, we applied the equivalent width technique [Chamberlain and Hunten, 1987; Krasnopolsky, 1986] to derive a new set of CO and CO<sub>2</sub> column densities, respectively, with the observed atmospheric transmittance spectra by NOMAD solar occultation. The absorption lines centered at 4285.0, 4288.2, and 4291.5 cm<sup>-1</sup> for CO (2-0) band and 3358.7, 3364.9, and 3366.4 cm<sup>-1</sup> for CO<sub>2</sub> (21102-00001) band are carefully selected for retrievals due to the contribution of nearby and central orders [cf. Liuzzi et al., 2019]. It is noted that the line strengths of the selected CO<sub>2</sub> have high sensitivity to the background temperature. In this study, we applied the vertical profiles of temperature simulated in the GEM-Mars model [Neary et al., 2018; Daerden et al., 2019]. We retrieve the CO and CO<sub>2</sub> slant column densities between 60 and ~100 km altitudes because those slant opacities are saturated below 60 km altitude. The CO and CO<sub>2</sub> spectra observed from April 2018 to September 2020, corresponding to from MY 34 Ls ~ 150 to MY 35 Ls ~ 280, are investigated.</p> <p>We found that the retrieved CO/CO<sub>2</sub> ratio between 60 and ~100 km increases with altitude. A behavior of the decrease in the CO/CO<sub>2</sub> ratio during the global dust storm corresponds to the previous observations [Olsen et al., 2021]. However, the CO/CO<sub>2</sub> profiles also vary with season and latitude. For interpretation, the 1D photochemical model will be compared with newly obtained CO/CO<sub>2</sub> profiles, especially in order to discuss the contributions from the variations in eddy diffusion coefficient and photochemistry in the MLT region on Mars.</p>

Water vapor vertical distributions on Mars: Results from three years of TGO/NOMAD science operations

<div> <p>Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO) started  science measurements on 21 April, 2018. Here, we present results on the retrievals of water vapor vertical distributions in the Martian atmosphere from three years of TGO/NOMAD science operations.</p> </div> <p><strong> </strong></p> <p>NOMAD is a spectrometer operating in the spectral ranges between 0.2 and 4.3 μm onboard ExoMars TGO. NOMAD has 3 spectral channels: a solar occultation channel (SO – Solar Occultation; 2.3–4.3 μm), a second infrared channel capable of nadir, solar occultation, and limb sounding (LNO – Limb Nadir and solar Occultation; 2.3–3.8 μm), and an ultraviolet/visible channel (UVIS – Ultraviolet and Visible Spectrometer, 200–650 nm). The infrared channels (SO and LNO) have high spectral resolution (λ/dλ~10,000–20,000) provided by an echelle grating used in combination with an Acousto Optic Tunable Filter (AOTF) which selects diffraction orders. The sampling rate for the solar occultation measurement is 1 second, which provides a good vertical sampling step (~1 km) with higher resolution (~2 km) from the surface to 200 km. Thanks to the instantaneous change of the observing diffraction orders achieved by the AOTF, the SO channel is able to measure five or six different diffraction orders per second in solar occultation mode. In this study, we analyze the solar occultation measurements at diffraction order 134 (3011-3035 cm<sup>-1</sup>), order 136 (3056-3080 cm<sup>-1</sup>), order 168 (3775-3805 cm<sup>-1</sup>), and order 169 (3798-3828 cm<sup>-1</sup>) acquired by the SO channel in order to investigate water vapor vertical distributions.</p> <p>Knowledge of the water vapor vertical profile is important to understand the water cycle and its escape process. Solar occultation measurements by two new spectrometers onboard TGO - NOMAD and Atmospheric Chemistry Suite (ACS) - allows us to daily monitor the water vapor vertical distributions through the whole Martian Year and obtain a good latitudinal coverage for every ~20° of Ls. In 2018, for the first time after 2007, a global dust storm occurred on Mars. It lasted for more than two months (from June to August). Moreover, following the global dust storm, a regional dust storm occurred in January 2019. The NOMAD and ACS observations therefore fully cover the majority of the global and regional dust storms and offer a unique opportunity to study the trace gases distributions during the dust storms. We analyzed those datasets and found a significant increase of water vapor abundances in the middle atmosphere (40-100 km) during the global dust storm from June to mid-September 2018 and the regional dust storm in January 2019. In particular, water vapor reaches very high altitude, at least 100 km, during the global dust storm (Aoki et al., 2019, Journal of Geophysical Research, Volume124, Issue12, Pages 3482-3497, doi:10.1029/2019JE006109). A GCM simulation explained that dust storm related increases of atmospheric temperatures suppress the hygropause, hence reducing ice cloud formation and so allowing water vapor to extend into the middle atmosphere (Neary et al., 2020, Geophysical Research Letters, accepted, Volume47, Issue7, e2019GL084354, doi: 10.1029/2019GL084354). This study presents the results with the extended dataset, which covers a full Mars year. The extended dataset newly includes aphelion season that involves interesting phenomena such as sublimation of water vapor from the northern polar cap and formation of the equatorial cloud belt, which are known as key periods to understand the large north-south hemispheric asymmetries of Mars water vapor. Yet, only a few papers report the water vapor vertical distributions in the aphelion season. The extended dataset also includes the southern summer season (dusty season) in MY 35, which will allow us to compare the water vapor distributions in the global dust storm year with those in the non-global dust storm year. In the presentation, we will discuss the water vapor vertical profiles as well as the aerosols vertical distributions retrieved from the three-year measurements of the TGO/NOMAD.</p>

CH4 abundance in Jupiter's upper atmosphere: A re-analysis of the ISO/SWS 3.3 µm non-LTE emission

<p>CH<sub>4</sub> plays a key role in the thermal structure of Jupiter's upper atmosphere and hence knowing its vertical distribution is crucial for its understanding. Methane concentrations have been inferred previously from the analyses of solar occultation, He and Ly-α airglow, and the ISO/SWS radiance measurements around 3.3 µm, showing all rather different values, particularly around the homopause. Even different analyses of the same ISO/SWS radiance spectra yield very different CH<sub>4</sub> volume mixing ratio profiles. Here, we present a new analysis of the ISO/SWS radiance spectra by using a comprehensive non-Local Thermodynamic Equilibrium (non-LTE) model and the most recent collisional rates measured in the laboratory. Further, we briefly discuss the potential effects of non-LTE on CH<sub>4 </sub>3.3 µm emission of temperate Jupiter exoplanets.</p>

Calibration of the Nomad SO Channel on ExoMars Trace Gas Orbiter

<p><strong>Introduction</strong></p> <p>NOMAD is a three-channel spectrometer on the ExoMars 2016 Trace Gas Orbiter. The NOMAD solar occultation (SO) channel has been operating around Mars since April 2018 [1]. In the past three years of science operations, we have performed many calibrations and have taken millions of spectra. This huge dataset allows us to continue to refine the calibration, through additional characterisation of the optical elements, detector performance and temperature-induced effects.</p> <p><strong>SO channel</strong></p> <p>By using an Acousto-Optic Tunable Filter (AOTF) in combination with an echelle grating spectrometer, the SO channel is able to operate at unprecedented spectral resolution – typically 0.15 to 0.25 cm<sup>-1</sup> [2] – and is therefore able to measure trace gases and set stringent upper limits on the presence (or non-presence) of organic molecules in the Martian atmosphere [3].</p> <p>In solar occultation mode, the Sun is used as the illumination source, meaning that very high Signal-to-Noise Ratios (SNRs) can be achieved, typically up to 2,000 for a single spectrum at the top of the atmosphere. The SO channel records 24 spectra per second in solar occultation mode, with a typical vertical sampling resolution (i.e. altitude difference between consecutive spectra) of around 50-150m [4]. Therefore, by binning multiple spectra together, the SNR can be increased significantly whilst still keeping a high vertical sampling resolution.</p> <p><strong>Calibration</strong></p> <p>However the AOTF-grating combination also presents additional challenges, as the optical properties of both elements must be independently characterised in order to correctly retrieve atmospheric gas concentrations. Analysis of the AOTF shape and sidelobes, and the Instrument Line Shape (ILS), are ongoing in several of the groups within the NOMAD team. Additional systematic effects have also been identified in the spectra e.g. [5] which hamper efforts to further increase SNR and thus reduce detection limits via binning. Whilst much calibration work on this has already been performed [2, 4], the calibration can still be improved and work is ongoing to understand these effects and how to remove them from the data.</p> <p><strong>Applications</strong></p> <p>Since the beginning of the mission, a particular emphasis has been placed on making observations of CH<sub>4</sub> around Gale Crater, in both nadir and solar occultation modes. HCl detections observations are now also made regularly, following the discovery of HCl in the atmosphere [7, 8] and its isotopologue [5], which are close to the detection limit of the channel. Calibration improvements will allow us to further constrain the CH<sub>4</sub> upper detection limit, observe lower concentrations of HCl, and further constrain the isotopic ratio.</p> <p><strong>References</strong></p> <p>[1] Vandaele, A. C. et al (2015) Planet. Space Sci., 119 ; [2] Liuzzi, G. et al. (2019) Icarus, 321 ; [3] Knutsen, E. W. et al. (2021), Icarus, 357 ; [4] Thomas, I. R. et al. (submitted 2021) “Calibration of NOMAD on ExoMars Trace Gas Orbiter: Part 1 – the SO channel”; [5] Liuzzi G. et al (2021) GRL 48(9); [6] Korablev, O. et al. (2021) Science Advances, 7(7); [8] Aoki, S. et al. (accepted 2021) “Annual appearance of hydrogen chloride on Mars and a striking similarity with the water vapor vertical distribution observed by TGO/NOMAD”, GRL.</p>

A Concept of 2U Spaceborne Multichannel Heterodyne Spectroradiometer for Greenhouse Gases Remote Sensing

We present the project of a 2U CubeSat format spaceborne multichannel laser heterodyne spectroradiometer (MLHS) for studies of the Earth’s atmosphere upper layers in the near-infrared (NIR) spectral range (1258, 1528, and 1640 nm). A spaceborne MLHS operating in the solar occultation mode onboard CubeSat platform, is capable of simultaneous vertical profiling of CO2, H2O, CH4, and O2, as well as Doppler wind measurements, in the tangent heights range of 5–50 km. We considered the low Earth orbit for the MLHS deployment and analyzed the expected surface coverage and spatial resolution during one year of operations. A ground-based prototype of the MLHS for CO2 and CH4 molecular absorption measurements with an ultra-high spectral resolution of 0.0013 cm−1 is presented along with the detailed description of its analytical characteristics and capabilities. Implementation of a multichannel configuration of the heterodyne receiver (four receivers per one spectral channel) provides a significant improvement of the signal-to-noise ratio with the reasonable exposure time typical for observations in the solar occultation mode. Finally, the capability of building up a tomographic picture of sounded gas concentration distributions provided by high spectral resolution is discussed.