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As a potential biomarker, Martian methane has attracted attention through several reports of its detection over the last 20 years. However, the very existence of this gas has been continuously questioned, in particular because the observed lifetime should be several orders of magnitude shorter than the 300 years predicted by photochemical models. Although several fast removal processes have been hypothesized to explain the observations, none of them has met a large consensus. It is in this context that the ESA-Roscomos ExoMars Trace Gas Orbiter (TGO) mission started its science operations in April 2018. ACS and NOMAD, two instruments onboard the TGO, have been collecting hundreds of highly sensitive measurements in solar occultation. No methane has been detected so far and an upper limit of 0.02 ppbv has been derived. The implications of this result on the methane problem on Mars will be addressed in this work. This upper limit is a strong constraint on the background level and, in turn, on the potential emission scenarios making the reported methane detections consistent with the TGO results. While several model studies aimed at identifying them, we will here adopt a probabilistic approach to the problem in order to question the plausibility of those detections and estimate the lifetime required to make them plausible from a probabilistic standpoint.

Introduction: The NOMAD-UVIS instrument on board the ExoMars Trace Gas Orbiter has been investigating the Martian atmosphere with the occultation technique since April 2018 [1]. In the solar occultation mode, it is mainly devoted to studying the climatology of ozone and aerosols content [2,3,4]. We analyzed almost two Mars Years of ozone vertical distributions acquired at the day-night terminator, corresponding to more than 8300 solar occultations, acquired between April 2018 (MY 34, LS=163°) and November 2021 (MY 36, LS=132°). Retrieval method(s): As in the work of [5], the NOMAD-UVIS ozone retrievals proved more difficult than expected due to the presence of spurious detections of ozone caused by instrumental effects, high dust content, and very low values of ozone. This led us to compare the results from three different retrieval approaches: an onion peeling method (OP); a full occultation Optimal Estimation Method (FOEM), and a direct onion peeling method (DOP). The OP method is similar to that used for Mars and Venus stellar occultations [6,7]. The FOEM and DOP approaches are based on ASIMUT-ALVL, the BIRA-IASB radiative code [8,9]. The main challenge was to find reliable criteria to exclude spurious detections of O3, and we finally adopted two criteria for filtering: i) a detection limit, and ii) the Δχ2 criterion. Both criteria exclude spurious O3 values especially near the perihelion, where based on the simulations from a general circulation model, we do expect very low values of ozone. Comparison of filtering methods between UVIS and SPICAM: We compared the results of filtering with SPICAM-UVIS observations. The SPICAM team applied very similar criteria for filtering their data to the ones implemented here [5]. Even if the two instruments observed during different Martian Years, the agreement on the filtered O3 retrievals is very good, and both filtering approaches lead to very similar results (see Figure 1). Figure 1: An example of the effect of the selection criteria for SPICAM-UV and NOMAD-UVIS obser-vations for a latitude band (-60°;-30°). Left panels show the ozone vertical distribution before applying the filtering; right panels shows O3 profiles after the selection criteria. The O3-H2O relationship: Water vapor was observed by the infrared channel of the NOMAD SO. The results from a first analysis can be found in [10], while an extended dataset is presented in a companion abstract [11]. Water vapor and ozone are measured simultaneously, which allows us to investigate the water-ozone correlation, the key to addressing the atmospheric chemistry on Mars. We present correlation plots of O3 vs. H2O at high latitudes (60°-90°, both hemispheres), and at the equator (30°S-30°N). It is important to notice that during a solar occultation experiment at the terminator, ozone may exhibit rapid changes due to photolysis that are uncorrelated to water vapor. As an example, we show the 60°N-90°N latitude region (Figure 2): a clear anti-correlation is observed at lower altitudes, up to 40 km. O3 is roughly proportional to (H2O)-1.0 up to 30 km and a variation with Ls seems also present. Figure 2: O3 (cm-3) vs. H2O (cm-3) vertical profiles measured simultaneously by NOMAD-UVIS at high latitudes in the Northern hemisphere (60°N-90°N). (a) 10 – 20 km; (b) 20 – 30 km; (c) 30 – 40 km; (d) 40 – 50 km. Colours indicate the Ls interval. The black line shows the function O3 =H2Ox, with x varying with the altitude range. Impact of gradients at the Martian terminator: Rapid variations in species concentration at the terminator have the potential to cause asymmetries in the species distributions along the line of sight (LOS) of a solar occultation experiment. Ozone, in particular, displays steep gradients across the terminator of Mars due to photolysis [12]. Nowadays, most of the retrieval algorithms for solar and stellar occultations rely on the assumption of a spherically symmetrical atmosphere. However, photochemically induced variations near sunrise/sunset conditions need to be taken into account in the retrieval process in order to prevent inaccuracies. We investigated the impact of gradients along the LOS for the retrieval of ozone under sunrise/sunset conditions. We used the diurnal variations in the ozone concentration obtained from photochemical model calculations together with an adapted radiative transfer code. References: [1] Vandaele et al. (2015), PSS. [2] Patel et al. (2021), JGR (Planets). [3] Khayat et al. (2021), JGR (Planets). [4] Neefs, E., et al. (2015) Applied Optics. [5] Määttänen et al. Icarus, in review. [6] Quémerais et al. (2006), JGR (Planets). [7] Piccialli et al. (2015), PSS. [8] Vandaele et al., (2008), JGR (Planets). [9] Piccialli et al. (2021), Icarus. [10] Aoki et al. (2019), JGR (Planets). [11] Aoki et al., (2021) EPSC. [12] Lefèvre, et al. (2008), Nature.

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, 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). NOMAD performs solar occultation, nadir and limb observations dedicated to the determination of the composition and the structure of the Martian atmosphere. TGO started its science phase in April 2018 and instruments have now been accumulating data for more than two Martian years. We will present selected results obtained by the NOMAD instrument covering the atmosphere composition with observations of several trace gases, dust, and clouds. 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, ozone vertical profiles, climatology and relation with water, airglow observations, detection of CO2 ice clouds, surface ices and in general advances in the analysis of the spectra recorded by the three channels of NOMAD. References [1] Vandaele, A.C., et al., 2015. Planet. Space Sci. 119, 233-249. [2] Vandaele et al., 2018. Space Sci. Rev., 214:80, doi.org/10.1007/s11214-11018-10517-11212.

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.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.References[1] Vandaele, A.C., et al., 2015. Planet. Space Sci. 119, 233-249.[2] Vandaele et al., 2018. Space Sci. Rev., 214:80, doi.org/10.1007/s11214-11018-10517-11212.

Hydrogen chloride (HCl) was recently discovered in the atmosphere of Mars by two spectrometers onboard the ExoMars Trace Gas Orbiter. The reported detection made in Martian Year 34 was transient, present several months after the global dust storm during the southern summer season. Here, we present the full data set of vertically resolved HCl detections obtained by the NOMAD instrument, which covers also Martian year 35. We show that the particular increase of HCl abundances in the southern summer season is annually repeated, and that the formation of HCl is independent from a global dust storm event. We also find that the vertical distribution of HCl is strikingly similar to that of water vapor, which suggests that the uptake by water ice clouds plays an important role. The observed rapid decrease of HCl abundances at the end of the southern summer would require a strong sink independent of photochemical loss. © 2021. American Geophysical Union., ExoMars is a space mission of the European Space Agency and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB- BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office, with the financial and contractual coordination by the European Space Agency Prodex Office (PEA 4000103401 and 4000121493), by the Spanish MICINN through its Plan Nacional and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1 ST/S00145X/1 ST/S00145X/1 and ST/T002069/1, and Italian Space Agency through grant 2018-2-HH.0. This work was supported by NASA's Mars Program Office under WBS 604796, "Participation in the TGO/NOMAD Investigation of Trace Gases on Mars." The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). This work was supported by the Belgian Fonds de la Recherche Scientifique-FNRS under grant numbers 30442502 (ET_HOME) and T.0171.16 (CRAMIC) and Belgian Science Policy Office BrainBe SCOOP and MICROBE Projects. S. A. is "Charge de Recherches" at the F.R.S.-FNRS. U.S. investigators were supported by the National Aeronautics and Space Administration. We acknowledge support from the Institut National des Sciences de l'Univers" (INSU), the "Centre National de la Recherche Scientifique" (CNRS) and "Centre National d'Etudes Spatiales" (CNES) through the "Programme National de Planetologie".

Following the recent detection of HCl in the atmosphere of Mars by ExoMars/Trace Gas Orbiter, we present here the first measurement of the 37Cl/35Cl isotopic ratio in the Martian atmosphere using a set of Nadir Occultation for MArs Discovery (NOMAD) observations. We determine an isotopic anomaly of −6 ± 78‰ compared to Earth standard, consistent with the −51‰–−1‰ measured on Mars’ surface by Curiosity. The measured isotopic ratio is also consistent with surface measurements, and suggests that Cl reservoirs may have undergone limited processing since formation in the Solar Nebula. The examination of possible sources and sinks of HCl shows only limited pathways to short-term efficient Cl fractionation and many plausible reservoirs of “light” Cl. © 2021. American Geophysical Union., ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by the Spanish MICINN through its Plan Nacional and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1 and ST/S00145X/1 and Italian Space Agency through grant 2018-2-HH.0. The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709), and the CBK PAN team from the EXOMHYDR project, carried out within the TEAM program of the Foundation for Polish Science cofinanced by the European Union under the European Regional Development Fund (TEAM/2016-3/20). This work was supported by NASA's Mars Program Office under WBS 604796, "Participation in the TGO/NOMAD Investigation of Trace Gases on Mars" and by NASA's SEEC initiative under Grant Number NNX17AH81A, "Remote sensing of Planetary Atmospheres in the Solar System and Beyond." U.S. investigators were supported by the National Aeronautics and Space Administration. S. Viscardy acknowledges support from the Belgian Fonds de la Recherche Scientifique-FNRS under grant numbers 30442502 (ET_HOME) and Belgian Science Policy Office BrainBe MICROBE Projects. S. Aoki is "Charge de Recherches" at the F.R.S-FNRS. F. Schmidt acknowledges support from the "Institut National des Sciences de l'Univers" (INSU), the "Center National de la Recherche Scientifique" (CNRS) and "Center National d'Etudes Spatiales" (CNES) through the "Program National de Planetologie". S. Robert thanks BELSPO for the FED-tWIN funding (Prf-2019-077-RT-MOLEXO).

General Circulation Models with interactive physical and chemistry processes are the state-of-the-art tools for an integrated view and understanding of the Martian atmosphere and climate system. The GEM-Mars model currently includes 16 tracers for chemical composition and applies a fully online, interactive calculation of the photo- and gas phase chemistry of carbon dioxide (CO2) and water vapor (H2O). These species largely control the chemical composition of the neutral Mars atmosphere through their photolysis products and their subsequent interactions. Water vapor undergoes a complex cycle on Mars as it is transported and interacts with ice reservoirs both at the planet's surface and at water ice clouds, which in turn provide radiative feedbacks. In the photochemical cycles involving CO2 and H2O, the abundances of 5 species have been reported by previous investigations with significant spatio-temporal coverage: CO2, H2O, CO, O3 and H2O2. This paper presents the current status of the atmospheric chemistry simulations in GEM-Mars by comparing them to a selection of these observational datasets as well as to oxygen dayglow emission from O2(a1∆g). The results are consistent with previous model-data comparisons and illustrate that the water cycle and the photochemistry are well implemented in the model. In particular, the simulation of the key reservoir species H2O2 provides a good match to the available data. Model-data biases for ozone columns and oxygen airglow are related to the simulated water vapor vertical profile, as these species have important column contributions from vertical layers at the top of the hygropause.

Methane (CH4) on Mars has attracted a great deal of attention since it was first detected in January 2003. As methane is considered a potential marker for past/present biological or geological activity, any possible detection would require evidence with strong statistical significance. Ethane (C2H6) and ethylene (C2H4) are also relevant chemical species as their shorter lifetimes in the Martian atmosphere make them excellent tracers for recent and ongoing releases. If detected, a CH4/C2Hn ratio could aid in constraining the potential source of organic production. Here we present the results of an extensive search for hydrocarbons in the Martian atmosphere in 240,000 solar occultation measurements performed by the ExoMars Trace Gas Orbiter/NOMAD instrument from April 2018 to April 2019. The observations are global, covering all longitudes and latitudes from 85°N to 85°S, and sampled from 6 to 100 km altitude with a typical vertical resolution of 2 km. There were no statistically significant detections of organics and new stringent upper limits for global ethane and ethylene were set at 0.1 ppbv and 0.7 ppbv, respectively. No global background level of methane was observed, obtaining an upper limit of 0.06 ppbv, in agreement with early results from ExoMars (Korablev et al., 2019). Dedicated searches for localized plumes at more than 2000 locations provided no positive detections, implying that if methane were released in strong and rapid events, the process would have to be sporadic. © 2020 Elsevier Inc. All rights reserved., The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAACSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1 and ST/S00145X/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique -FNRS under grant number 30442502 (ET_HOME). The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). Canadian investigators were supported by the Canadian Space Agency. This work was supported by NASA's Mars Program Office under WBS 604796, "Participation in the TGO/NOMAD Investigation of Trace Gases on Mars". MC is supported by the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Universities Space Research Association (USRA) under contract with NASA. S. A. is "Charge de Recherches" at the F.R.S.-FNRS. We would also like to thank Dr. Sara Faggi for valuable advice, expertise and continuous support during this study.

We will present the vertical distribution of ozone obtained from NOMAD-UVIS solar occultations and we will compare the results of three retrieval schemes.NOMAD (Nadir and Occultation for MArs Discovery) is a spectrometer composed of 3 channels: 1) a solar occultation channel (SO) operating in the infrared (2.3-4.3 μm); 2) a second infrared channel LNO (2.3-3.8 μm) capable of doing nadir, as well as solar occultation and limb; and 3) an ultraviolet/visible channel UVIS (200-650 nm) that can work in the three observation modes [1,2].The UVIS channel has a spectral resolution ozone and aerosols content [3].Since the beginning of operations, on 21 April 2018, NOMAD UVIS acquired more than 4000 solar occultations with an almost complete coverage of the planet.NOMAD-UVIS spectra are simulated using three different retrieval scheme:1) An onion peeling approach based on [4,5] deriving slant columns at the different altitudes sounded, from which local densities are obtained;2) The line-by-line radiative transfer code ASIMUT-ALVL developed at IASB-BIRA [6] using the Optimal Estimation Method to derive the local density profile in one go (on all transmittances of one occultation observation);3) A direct onion peeling method deriving sequentially from top to bottom the local densities in the different layers probed.We will compare results obtained from the different retrieval methods as well as their uncertainties and we will discuss the advantages and difficulties of each method.References[1] Vandaele, A.C., et al., Planetary and Space Science, Vol. 119, pp. 233–249, 2015.[2] Neefs, E., et al., Applied Optics, Vol. 54 (28), pp. 8494-8520, 2015.[3] M.R. Patel et al., In: Appl. Opt. 56.10 (2017), pp. 2771–2782. DOI: 10.1364/AO.56.002771.[4] Quémerais, E.,et al. J.Geophys. Res. (Planets)111, 9, 2006.[5] Piccialli, A. et al., Planetary and Space Science, 113-114(2015) 321–335[6] Vandaele, A.C., et al., JGR, 2008. 113 doi:10.1029/2008JE003140.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited., Isotopic ratios and, in particular, the water D/H ratio are powerful tracers of the evolution and transport of water on Mars. From measurements performed with ExoMars/NOMAD, we observe marked and rapid variability of the D/H along altitude on Mars and across the whole planet. The observations (from April 2018 to April 2019) sample a broad range of events on Mars, including a global dust storm, the evolution of water released from the southern polar cap during southern summer, the equinox phases, and a short but intense regional dust storm. In three instances, we observe water at very high altitudes (>80 km), the prime region where water is photodissociated and starts its escape to space. Rayleigh distillation appears the be the driving force affecting the D/H in many cases, yet in some instances, the exchange of water reservoirs with distinctive D/H could be responsible. © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science., This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401 and 4000121493), by the Spanish MICINN through its Plan Nacional, by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), and by the Spanish Science Ministry Centro de Excelencia Severo Ochoa Program under grant SEV-2017-0709, as well as by the U.K. Space Agency through grants ST/R005761/1, ST/P001262/1, ST/R001405/1, and ST/S00145X/1 and the Italian Space Agency through grant 2018-2-HH.0. This work was supported by NASA’s Mars Program Office under WBS 604796, “Participation in the TGO/NOMAD investigation of trace gases on Mars” and by NASA’s SEEC initiative under grant number NNX17AH81A, “Remote sensing of planetary atmospheres in the solar system and beyond.” M.J.C. was supported by the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Universities Space Research Association (USRA) under contract with NASA. S.A. is postdoctoral researcher of the Belgian Fund for Scientific Research (FNRS).

1. IntroductionRapid variations in species concentration at the terminator have the potential to cause asymmetries in the species distributions along the line of sight (LOS) of a solar occultation experiment. Ozone, in particular, displays steep gradients across the terminator of Mars due to photolysis [1]. Nowadays, most of the retrieval algorithms for solar and stellar occultations rely on the assumption of a spherically symmetrical atmosphere. However, photochemically induced variations near sunrise/sunset conditions need to be taken into account in the retrieval process in order to prevent inaccuracies.Here, we investigated the impact of gradients along the LOS of the solar occultation experiment TGO/NOMAD-UVIS for the retrieval of ozone under sunrise/sunset conditions. We used the diurnal variations in the ozone concentration obtained from photochemical model calculations together with an adapted radiative transfer code.2. The NOMAD UVIS channelNOMAD is a spectrometer composed of 3 channels: 1) a solar occultation channel (SO) operating in the infrared (2.3-4.3 μm); 2) a second infrared channel LNO (2.3-3.8 μm) capable of doing nadir, as well as solar occultation and limb; and 3) an ultraviolet/visible channel UVIS (200-650 nm) that can work in the three observation modes [2,3].The UVIS channel has a spectral resolution ozone and aerosols content [4].Since the beginning of operations, on 21 April 2018, NOMAD UVIS acquired more than 3000 solar occultations with an almost complete coverage of the planet.3. Retrieval techniqueNOMAD-UVIS spectra are simulated using the line-by-line radiative transfer code ASIMUT-ALVL developed at IASB-BIRA [5]. In a preliminary study based on SPICAM-UV solar occultations (see [6]), ASIMUT was modified to take into account the atmospheric composition and structure at the day-night terminator. As input for ASIMUT, we used gradients predicted by the 3D GEM-Mars v4 Global Circulation Model (GCM) [7,8]. UVIS ozone profiles will also be compared to SPICAM-UV retrievals. 4. Summary and future workWe will present ozone vertical profiles retrieved from the first Martian year of observations from TGO/NOMAD-UVIS. In addition, we plan to compare our retrievals to SPICAM-UV observations. As first step, we will retrieve O3 profiles without taking in account gradients. Then, we will investigate the effects of ozone density gradients on the retrieval of ozone.References[1] Lefèvre, F., Bertaux, J.L., Clancy, R. T., Encrenaz, T., Fast, K., Forget, F., Lebonnois, S., Montmessin, F., Perrier, S., Aug. 2008. Heterogeneous chemistry in the atmosphere of Mars. Nature 454, 971–975.[2] Vandaele, A.C., et al., Planetary and Space Science, Vol. 119, pp. 233–249, 2015.[3] Neefs, E., et al., Applied Optics, Vol. 54 (28), pp. 8494-8520, 2015.[4] M.R. Patel et al., In: Appl. Opt. 56.10 (2017), pp. 2771–2782. DOI: 10.1364/AO.56.002771.[5] Vandaele, A.C., et al., JGR, 2008. 113 doi:10.1029/2008JE003140.[6] Piccialli, A., Icarus, 2019, [7] Neary, L., and F. Daerden (2018), Icarus, 300, 458–476, doi:10.1016/j.icarus.2017.09.028.[8] Daerden et al., 2019, Icarus 326, https://doi.org/10.1016/j.icarus.2019.02.030 How to cite: Piccialli, A., Vandaele, A. C., Willame, Y., Aoki, S., Depiesse, C., Trompet, L., Neary, L., Viscardy, S., Daerden, F., Erwin, J., Thomas, I. R., Ristic, B., Mason, J. P., Patel, M., Bellucci, G., and Lopez-Moreno, J.-J.: Impact of gradients at the Martian terminator on the retrieval of ozone from TGO/NOMAD-UVIS, Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-670, 2020

Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO) started the science measurements on 21 April, 2018. We present results on the retrievals of water vapor vertical profiles in the Martian atmosphere from the first Mars year measurements of the TGO/NOMAD.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 concept of the infrared channels are derived from the Solar Occultation in the IR (SOIR) instrument onboard Venus Express (VEx). The sampling rate for the solar occultation measurement is 1 second, which provides better 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-1), order 136 (3056-3080 cm-1) and 168 (3775-3805 cm-1) acquired by the SO channel in order to investigate H2O vertical profiles.Knowledge of the water vapor vertical distribution is important to understand the water cycle and escape processes. Solar occultation measurements by the two spectrometers onboard TGO - NOMAD and Atmospheric Chemistry Suite (ACS) - allow us to monitor daily the water vapor vertical profiles through one whole Martian Year and obtain a latitudinal map 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. TGO began its science operations on 21 April 2018. NOMAD observations therefore fully cover the period before/during/after the global and regional dust storms and offer a unique opportunity to study the trace gases distributions during such events. We have analyzed those datasets and found a significant increase of water vapor abundance 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 altitudes, 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, 47, e2019GL084354., doi: 10.1029/2019GL084354). The current study presents the results obtained when considering the extended dataset, which covers a full Martian year. The extended dataset includes the recent aphelion season that involves interesting phenomena such as sublimation of water vapor from the northern polar cap and formation of the equatorial cloud belt, and is known as a key period to understand the large north-south hemispheric asymmetries of Mars water vapor. Yet, until now, only few papers reported the water vapor vertical distribution during the aphelion season. The extended dataset also includes the period when the global dust storm occurred the year before; this will allow us to compare the water vapor distributions under global dust storm conditions with those found during non-global dust storm years. In the presentation, we will discuss the H2O vertical profiles as well as the aerosols vertical distribution retrieved from the first full Martian year measurements of the TGO/NOMAD. How to cite: Aoki, S., Vandaele, A., Daerden, F., Villanueva, G., Thomas, I., Erwin, J., Trompet, L., Robert, S., Neary, L., Viscardy, S., Piccialli, A., Liuzzi, G., Crismani, M., Clancy, T., Smith, M., Ristic, B., Lopez-Valverde, M.-A., Patel, M., Bellucci, G., and Lopez-Moreno, J.-J.: Water vapor vertical profiles on Mars: Results from the first full Mars Year of TGO/NOMAD science operations, Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-392, 2020

The NOMAD instrument suite on the ESA-Roskosmos ExoMars Trace Gas Orbiter (TGO) observes the physical and chemical composition of the Martian atmosphere with highly resolved vertical profiles and nadir sounding in the IR and UV-vis domains. Vertically resolved profiles of, amongst other species, water vapor, HDO, ozone, CO, CO2, oxygen airglow, dust and clouds were obtained for more than one Martian year [1-5]. During its first year of operations, NOMAD witnessed the 2018 Global Dust Storm (GDS) during its onset, peak and decline. The redistribution of water vapor to high altitudes and latitudes observed during the GDS was explained using the GEM-Mars General Circulation Model (GCM) [6-8]. The GCM was driven by the dust optical depths for Mars Year 34 provided by [9]. The photolysis products of water vapor are a major driver for the atmospheric chemistry on Mars. As water vapor is redistributed over the atmosphere, it is expected to have considerable impact on many other species. GEM-Mars contains routines for atmospheric chemistry and here we present some results of the simulated impact of the GDS on atmospheric chemistry and on several of the observed species. GEM-Mars now also includes the simulation of HDO and the fractionation of water vapor upon cloud formation. The simulations will be compared with the vertical profiles of the D/H ratio obtained from NOMAD observations. The impact of the GDS on D/H can be estimated from these simulations. References [1] Vandaele, A. C. et al. (2019), Nature, 568, 7753, 521-525, doi: 10.1038/s41586-019-1097-3.[2] Aoki, S. et al. (2019), J. Geophys. Res.: Planets, 124, 3482–3497. https://doi.org/10.1029/2019JE006109[3] Gérard et al. (2020), Nature Astronomy, https://doi.org/10.1038/s41550-020-1123-2[4] Villanueva et al., submitted.[5] Korablev et al., 2020, in rev.[6] Neary, L. and F. Daerden (2018), Icarus, 300, 458–476, https://doi.org/10.1016/j.icarus.2017.09.028[7] Daerden, F. et al. (2019), Icarus, 326, 197-224, doi: 10.1016/j.icarus.2019.02.030.[8] Neary, L. et al. (2020), Geophys. Res. Lett., 47, e2019GL084354. https://doi.org/10.1029/2019GL084354[9] Montabone, L. et al. (2019), J. Geophys. Res.: Planets. doi: 10.1029/2019JE006111. 2.11.0.0 BIRA-IASB NOMAD team (continued): S. Robert (1), L. Trompet (1), A. Mahieux (1), C. Depiesse (1), E. Neefs (1) and B. Ristic (1). BIRA-IASB NOMAD team (continued): S. Robert (1), L. Trompet (1), A. Mahieux (1), C. Depiesse (1), E. Neefs (1) and B. Ristic (1). How to cite: Daerden, F., Neary, L., Villanueva, G., Aoki, S., Viscardy, S., Thomas, I., Vandaele, A. C., Liuzzi, G., Crismani, M., Khayat, A., Smith, M. D., Clancy, R. T., Wolff, M. J., Sandor, B. J., Whiteway, J. A., Mumma, M. J., Erwin, J., Willame, Y., and Piccialli, A. and the BIRA-IASB NOMAD team (continued): Simulating Mars D/H and atmospheric chemistry during the 2018 Global Dust Storm and comparing with NOMAD observations , Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-371, 2020

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. Here we report on the different discoveries highlighted by the instrument during its first full Martian year of observations: investigation of the 2018 Global dust storm and its impact on the water uplifting and escape, on temperature and pressure increases within the atmosphere; dust and ice clouds distribution; ozone measurements; dayglow observations; detection of HCl vertical profiles and in general advances in the analysis of the spectra recorded by the three channels of NOMAD. References [1] Vandaele, A.C., et al., 2015. Planet. Space Sci. 119, 233-249. [2] Vandaele et al., 2018. Space Sci. Rev., 214:80, doi.org/10.1007/s11214-11018-10517-11212.

The Nadir and Occultation for MArs Discovery (NOMAD) instrument on board ExoMars Trace Gas Orbiter (TGO) measured a large increase in water vapor at altitudes in the range of 40‐100 km during the 2018 global dust storm on Mars. Using a three‐dimensional general circulation model, we examine the mechanism responsible for the enhancement of water vapor in the upper atmosphere. Experiments with different prescribed vertical profiles of dust show that when more dust is present higher in the atmosphere the temperature increases and the amount of water ascending over the tropics is not limited by saturation until reaching heights of 70‐100 km. The warmer temperatures allow more water to ascend to the mesosphere. Photochemical simulations show a strong increase in high‐altitude atomic hydrogen following the high‐altitude water vapor increase by a few days.

As a potential biomarker, Martian methane has attracted attention through several reports of its detection over the last 15 years. Photochemical models predict that the lifetime of atmospheric methane should be of the order of 300 years, which implies that any detection would point to recent emissions. However, the very existence of this gas has been continuously questioned, in particular because the observed lifetime would be several orders of magnitude shorter than expected. Several fast removal processes have been hypothesized to explain the observations, but none of them has met a large consensus so far. It is in this context that the ESA-Roscomos ExoMars Trace Gas Orbiter (TGO) mission started its science operations in April 2018. The first highly sensitive measurements of methane in solar occultation were reported last year. No methane was detected over the first months of the TGO mission and an upper limit of 0.05 ppbv was obtained. The implications of this result on the methane problem on Mars will be addressed in this work.Several model studies investigated the transport of methane in Mars’ atmosphere. In particular, simulations of surface emissions of the gas using General Circulation Models (GCM) for Mars predicted the formation of layers during the first weeks after the release. Therefore, any detection of a layer by TGO would point to a recent emission. As a corollary to this, methane should be found within a few days at higher altitudes after its emission from the surface.The reported detection limit of 0.05 ppbv is a strong constraint on the background level of methane, i.e. on the total amount of the gas present in the atmosphere for a time exceeding the transport timescale (~3 months). However, locally, the retrieved detection limit from TGO strongly depends on the amount of atmospheric dust and, thus, on several factors such as the season, latitude, and altitude, which makes the problem much more complicated.Hence, what are the surface emission scenarios that are consistent with the TGO results? To answer this question, a statistical analysis of GEM-Mars GCM simulations including a large range of theoretical lifetimes will be conducted to determine the realms of scenarios in agreement with the multifactor-dependent TGO upper limits. The positive detections reported over the last 15 years will also be discussed in the light of the results obtained from our study.

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 probing the ultraviolet and infrared regions covering large parts of the 0.2-4.3 µm spectral range [1,2].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. Here we report on the different discoveries highlighted by the instrument: investigation of the 2018 Global dust storm and its impact on the water uplifting and escape, its impact on temperature increases within the atmosphere as inferred by GCM modeling and observations, the dust and ice clouds distribution during the event, ozone measurements, dayglow observations and in general advances in the analysis of the spectra recorded by the three channels of NOMAD.References[1] Vandaele, A.C., et al., 2015. Planet. Space Sci. 119, 233-249.[2] Vandaele et al., 2018. Space Sci. Rev., 214:80, doi.org/10.1007/s11214-11018-10517-11212.

Using the GEM-Mars three-dimensional general circulation model (GCM), we examine the mechanism responsible for the enhancement of water vapour in the upper atmosphere as measured by the Nadir and Occultation for MArs Discovery (NOMAD) instrument onboard ExoMars Trace Gas Orbiter (TGO) during the 2018 global dust storm on Mars.Experiments with different prescribed vertical profiles of dust show that when more dust is present higher in the atmosphere, the temperature increases and the amount of water ascending over the tropics is not limited by saturation until reaching heights of 70-100 km. The warmer temperatures allow more water to ascend to the mesosphere. The simulation of enhanced high-altitude water abundances is very sensitive to the vertical distribution of the dust prescribed in the model.The GEM-Mars model includes gas-phase photochemistry, and these simulations show how the increased water vapour over the 40-100 km altitude range results in the production of high-altitude atomic hydrogen which can be linked to atmospheric escape.

We pursued our ground-based seasonal monitoring of hydrogen peroxide on Mars using thermal imaging spectroscopy, with two observations of the planet near opposition, in May 2016 (solar longitude Ls = 148.5°, diameter = 17 arcsec) and July 2018 (Ls = 209°, diameter = 23 arcsec). Data were recorded in the 1232–1242 cm−1 range (8.1 μm) with the Texas Echelon Cross Echelle Spectrograph (TEXES) mounted at the 3 m Infrared Telescope Facility (IRTF) at the Mauna Kea Observatories. As in the case of our previous analyses, maps of H2O2 were obtained using line depth ratios of weak transitions of H2O2 divided by a weak CO2 line. The H2O2 map of April 2016 shows a strong dichotomy between the northern and southern hemispheres, with a mean volume mixing ratio of 45 ppbv on the north side and less than 10 ppbv on the south side; this dichotomy was expected by the photochemical models developed in the LMD Mars Global Climate Model (LMD-MGCM) and with the recently developed Global Environmental Multiscale (GEM) model. The second measurement (July 2018) was taken in the middle of the MY 34 global dust storm. H2O2 was not detected with a disk-integrated 2σ upper limit of 10 ppbv, while both the LMD-MGCM and the LEM models predicted a value above 20 ppbv (also observed by TEXES in 2003) in the absence of dust storm. This depletion is probably the result of the high dust content in the atmosphere at the time of our observations, which led to a decrease in the water vapor column density, as observed by the PFS during the global dust storm. GCM simulations using the GEM model show that the H2O depletion leads to a drop in H2O2, due to the lack of HO2 radicals. Our result brings a new constraint on the photochemistry of H2O2 in the presence of a high dust content. In parallel, we reprocessed the whole TEXES dataset of H2O2 measurements using the latest version of the GEISA database (GEISA 2015). We recently found that there is a significant difference in the H2O2 line strengths between the 2003 and 2015 versions of GEISA. Therefore, all H2O2 volume mixing ratios up to 2014 from TEXES measurements must be reduced by a factor of 1.75. As a consequence, in four cases (Ls around 80°, 100°, 150°, and 209°) the H2O2 abundances show contradictory values between different Martian years. At Ls = 209° the cause seems to be the increased dust content associated with the global dust storm. The inter-annual variability in the three other cases remains unexplained at this time.

Reports of methane detection in the Martian atmosphere have been intensely debated. The presence of methane could enhance habitability and may even be a signature of life. However, no detection has been confirmed with independent measurements. Here, we report a firm detection of 15.5 ± 2.5 ppb by volume of methane in the Martian atmosphere above Gale Crater on 16 June 2013, by the Planetary Fourier Spectrometer onboard Mars Express, one day after the in situ observation of a methane spike by the Curiosity rover. Methane was not detected in other orbital passages. The detection uses improved observational geometry, as well as more sophisticated data treatment and analysis, and constitutes a contemporaneous, independent detection of methane. We perform ensemble simulations of the Martian atmosphere, using stochastic gas release scenarios to identify a potential source region east of Gale Crater. Our independent geological analysis also points to a source in this region, where faults of Aeolis Mensae may extend into proposed shallow ice of the Medusae Fossae Formation and episodically release gas trapped below or within the ice. Our identification of a probable release location will provide focus for future investigations into the origin of methane on Mars. A methane spike 15.5 ± 2.5 parts per billion by volume was detected in the Martian atmosphere above Gale Crater on 16 June 2013 by Mars Express, independently confirming the debated in situ observation by the Curiosity rover a day earlier.

The detection of methane on Mars has been interpreted as indicating that geochemical or biotic activities could persist on Mars today

Global dust storms on Mars are rare

International audience; Rapid variations of pressure, temperature and illumination at the day-night terminator have the potential to cause asymmetries in the abundance distribution of the atmosphere constituents along the line of sight (LOS) of a solar occultation experiment. Ozone, in particular, displays steep density gradients across the terminator of Mars due to photolysis. Nowadays, most of the retrieval algorithms for solar and stellar occultations rely on the assumption of a spherically symmetrical atmosphere. However, photochemically induced variations near sunrise/sunset conditions need to be taken into account in the retrieval technique in order to prevent inaccuracies.We investigated the impact of gradients along the LOS of the solar occultation experiment SPICAM/Mars Express for the retrieval of ozone under sunrise/sunset conditions. In order to test the impact of different gradients, we selected four occultations at sunrise and at sunset each. Sunset occultations are located near the equator, while sunrise observations are situated at high latitudes in the South, because of the geometry of the orbit.We used the diurnal variations in the ozone concentration obtained from a three-dimensional General Circulation Model (GEM-Mars) together with an adapted radiative transfer code (ASIMUT). The General Circulation Model (GCM) suggests that ozone variations strongly depend on latitude, altitude, and season. As shown by the model, near the equator and below 25 km, ozone changes only slightly with local time. Around 45 km, the density changes by several orders of magnitude across the terminator. At high latitudes in the South, during northern winter time, ozone variations at the terminator are negligible.The impact of gradients on ozone retrievals is strongly related to the local atmospheric structure as predicted by the GCM. Sunset ozone retrievals are smaller than retrievals obtained assuming a spherically symmetrical atmosphere , with a maximum change of about 20%. At sunrise, the impact of gradients on the retrievals is negligible. This behavior can be explained by the specific geometry of sunrise observations, all situated at high latitudes in the South.

International audience; Recent measurements of methane (CH4) by the Mars Science Laboratory (MSL) now confront us with robust data that demand interpretation. Thus far, the MSL data have revealed a baseline level of CH4 (*0.4 parts per billion by volume [ppbv]), with seasonal variations, as well as greatly enhanced spikes of CH4 with peak abundances of *7 ppbv. What do these CH4 revelations with drastically different abundances and temporal signatures represent in terms of interior geochemical processes, or is martian CH4 a biosignature? Discerning how CH4 generation occurs on Mars may shed light on the potential habitability of Mars. There is no evidence of life on the surface of Mars today, but microbes might reside beneath the surface. In this case, the carbon flux represented by CH4 would serve as a link between a putative subterranean biosphere on Mars and what we can measure above the surface. Alternatively, CH4 records modern geochemical activity. Here we ask the fundamental question: how active is Mars, geochemically and/or biologically? In this article, we examine geological, geochemical, and biogeo- chemical processes related to our overarching question. The martian atmosphere and surface are an over- whelmingly oxidizing environment, and life requires pairing of electron donors and electron acceptors, that is, redox gradients, as an essential source of energy. Therefore, a fundamental and critical question regarding the possibility of life on Mars is, ‘‘Where can we find redox gradients as energy sources for life on Mars?’’ Hence, regardless of the pathway that generates CH4 on Mars, the presence of CH4, a reduced species in an oxidant-rich environment, suggests the possibility of redox gradients supporting life and habitability on Mars. Recent missions such as ExoMars Trace Gas Orbiter may provide mapping of the global distribution of CH4. To discriminate between abiotic and biotic sources of CH4 on Mars, future studies should use a series of diagnostic geochemical analyses, preferably performed below the ground or at the ground/atmosphere interface, including measurements of CH4 isotopes, methane/ethane ratios, H2 gas concentration, and species such as acetic acid. Advances in the fields of Mars exploration and instrumentation will be driven, augmented, and supported by an improved un- derstanding of atmospheric chemistry and dynamics, deep subsurface biogeochemistry, astrobiology, planetary geology, and geophysics. Future Mars exploration programs will have to expand the integration of complementary areas of expertise to generate synergistic and innovative ideas to realize breakthroughs in advancing our under- standing of the potential of life and habitable conditions having existed on Mars. In this spirit, we conducted a set of interdisciplinary workshops. From this series has emerged a vision of technological, theoretical, and meth- odological innovations to explore the martian subsurface and to enhance spatial tracking of key volatiles, such as CH4.

The surname of author Cathy Quantin-Nataf was misspelled ‘Quantin-Nata’, authors Ehouarn Millour and Roland Young were missing from the ACS and NOMAD Science Teams list, and minor changes have been made to the author and affiliation lists; see accompanying Amendment. These errors have been corrected online.

The surname of author Cathy Quantin-Nataf was misspelled ‘Quantin-Nata’ , authors Ehouarn Millour and Roland Young were missing from the ACS Science Team list, and minor changes have been made to the author and affiliation lists; see accompanying Amendment. These errors have been corrected online.

The stratosphere is thought to play a central role in the atmospheric response to solar irradiance variability. Recent observations suggest that the spectral solar irradiance (SSI) variability involves significant time-dependent spectral variations, with variable degrees of correlation between wavelengths, and new reconstructions are being developed. In this paper, we propose a simplified modelling framework to characterise the effect of short term SSI variability on stratospheric ozone. We focus on the pure photochemical effect, for it is the best constrained one. The photochemical effect is characterised using an ensemble simulation approach with multiple linear regression analysis. A photochemical column model is used with interactive photolysis for this purpose. Regression models and their coefficients provide a characterisation of the stratospheric ozone response to SSI variability and will allow future inter-comparisons between different SSI reconstructions. As a first step in this study, and to allow comparison with past studies, we take the representation of SSI variability from the Lean (1997) solar minimum and maximum spectra. First, solar maximum-minimum response is analysed for all chemical families and partitioning ratios, and is compared with past studies. The ozone response peaks at 0.18 ppmv (approximately 3%) at 37 km altitude. Second, ensemble simulations are regressed following two linear models. In the simplest case, an adjusted coefficient of determination R2 larger than 0.97 is found throughout the stratosphere using two predictors, namely the previous day's ozone perturbation and the current day's solar irradiance perturbation. A better accuracy (R2 larger than 0.9992) is achieved with an additional predictor, the previous day's solar irradiance perturbation. The regression models also provide simple parameterisations of the ozone perturbation due to SSI variability. Their skills as proxy models are evaluated independently against the photochemistry column model. The bias and RMS error of the best regression model are found smaller than 1% and 15% of the ozone response, respectively. Sensitivities to initial conditions and to magnitude of the SSI variability are also discussed.

We present the analyses of UARS MLS ozone data obtained by the Belgian Assimilation System for Chemical ObsErvations (BASCOE). This system, based on the 4D-var method, is dedicated to the assimilation of stratospheric chemistry observations. It uses a 3-D Chemical Transport Model (3D-CTM) including 57 chemical species with explicit calculation of stratospheric chemistry. The CTM is driven by ECMWF ERA-40 analyses of winds and temperature, with a horizontal grid of 3.75 in latitude by 5 in longitude, and with 37 pressure levels from the surface to 0.1 hPa. BASCOE has assimilated UARS MLS observations acquired during the period 1992-1997. We discuss how BASCOE is able to reproduce MLS data, and we evaluate the BASCOE analyses with respect to independent observations from UARS HALOE, ozonesondes, and ground-based lidars. An excellent agreement is found with independent observations (bias usually less than 10%), except in the lowermost stratosphere and in the Antarctic ozone hole. The performances of BASCOE ozone analyses are also compared to those of two other long-term ozone reanalyses; namely, ERA-40 and ERA-Interim, both from ECMWF. Finally, sensitivity test based on BASCOE free model runs suggest that ozone analyses during the ozone hole period would be greatly improved by driving BASCOE with the dynamical fields of the new ECMWF reanalyses ERA-Interim. This work is part of the Stratospheric Ozone Profile Record service raised by the GMES Service Element PROMOTE.

The photochemical response of the stratosphere to short-term solar variability is investigated using a photochemistry column model with interactive photolysis calculation. The solar variability is here simply represented using the Lean (1997) solar minimum and maximum spectra. In order to isolate the photochemistry effect, simulations are devoid of diffusion or any other external forcing and the temperature is held constant. The solar mininum/maximum response is estimated for all chemical families and partitioning ratios, and the underlying photochemical mechanisms are described in detail. The ozone response peaks at 0.18 ppmv (approximatively 3%) at 37 km altitude. In an attempt to find the simplest statistical model able to represent the effect of solar variability in the stratosphere, the diurnal-average response of ozone from an ensemble of 200 simulations is regressed linearly following two auto-regressive models. In the simplest case, an adjusted coefficient of determination R2 larger than 0.97 is found throughout the stratosphere using two predictors, namely the previous day's ozone perturbation and the current day's solar irradiance perturbation. A better accuracy (R2 larger than 0.9992) is achieved with an additional predictor, the previous day's solar irradiance perturbation. The skills of the two auto-regressive models at representing the effect of solar variability are then evaluated independently when coupled either on-line or off-line with the comprehensive photochemistry column model driven by the solar average spectrum. In all cases, the magnitude of the bias and the RMS error are found smaller than 5% and 20% of the ozone response, respectively. When used on-line, the 3-predictor model captures the ozone response to solar variability throughout the stratosphere with bias and RMS error lower than 1% and 15% of the ozone response, respectively. The results are found to be insensitive to an increase in the magnitude of the solar variability by a factor three, when this increase is applied uniformly throughout the solar spectrum. These statistical models offer accurate, computationally inexpensive parameterisations of the effect of solar variability in the stratosphere for climate-chemistry models with simplified chemistry that can be driven by any solar variability index. Finally, the statistical approach introduced here, based on ensemble photochemical simulations, provides an effective gauge to measure the effects of using more realistic solar variability spectra on the ozone response.

The record-breaking major stratospheric warming of northern winter 2009 (January–February) is studied using BASCOE (Belgian Assimilation System for Chemical ObsErvation) stratospheric water vapour analyses and MLS (Microwave Limb Sounder) water vapour observations, together with meteorological data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and potential vorticity (PV) derived from ECMWF meteorological data. We focus on the interaction between the cyclonic wintertime stratospheric polar vortex and subsidiary anticyclonic stratospheric circulations during the build-up, peak and aftermath of the major warming. We show dynamical consistency between the water vapour analysed fields and the meteorological and PV fields. Using various approaches, we use the analysed water vapour fields to estimate descent in the polar vortex during this period of between ~0.5 km day−1 and ~0.7 km day−1. New results include the analysis of water vapour during the major warming and demonstration of the benefit of assimilating MLS satellite data into the BASCOE model.

The thermal conductivity is calculated with the Helfand-moment method in the Lennard-Jones fluid near the triple point. The Helfand moment of thermal conductivity is here derived for molecular dynamics with periodic boundary conditions. Thermal conductivity is given by a generalized Einstein relation with this Helfand moment. We compute thermal conductivity by this new method and compare it with our own values obtained by the standard Green-Kubo method. The agreement is excellent., Comment: Submitted to the Journal of Chemical Physics

We apply the escape-rate formalism to compute the shear viscosity in terms of the chaotic properties of the underlying microscopic dynamics. A first passage problem is set up for the escape of the Helfand moment associated with viscosity out of an interval delimited by absorbing boundaries. At the microscopic level of description, the absorbing boundaries generate a fractal repeller. The fractal dimensions of this repeller are directly related to the shear viscosity and the Lyapunov exponent, which allows us to compute its values. We apply this method to the Bunimovich-Spohn minimal model of viscosity which is composed of two hard disks in elastic collision on a torus. These values are in excellent agreement with the values obtained by other methods such as the Green-Kubo and Einstein-Helfand formulas., Comment: 16 pages, 16 figures (accepted in Phys. Rev. E; October 2003)

We report a study of viscosity by the method of Helfand moment in systems with periodic boundary conditions. We propose a new definition of Helfand moment which takes into account the minimum image convention used in molecular dynamics with periodic boundary conditions. Our Helfand-moment method is equivalent to the method based on the Green-Kubo formula and is not affected by ambiguities due to the periodic boundary conditions. Moreover, in hard-ball systems, our method is equivalent to the one developed by B. J. Alder, D. M. Gass, and T. E. Wainwright [{\it J. Chem. Phys.} {\bf 53}, 3813 (1970)]. We apply and verify our method in a fluid composed of $N\geq 2$ hard disks in elastic collisions. We show that the viscosity coefficients already take values in good agreement with Enskog's theory for N=2 hard disks in a hexagonal geometry., Comment: 30 pages, 32 figures, accepted in August 2003 (Phys.Rev. E)

We propose a new method, the Helfand-moment method, to compute the shear viscosity by equilibrium molecular dynamics in periodic systems. In this method, the shear viscosity is written as an Einstein-like relation in terms of the variance of the so-called Helfand moment. This quantity, is modified in order to satisfy systems with periodic boundary conditions usually considered in molecular dynamics. We calculate the shear viscosity in the Lennard-Jones fluid near the triple point thanks to this new technique. We show that the results of the Helfand-moment method are in excellent agreement with the results of the standard Green-Kubo method., Comment: Submitted to the Journal of Chemical Physics

This paper discusses the global analyses of stratospheric ozone (O3) and nitrogen dioxide (NO2) obtained by the Belgian Assimilation System for Chemical Observations from Envisat (BASCOE). Based on a chemistry transport model (CTM) and the 4-dimensional variational (4D-Var) method, BASCOE has assimilated chemical observations of O3, NO2, HNO3, N2O, CH4 and H2O, made between July 2002 and March 2004 by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard the European Space Agency (ESA) Environment Satellite (ENVISAT). This corresponds to the entire period during which MIPAS was operating at its nominal resolution. Our analyses are evaluated against assimilated MIPAS data and independent HALOE (HALogen Occultation Experiment) and POAM-III (Polar Ozone and Aerosol Measurement) satellite data. A good agreement is generally found between the analyses and these datasets, in both cases within the estimated error bars of the observations. The benefit of data assimilation is also evaluated by comparing a BASCOE free model run with MIPAS observations. For O3, the gain from the assimilation is significant during ozone hole conditions, and in the lower stratosphere. Elsewhere, the assimilation does not provide significant improvement. For NO2, the gain from the assimilation is realized through most of the stratosphere. Using the BASCOE analyses, we estimate the differences between MIPAS data and independent data from HALOE and POAM-III, and find results close to those obtained by classical validation methods involving only direct measurement-to-measurement comparisons. Our results extend and reinforce previous MIPAS data validation efforts by taking into account a much larger variety of atmospheric states and measurement conditions. This study discusses possible further developments of the BASCOE data assimilation system; these concern the horizontal resolution, a better filtering of NO2 observations, and the photolysis calculation near the lid of the model. The ozone analyses are part of the PROMOTE project and are publicly available via the BASCOE website (http://www.bascoe.oma.be/promote/).