Search

Aviation NOx emissions have not only an impact on global climate by changing ozone and methane levels but also contribute to deteriorate local air quality. In order to properly assess the co-benefit with air quality improvement and the trade-off with the climate change associated with CO2, it appears essential to better quantify the climate impact of aircraft NOx emissions. A new version of the LMDZ-INCA global model, including both tropospheric and stratospheric chemistry and the sulfate-nitrate-ammonium cycle, is applied to re-evaluate the impact of aircraft NOx and aerosol emissions on climate. The results confirm that the efficiency of NOx to produce ozone is very much dependent on the injection height. For the baseline simulation and reference scenario this efficiency is 6.6 TgO3/TgN. The efficiency increases with the background methane and NOx concentrations and with decreasing aircraft NOx emissions. The same findings translate to the associated ozone radiative forcing which exhibits a fairly constant value per ozone mass change of 3.4 mW/m2/TgO3. The methane lifetime variation is less sensitive to the aircraft NOx emission location than the ozone change. The change in CH4 mixing ratio itself represents 75 % of the total methane forcing. The indirect changes through long-term tropospheric ozone, stratospheric water vapour and methane oxidation to CO2 contribute for 19 %, 4 % and 1 %, respectively. The net NOx radiative forcing (O3 + CH4) is largely affected by the revised CH4 radiative forcing formula which increases the total CH4 negative forcing by 15 %. As a consequence, the ozone positive forcing and the methane negative forcing largely offset each other resulting in a slightly positive forcing for the present-day. However, in the future, the net forcing turns to negative due essentially to higher methane background concentrations. Additional radiative forcings involving particle formation arise from aircraft NOx emissions since the increased OH concentrations are responsible for an enhanced conversion of SO2 to sulfate particles. Aircraft NOx emissions also increase the formation of nitrate particles in the lower troposphere. However, in the upper-troposphere, increased sulfate concentrations favor the titration of ammonia leading to lower ammonium nitrate concentrations. The total forcing from sulfate and nitrate aerosols associated with NOx emissions is negative and is estimated to -3.0 mW/m2/TgN for the present-day. When these aerosol radiative forcings are considered, the total NOx forcing turns from a positive value to a negative value even for present-day conditions. Hence, total radiative forcing from aircraft emissions associated with changes in atmospheric chemistry and direct aerosols forcings is negative for both present-day and future (2050) conditions. NOx emissions only cause a negative forcing representing about 45 % of the total forcing. The negative forcing associated with sulfates largely dominates the effect of the other particles. The sulfate direct radiative forcing is estimated to be associated for about 50 % with the direct SO2 and SO4 aircraft emissions and for about 50 % with the increased conversion of SO2 to SO4 at higher OH concentrations, and hence related to the NOx emissions. The net effect of decreasing (resp. increasing) the flight altitude by 2000 ft (about 610 m) is to increase (resp. decrease) the total negative forcing by 57 % (resp. 65 %). The variation of the total forcing with flight altitude is dominated by the high sensitivity of the ozone positive forcing to the altitude of the perturbation. Several mitigation options involving flight aircraft operation and cruise altitude changes, traffic growth, engine technology, and fuel type, exist to reduce the climate impact of aircraft NOx emissions. However, the climate forcing of aircraft NOx emissions is likely to be small or even switch to negative (cooling) depending on atmospheric NOx or CH4 future background concentrations or when the NOx impact on sulfate and nitrate particles is considered. There remain large uncertainties on the NOx net impact on climate effect calculation. Nevertheless, the results suggest that reducing aircraft NOx emissions is primarily beneficial for improving air quality. For climate consideration, one option to reduce uncertainties in mitigation strategies might be to prioritize the reduction of CO2 aircraft emissions which have a well-established and long-term impact on climate, however this reduces the overall mitigation potential.

The Atmospheric Chemistry Suite (ACS) on the ExoMars Trace Gas Orbiter (TGO) took its first science observation in April 2018, right before the onset of the Mars Year (MY) 34 global dust storm. One of the main objectives of the TGO mission is to search for as-yet undetected trace gases that can tell us about contemporary volcanism on Mars, or its present and past habitability. In the data collected those first months, heavily impacted by dust activity, the first novel trace gas was discovered: hydrogen chloride. MY 37 has just begun and we have recently finished observing our third full dusty season on Mars with TGO and ACS (around perihelion, spring and summer in the southern hemisphere). HCl in the atmosphere of Mars is a seasonal phenomenon, having appeared coincidentally with the start of dust activity in each MY. HCl was thought to be an indication of contemporary volcanism, but its widespread distribution across both hemispheres and recurring seasonality are suggestive of a photochemical source. Here, we present the climatology of HCl after three Martian perihelion periods, as well as a comparison with other parameters measured with ACS, such as water, temperature, and aerosols. From coincident measurements made with the Mars Climate Sounder (MCS) on Mars Reconnaissance Orbiter (MRO), we can also compare the climatology of HCl with those of dust and water ice. HCl is strongly correlated to water vapour, which is itself correlated to atmospheric temperatures. While HCl only appears in the presence of suspended dust aerosols, the measured abundances of these two quantities are poorly correlated. The disappearance of HCl towards the autumnal equinox may be related to changes in temperature. The cooling atmosphere removes water vapour from the gas phase, necessary for formation of HCl, and promotes ice formation, which HCl may adhere to. We will show the evolution of HCl abundance over three Martian years in both hemispheres, and show how they fit into the seasonality of Martian dust, the water cycle, and ice formation, and discuss the possible mechanisms of its formation and destruction.

Ozone depletion over the polar regions is monitored each year by satellite and ground-based instruments. In this study, the vortex-averaged ozone loss over the last three decades is evaluated for both polar regions using the passive ozone tracer of the chemical transport model TOMCAT/SLIMCAT and total ozone observations from Système d'Analyse par Observation Zénithale (SAOZ) ground-based instruments and Multi-Sensor Reanalysis (MSR2). The passive tracer method allows us to determine the evolution of the daily rate of column ozone destruction, and the magnitude of the cumulative loss at the end of the winter. Three metrics are used to estimate the linear trend since 2000 and to assess the current situation of ozone recovery over both polar regions: 1) The maximum ozone loss at the end of the winter; 2) the onset day of ozone loss at a specific threshold and 3) the ozone loss residuals computed from the differences between annual ozone loss and ozone loss values regressed with respect to sunlit volume of polar stratospheric clouds (VPSC). This latter metric is based on linear and parabolic regressions for ozone loss in the Northern and Southern Hemispheres, respectively. In the Antarctic, metrics 1, and 3, yield trends of -2.3 and -1.8 % dec-1 for the 2000–2021 period, significant at 1 and 2 standard error (σ), respectively. For metric 2, various thresholds were considered, all of them showing a time delay for when they are reached. The trends are significant at the 2σ level and vary from 3.5 to 4.2 day dec-1 between the various thresholds. In the Arctic, metric 1 exhibits large interannual variability and no significant trend is detected; this result is highly influenced by the record ozone losses in 2011 and 2020. Metric 2 is not applied in the Northern Hemisphere due to the difficulty of finding a threshold value in a consistent number of winters. Metric 3 shows a negative trend in Arctic ozone loss residuals of -1.7 ±1 % dec-1, significant at 1σ level. This is therefore the first quantitative detection of ozone recovery in the Arctic springtime lower stratosphere.

We present here three-dimensional simulations of the Venus photochemistry and clouds from the ground to the bottom of the thermosphere. For that purpose, we have implemented a state-of-the-art photochemical and equilibrium cloud model in the Venus Planetary Climate Model (Venus PCM). The interactive coupling between dynamics, radiation, chemistry and clouds allows a comprehensive description of the CO, CO, sulfur, chlorine, oxygen, and hydrogen species, with tracking of the condensed phase. Regarding the clouds, the Venus PCM calculates the composition, number density, and sedimentation rates of the binary HSO-HO liquid aerosols, based on observed altitude-dependent size distributions. The article describes in detail the new components implemented in the Venus PCM. It then presents an overview of the results concerning clouds and atmospheric chemistry, which are compared with a wide range of observations. The modeled cloud characteristics and vertical profiles of minor species are found to be in broad agreement with most of the measurements available between 30 and 100 km. In particular, the Venus PCM reproduces the steep decrease of HO and SO mixing ratio inside the cloud layer, as well as the observed vertical distribution of species well identified above the clouds, such as CO and O3. The model also agrees with the ground-based measurements of HCl, but not with the conflicting HCl vertical profiles derived from Venus Express. On the quasi-horizontal plane, latitudinal contrasts in the modeled trace species mostly result from the Hadley-type mean meridional circulation. Large-scale longitudinal variations are essentially created by the diurnal thermal tide above the clouds, and by photolysis above 80 km. © 2023 Elsevier Inc. All rights reserved., The Venus PCM simulations were done thanks to the High-Performance Computing (HPC) resources of Centre Informatique National de l’Enseignement Supérieur (CINES) under the allocation A0100110391 made by Grand Equipement National de Calcul Intensif (GENCI)., With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2021-001131-S).

The atmosphere of Mars is enriched in heavy isotopes with respect to Earth as a result of the escape of the atmosphere to space over billions of years. Estimating the enrichment in heavy isotopes due to atmospheric escape requires a rigorous understanding of all atmospheric processes that contribute to the evolution of isotopic ratios between the lower and upper atmosphere, where escape processes take place. Using a combination of vertical profiles from the Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) with the predictions of a photochemical model, we provide evidence for a new process of photochemistry-induced fractionation that depletes the heavy isotopes of C and O in CO. Accounting for this new source of fractionation, we find that only a fraction of ≈12% of the atmosphere needs to have been lost to space through CO photochemical escape to explain the enrichment of 13C/12C in CO2 measured by the Curiosity Rover.

From the Archean toward the Proterozoic, the Earth's atmosphere underwent a major shift from anoxic to oxic conditions, around 2.4 to 2.1 Gyr, known as the Great Oxidation Event (GOE). This rapid transition may be related to an atmospheric instability caused by the formation of the ozone layer. Previous works were all based on 1D photochemical models. Here, we revisit the GOE with a 3D photochemical-climate model to investigate the possible impact of the atmospheric circulation and the coupling between the climate and the dynamics of the oxidation. We show that the diurnal, seasonal and transport variations do not bring significant changes compared to 1D models. Nevertheless, we highlight a temperature dependence for atmospheric photochemical losses. A cooling during the late Archean could then have favored the triggering of the oxygenation. In addition, we show that the Huronian glaciations, which took place during the GOE, could have introduced a fluctuation in the evolution of the oxygen level. Finally, we show that the oxygen overshoot which is expected to have occurred just after the GOE, was likely accompanied by a methane overshoot. Such high methane concentrations could have had climatic consequences and could have played a role in the dynamics of the Huronian glaciations.

Carbon monoxide is a non-condensable gas in the Martian atmosphere produced by the photolysis of CO2. Its abundance responds to the condensation and sublimation of CO2 from the polar caps, resulting in seasonal variations of the CO mixing ratio. ACS onboard the ExoMars Trace Gas Orbiter have measured CO in infrared bands by solar occultation. Here we provide the first long-term monitoring of the CO vertical distribution at the altitude range from 0 to 80 km for 1.5 Martian years from Ls = 163° of MY34 to the end of MY35. We obtained a mean CO mixing ratio of ∼960 ppmv at latitudes from 45°S to 45°N and altitudes below 40 km, mostly consistent with previous observations. We found a strong enrichment of CO near the surface at Ls = 100–200° in high southern latitudes with a layer of 3,000–4,000 ppmv, corresponding to local depletion of CO2. At equinoxes we found an increase of the CO mixing ratio above 50 km to 3,000–4,000 ppmv at high latitudes of both hemispheres explained by the downwelling flux of the Hadley circulation on Mars, which drags the CO enriched air. General circulation models tend to overestimate the intensity of this process, bringing too much CO. The observed minimum of CO in the high and mid-latitudes southern summer atmosphere amounts to 700–750 ppmv, agreeing with nadir measurements. During the global dust storm of MY34, when the H2O abundance peaks, we see less CO than during the calm MY35, suggesting an impact of HOx chemistry on the CO abundance.

Introduction The thermal (Jeans) escape of Hydrogen accumulated during the history of Mars has been one of the major mechanisms explaining the transition of Mars from a thicker and wetter atmosphere in the past to the current thin and dry atmosphere (Brain et al., 2017). Recent observations (Heavens et al., 2018, Chaffin et al., 2021) have revealed a clear link between the water cycle in the lower atmosphere, the transport of water to the middle/upper atmosphere, and the thermal escape of Hydrogen. However, many unknowns remain, including the role of the different processes responsible of transporting water from the lower to the upper atmosphere and converting it to Hydrogen atoms, or the effects of global dust storms (GDS hereafter) compared to the regular seasonal variability. While different 1D models have been used to reproduce and understand some of the observations (e.g. Chaffin et al., 2017), until now global models have failed to reproduce the observed variability of the H escape. In particular, a recent study with the Laboratoire de Météorologie Dynamique Mars Global Climate Model (LMD-MGCM hereafter) evidences that the model significantly underestimates the H escape rate when comparing with Mars Express SPICAM observations, in particular during the perihelion season (Chaufray et al., 2021). In this work we will summarize the recent improvements that we have included in the LMD-MGCM in order to better reproduce the observed Hydrogen escape rate, and will discuss some of the results obtained with the improved model. Model description We have included three improvements with respect to the version of the LMD-MGCM used in Chaufray et al., 2021. First, we have incorporated in the simulations a sophisticated model of the microphysics of water ice clouds allowing for the formation of supersaturated water layers (Navarro et al., 2014). Second, we have extended the photochemical model in the LMD-MGCM to incorporate the chemistry of H2O+ and derived ions, as well as of deuterated (both neutral and ion) species. Third, we have also included in the calculations an improved model of deuterium fractionation (Vals et al., 2022). While this allows us to study the D escape, we will focus here only on the H escape; simulations of the deuterium escape are discussed in Chaufray et al. (this issue). Preliminary results The incorporation in the calculations of the microphysical model allowing for the formation of supersaturated water layers significantly increases the amount of water in the upper atmosphere of the planet with respect to the previous calculations, producing a strong enhancement of up to one order of magnitude in the H escape rate. The incorporation of the chemistry of water-derived ions further increases the escape rate in between ~20 and ~40%, depending on the season. This results in a better agreement with observations of H escape (figure 1). However, significant differences still remain. In particular, the decrease in the rate of H escape at the end of the year is not well captured by the model, suggesting that, in the model, water remains in the upper atmosphere longer than observed. We study also the interannual variability of the simulated escape rate. While the solar activity seems to play a secondary role, dust storms in the lower atmosphere have a clear effect over the H escape rate. Our simulations show, for example, that the global dust storm in MY34 increased the annually integrated H escape rate in about 30%. This confirms the importance of taking into account the effects of GDSs when calculating the accumulated escape rate over Martian history. This work opens the doors to studying the H escape rate at past Mars conditions characterized by different orbital parameters (e.g. obliquity, time of perihelion, etc.). See Gilli et al., this issue, for a first study in this direction. Figure 1. H escape rate simulated for MY28 (light green line) and MY33 (orange line). The black thin line shows the escape rate for MY28 simulated with the previous model version, taken from Chaufray et al. (2021). The green and red symbols represent measured values of the H escape rate during MY28 and MY33, respectively, taken from Chaffin et al. (2014) and Heavens et al. (2018) Acknowledgements F.G-G. and G.G. are funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC FEDER funds under project RTI2018-100920-J-I00. AB and MALV were supported by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). The IAA team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the Center of Excellence “Severo Ochoa" award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) References Brain, D., et al., (2017), Solar Wind Interaction and Atmospheric Escape. Chapter 15 in “The Atmosphere and Climate of Mars”, Cambridge University Press, doi:10.1017/9781139060172.015 Chaffin, M., et al. (2021), Martian water loss to space enhanced by regional dust storms, Nat. Astron. doi:10.1038/s41550-021-01425-w Chaufray, J.-Y., et al. (2021), Study of the hydrogen escape rate at Mars during martian years 28 and 29 from comparisons between SPICAM/Mars express observations and GCM-LMD simulations. Icarus, doi:10.1016/j.icarus.2019.113498 Heavens, N., et al. (2018). Hydrogen escape from Mars enhanced by deep convection in dust storms. Nat. Astron., doi:10.1038/s41550-017-0353-4 Navarro, T., et al. (2014), Global climate modeling of the Martian water cycle with improved microphysics andadiatively active water ice clouds. JGR (Planets), doi.org:10.1002/2013JE004550 Vals, M., et al. (2022), Improved modeling of Mars' HDO cycle using a Mars' Global Climate Model. Paper submitted to JGR-Planets.

The Martian mesosphere, and in particular the upper mesosphere, above about 80 km from the surface, remains poorly explored in comparison with the troposphere and the thermosphere/ionosphere. Observations by the NOMAD and ACS instruments on board the ExoMars Trace Gas Orbiter mission (TGO in what follows) are starting to fill this gap by providing measurements such as the abundance of mesospheric water and the effects of global dust storms (Vandaele et al., 2019; Belyaev et al., 2021; Brines et al., 2022), the CO variability (Modak et al., 2022) or the temperature and density structure (López-Valverde et al., 2022). For the first time we have a set of diverse atmospheric parameters derived simultaneously with good vertical resolution and from a single instrument, which is ideal for model validation purposes.In this work, we will compare the predictions of the LMD-Mars GCM (LMD-MGCM) in the mesosphere with the NOMAD results (abundances of CO2, CO, and H2O, as well as temperature) obtained by the retrieval scheme developed at the Instituto de Astrofisica de Andalucia-CSIC (IAA), in order to validate the model and to gain insight into the physical processes at the origin of the observed structures. In particular, we will focus on the comparison of the CO2 density and temperature structure observed below ~120 km (López-Valverde et al., 2022) during the second half of Mars Year 34, a period including the MY34 global dust storm. We will show that the model reproduces well the strong seasonal variations of the CO2 density observed by NOMAD. However, the model tends to overestimate the temperatures above ~50 km.AcknowledgmentsF.G-G. is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC FEDER funds under project RTI2018-100920-J-I00. MALV, AB, AM, and AS were supported by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). The IAA team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa" award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709)

The D/H ratio measured in the Martian atmosphere gives an estimation of the planet’s water escape rate. We present here a global circulation model including the water isotope HDO and the main physical processes of fractionation. Modeling the D/H cycle accompanies the recent measurements from the ACS spectrometer onboard the orbiter TGO. The comparison between the model outputs and the TGO/ACS observations reveal some discrepancies, in particular during the second half of the martian year, when the escape is supposed to be the most efficient. The present work aims at studying these main differences. Introduction The high value of the Martian D/H ratio, derived from the HDO/H2O abundance ratio, in comparison to that of the Earth’s oceans[1],[2],[3],[4], is an indicator of the large escape of water from Mars over time. Apart from the mass difference between both isotopes, the differential escape of H and D comes from the preferential photolysis of H2O over HDO[5] and the preferential condensation of HDO over H2O [6],[7],[8]. The NOMAD and ACS instruments onboard ExoMars Trace Gas Orbiter (TGO) have recently provided unprecedented observations of vertical variations in HDO abundance and D/H ratio in the martian atmosphere [9],[10],[11],[12], motivating model development [13],[14]. In particular, TGO data cover the second half (“dusty season”) of martian year (MY) 34, which includes a Global Dust Storm (GDS) (Ls 180°-230°) and a regional dust storm (Ls 315°-330°). These dust events have been proven to be of particular importance in the hydrogen escape inventory [12],[15],[16],[17],[18]. HDO modeling The D/H ratio measured in the Martian atmosphere gives an estimation of the planet’s water escape rate. We present here a global circulation model including the water isotope HDO and the main physical processes of fractionation. Modeling the D/H cycle accompanies the recent measurements from the ACS spectrometer onboard the orbiter TGO. The comparison between the model outputs and the TGO/ACS observations reveal some discrepancies, in particular during the second half of the martian year, when the escape is supposed to be the most efficient. The present work aims at studying these main differences. Based on Montmessin et al. 2005[19], Rossi et al. 2021[13] have introduced the HDO cycle in the Mars Planetary Climate Model (Mars PCM), formely known as the LMD Mars Global Climate Model[20]. Vals et al. 2022[21] have recently adapted these implementations to the complete representation of the water ice clouds, including the radiative effect of clouds[22] and microphysics, the latter allowing the modeling of supersaturation[23]. The effect of kinetics in the fractionation by condensation has also been included, as has the photodissociation of HDO and the photochemical reactions of the deuterated species [24],[25],[26]. Rossi et al. 2022[27] have used the improved model to compare with TGO/ACS/MIR retrievals of D/H profiles[28] and revealed some discrepancies, which account for the challenge of reproducing the D/H cycle, depending on key components of the Martian atmosphere such as the water and dust cycles. Preliminary results By comparing the simulations and the observations, we particularly notice an overestimation by the model of water vapor above ∼60 km occuring near the GDS in MY34, and an underestimation near perihelion in the southern hemisphere (see Figure 1). The difference in water vapor vertical distribution has a direct impact on the D/H profiles (see Figures 2 and 3). In this work, we want to analyse these differences to eventually propose some model improvements. Further comparisons and results for MY34 and 35 will be shown at the conference. Figure 1: Water vapor volume mixing ratio in ppmv as measured by TGO/ACS/NIR[16] (black) and as computed by the model (red) at different altitude ranges. Figure 2: Water vapor volume mixing ratio, temperature and saturation as measured by TGO/ACS/NIR[16], and D/H profiles as measured by TGO/ACS/MIR[28] (black) and as computed by the model (red) during MY34 between solar longitudes 235°, 245° and latitudes -25°N, 25°N. Figure 3: Same layout as Figure 2 between solar longitudes 270°, 300° and latitudes -60°N, 45° N. References [1] Owen et al.: Science, 240 (4860), 1767-1770, 1988 [2] Encrenaz et al.: A&A, 612 , A112, 2018. [3] Krasnopolsky: Icarus, 257 , 377 - 386, 2015 [4] Villanueva et al.: Science, 348 (6231), 218–221, 2015. [5] Cheng et al.: Geophysical Research Letters, 26 (24), 3657-3660, 1999 [6] Krasnopolsky: Icarus, (2), 597-602, 2000. [7] Fouchet et al.: Icarus, 144 , 114-123, 2000. [8] Bertaux et al.: Journal of Geophysical Research: Planets, 106 (E12), 32879-32884, 2001. [9] Vandaele et al.: Space Science Reviews, 214 (5), 80, 2018. [10] Vandaele et al.: Nature, 568 (7753), 521-525, 2019. [11] Korablev et al.: Space Science Reviews, 214 , 7, 2018. [12] Korablev et al.: Nature, 568 (7753),517-520, 2019. [13] Rossi et al.: grl , 48 (7), e90962, 2021. [14] Daerden et al.: Journal of Geophysical Research (Planets), 127 (2), e07079, 2022. [15] Aoki et al.: Journal of Geophysical Research (Planets), 124 (12), 3482-3497, 2019. [16] Fedorova et al.: Science, 367 (6475), 297-300, 2020. [17] Neary et al.: Geophysical Research Letters, 47 (7), e84354, 2020. [18] Chaffin et al.: Nature Astronomy, 5 , 1036-1042, 2021. [19] Montmessin et al.: Journal of Geophysical Research, 2005. [20] Forget et al.: JGR, 104:24,155–24,176, 1999. [21] Vals et al.: Journal of Geophysical Research (Planets), under revision, 2022. [22] Madeleine et al. : Geophys. Res. Lett., 39 , 23202, 2012. [23] Navarro et al.: Journal of Geophysical Research (Planets), 2014. [24] Lefèvre et al.: Journal of Geophysical Research (Planets), 126 (4), 2021. [25] Cheng et al.: Journal of Chemical Physics, 120 (1), 224-229, 2004. [26] Chung et al.: Nuclear Instruments and Methods in Physics Research A, 467 (2002), 1572-1576, 2001. [27] Rossi et al.: Journal of Geophysical Research (Planets), under revision, 2022. [28] Alday et al.: Nature Astronomy, 5, 943-950, 2021.

Summary The mid-infrared channel of the Atmospheric Chemistry Suite (ACS) [1] onboard ESA’s Trace Gas Orbiter (TGO) has performed a sensitive search of methane in the Martian atmosphere using the solar occultation technique for more than two Mars years (four terrestrial years). The first two reports [2,3] have concluded on the global absence of methane detection above a mean level of 50 then 20 parts-per-trillion-per-volume (pptv), contrasting with the 0.4 to 45 ppbv concentrations reported from Earth-ground-observations, Mars orbiters, and from Mars’ surface by the Curiosity rover [5,6,7,8,9]. Here we present an update of the ACS methane search compiling all the ACS data collected up to summer 2022. Introduction For several decades, methane on Mars has been the subject of a continuous and intense search for it may resonate with an ongoing geophysical or biogenic activity on a planet that has long been perceived as inert. After a succession of failed attempts and positive detections between 2004 and 2019 [4,5,6,7,8,9,21,22], TGO started its trace gas detection mission in April 2018 and has continued since then. With its two separate state-of-the-art infrared spectrometers specifically conceived to achieve outstanding detection performances [1,10] for the ultra-sensitive search of a list of trace gases, including methane, TGO was aimed at confirming the presence of methane at Mars, or alternatively to establish the smallest upper limits possible. The Atmospheric Chemistry Suite (ACS) developed by IKI with the support of CNES is one of the spectrometers of TGO. Together with NOMAD, they offer a complementary access to the entire wavelength range from UV to thermal IR. They are also both observing the mid-IR range and therefore provide a redundant and thus reliable exploration of the presence of a variety of trace gases. Among the greatest achievements of these two experiments, we shall mention the joint observation of HDO and H2O [11,12], the discovery of HCl [13,14,15], the close and joint monitoring of water and ozone [16,17,18], as well as the first CO profiles [19]. Method The mid-infrared range, in particular the region around the fundamental 3.3 μm band of the C-H bond, provides the most sensitive way to detect methane and higher-order hydrocarbons (Fig. 1). With ACS, this spectral region has been explored around Mars during the last four years and has revealed an absence of methane above a limit of 20 pptv (Fig. 2), while NOMAD similarly established such limit to 60 pptv [20]. The technique of solar occultation is known to be more sensitive than nadir since the Sun is a source at Mars that is ~600 000 times brighter that Mars itself. While the ubiquitous atmospheric dust prevents observing the near-surface region, the high-resolving power (30 000) and its high Signal-to-Noise Ratio (~10 000 at low level of dust) compensate for this limitation. We attempt to detect CH4 using the R2 region which features the most intense individual lines of the band system. This region was backed up by parallel search in three other regions to have confirmation in case of a positive detection. Results With two Martian years of survey, the dataset consists of more than 1 000 individual altitude profiles. Since [3], improvements in the data processing have been implemented. This 4-year dataset will be presented along with its implications in regard to other observational studies and model attempts to explain reconcile the various observations. [1] Korablev, O., et al. 2018. Space Sci. Rev. [2] Korablev, O., et al. 2019. Nature [3] Montmessin, F., et al. 2021. Astron. & Astrophys. [4] Lellouch, E., et al. 2000. Astron. & Astrophys. [5] Formisano, V., et al., 2004. Science [6] Krasnopolsky, V. et al., 2004. Icarus [7] Mumma, M. et al., 2009. Science [8] Webster, C.R., et al., 2015, Science [9] Webster, C.R., et al., 2018, Science [10] Vandaele, A.C., et al. 2018. Space Sci. Rev. [11] Villanueva, G. et al., 2020. Sci. Adv. [12] Alday, J. et al., 2021. Nat. Ast. [13] Korablev, O., et al., 2021. Sci. Adv. [14] Aoki, S., et al., 2021. Geophys. Res. Let. [15] Olsen, K. et al., 2021. Astron. & Astrophys. [16] Fedorova et al., A., et al., 2020, Science [17] Patel, M., et al., 2021, J. of Geophys. Res. [18] Belyaev, D., et al, 2021., Geophys. Res. Let. [19] Olsen, K. et al., 2021. Nat. Geo. [20] Knutsen, E., et al., 2021, Icarus.

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made., HDO and the D/H ratio are essential to understand Mars past and present climate, in particular with regard to the evolution through ages of the Martian water cycle. We present here new modeling developments of the HDO cycle with the Laboratoire de Météorologie Dynamique Mars Global Climate Model (GCM). The present study aims at exploring the behavior of the D/H ratio cycle and its sensitivity to the modeling of water ice clouds and the formulation of the fractionation by condensation. Our GCM simulations are compared with observations provided by the Atmospheric Chemistry Suite (ACS) on board the ESA/Roscosmos Trace Gas Orbiter (TGO), and reveal that the model quite well reproduces the temperature and water vapor fields, which offers a good basis for representing the D/H ratio cycle. The comparison also emphasizes the importance of modeling the state of supersaturation, resulting from the microphysical processes of water ice clouds, to correctly account for the water vapor and the D/H ratio of the middle-to-upper atmosphere. This work comes jointly with a detailed comparison of the measured D/H profiles by TGO/ACS and the model outputs, conducted in the companion paper of Rossi et al. (2022, https://doi.org/10.1029/2022JE007201) (this issue). © 2022. The Authors., ExoMars is the space mission of ESA and Roscosmos. The ACS experiment is led by IKI Space Research Institute in Moscow. The project acknowledges funding by Roscosmos and CNES. Science operations of ACS are funded by Roscosmos and ESA. M.V. acknowledges support from the DIM ACAV labelled by the Ile-de-France region in support for the research (Domaine d’Intérêt Majeur, Astrophysique et Conditions d’Apparition de la Vie). L.R. acknowledges support from CNES. F. G.-G. is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709).

The D/H ratio and its implications on the atmospheric escape, make it an essential observable to study the current and past inventory of water on Mars. With the arrival of the Trace Gas Orbiter around Mars, new measurements of the D/H ratio are now available and require tools to interpret the observations and understand the HDO cycle. We here present simulations of an updated version of the Laboratoire de Météorologie Dynamique Mars Global Climate Model which includes HDO and in particular the fractionation processes it undergoes. We compare our model simulations with the HDO observations in solar occultation from the Atmospheric Chemistry Suite mid-infrared channel on board the Trace Gas Orbiter (Alday et al., 2021; https://doi.org/10.5281/ZENODO.5100448). The model successfully reproduces the general trends of the D/H ratio, indicating that the main physical processes are captured by theory. A consistent simulation of condensation processes is found to be key in the representation of the D/H ratio. Improvements in the representation of clouds and on the water cycle will help improving the representation of the HDO cycle and better help extrapolate back in times the conditions of water escape on Mars. © 2022. American Geophysical Union. All Rights Reserved., L.R. acknowledges support from CNES. F.G.-G. is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). M.V. acknowledges support from the DIM ACAV labelled by the Ile-de-France region in support for the research (“Domaine d’Intérêt Majeur, Astrophysique et Conditions d’Apparition de la Vie).”

The detection of sulphur species in the Martian atmosphere would be a strong indicator of volcanic outgassing from the surface of Mars. We wish to establish the presence of SO2, H2S, or OCS in the Martian atmosphere or determine upper limits on their concentration in the absence of a detection. We perform a comprehensive analysis of solar occultation data from the mid-infrared channel of the Atmospheric Chemistry Suite instrument, on board the ExoMars Trace Gas Orbiter, obtained during Martian years 34 and 35. For the most optimal sensitivity conditions, we determine 1-sigma upper limits of SO2 at 20 ppbv, H2S at 15 ppbv, and OCS at 0.4 ppbv; the last value is lower than any previous upper limits imposed on OCS in the literature. We find no evidence of any of these species above a 3-sigma confidence threshold. We therefore infer that passive volcanic outgassing of SO2 must be below 2 ktons/day., 13 pages, 12 figures, received for production in A&A on the 19th of November 2021

The amplitude and rate of ozone depletion in the Arctic is monitored every year since 1994 by comparison between SAOZ UV-Vis ground-based network from NDACC and Multi-Sensor Reanalysis 2 (MSR-2) total ozone measurements over 8 stations in the Arctic and 3-D chemical transport model simulations in which ozone is considered as a passive tracer. The passive ozone method allows determining the cumulative loss at the end of the winter. The amplitude of the destruction varies between 0-10% in relatively warm and short vortex duration years to 25-38% in colder and longer ones, which the record winters estimated in 2010/2011 and 2019/2020.In this study, the interannual variability of 10-days average rate of 2021/2022 winter will be analyzed and compared to previous years. In addition, SAOZ NO2 data will be used to evaluate re-noxification in the Arctic. The long-term ozone loss series estimated from measurements will be compared to REPROBUS and SLIMCAT CTM simulations. Relationship with illuminated Polar Stratospheric Clouds will be also presented.

Our understanding of Venus and Mars climate has been noticeably improved thanks to progress with General Circulation Models (GCM) (e.g., Forget et al. 1999, Lebonnois et al. 2010, Gilli et al. 2021) and increasing measurements, both from space missions and ground-based telescopes. While there are 13 operational missions currently dedicated to Mars, a new era in the exploration of “our sister” planet Venus is coming in the next decades with the selection of 3 missions: DAVINCI and VERITAS by NASA, EnVision by ESA, in addition to the Indian orbiter mission, Shukrayyan-1 (planned for 2025). Nevertheless, our view of the upper layers of those planets (i.e., above approximately 80 km and 60 km on Venus and Mars, respectively) remains incomplete. The observed high variability of those regions (e.g., Gerard et al. 2014, Gonzalez-Galindo et al. 2015) is very challenging to predict by 3D models. Planetary waves (e.g., Kelvin waves) are suggested to play an important role in the variability in the so-called transition region on Venus (between super-rotation and day-to-night circulation) (Navarro et al. 2021) and gravity waves are recognized to produce a significant impact on the thermal tides of Mars (Gilli et al. 2020). In this talk, I will give a brief overview of recent 3D GCM developments done in collaboration with the Institut Pierre-Simon Laplace (IPSL) laboratories in France and the Instituto de Astrofisica de Andalucia (IAA) in Spain, such as the inclusion of a stochastic non-orographic gravity wave parameterization and improvements on the parameterization of non-LTE CO2 heating rates (Martinez et al. 2022, submitted), to provide a more realistic picture of those upper regions of the Venus and Mars atmosphere. References:Forget et al. 1999, JGR, 104, 155-24Lebonnois et al. 2010, JGR-Planets, 115, 6006Gilli et al. 2021, Icarus, Vol. 366, 114432Navarro et al. 2021, Icarus, Vol. 366, 114400Gilli et al. 2020, JGR-Planets, 125-3Gilli et al. 2017, Icarus, Vol.248, 478-498Gerard et al. 2014, Icarus, 236, 92-103Gonzalez-Galindo et al. 2015. JGR-Planets, 120, 2020-2035Martinez et al. 2022, submitted to Icarus Acknowledgments:GG is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC-FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). This research was also supported by Fundação para a Ciência e a Tecnologia (FCT) through the research grants UIDB/04434/2020, UIDP/04434/2020, P-TUGA PTDC/FIS-AST/29942/2017.

Monitoring CO2 from space is essential to characterize the spatiotemporal distribution of this major greenhouse gas and quantify its sources and sinks. The mixing ratio of CO2 to dry air can be derived from the CO2∕O2 column ratio. The O2 column is usually derived from its absorption signature on the solar reflected spectra over the O2 A band (e.g. Orbiting Carbon Observatory-2 (OCO-2), Thermal And Near infrared Sensor for carbon Observation (TANSO)/Greenhouse Gases Observing Satellite (GOSAT), TanSat). As a result of atmospheric scattering, the atmospheric path length varies with the aerosols' load, their vertical distribution, and their optical properties. The spectral distance between the O2 A band (0.76 µ m) and the CO2 absorption band (1.6 µ m) results in significant uncertainties due to the varying spectral properties of the aerosols over the globe. There is another O2 absorption band at 1.27 µ m with weaker lines than in the A band. As the wavelength is much closer to the CO2 and CH4 bands, there is less uncertainty when using it as a proxy of the atmospheric path length to the CO2 and CH4 bands. This O2 band is used by the Total Carbon Column Observing Network (TCCON) implemented for the validation of space-based greenhouse gas (GHG) observations. However, this absorption band is contaminated by the spontaneous emission of the excited molecule O2 * , which is produced by the photo-dissociation of O3 molecules in the stratosphere and mesosphere. From a satellite looking nadir, this emission has a similar shape to the absorption signal that is used. In the frame of the CNES (Centre National d'Etudes Spatiales – the French National Centre for Space Studies) MicroCarb project, scientific studies have been performed in 2016–2018 to explore the problems associated with this O2 * airglow contamination and methods to correct it. A theoretical synthetic spectrum of the emission was derived from an approach based on A21 Einstein coefficient information contained in the line-by-line high-resolution transmission molecular absorption (HITRAN) 2016 database. The shape of our synthetic spectrum is validated when compared to O2 * airglow spectra observed by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY)/Envisat in limb viewing. We have designed an inversion scheme of SCIAMACHY limb-viewing spectra, allowing to determine the vertical distribution of the volume emission rate (VER) of the O2 * airglow. The VER profiles and corresponding integrated nadir intensities were both compared to a model of the emission based on the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model. The airglow intensities depend mostly on the solar zenith angle (both in model and data), and the model underestimates the observed emission by ∼15 %. This is confirmed with SCIAMACHY nadir-viewing measurements over the oceans: in such conditions, we have disentangled and retrieved the nadir O2 * emission in spite of the moderate spectral resolving power ( ∼860 ) and found that the nadir SCIAMACHY intensities are mostly dictated by solar zenith angle (SZA) and are larger than the model intensities by a factor of ∼1.13 . At a fixed SZA, the model airglow intensities show very little horizontal structure, in spite of ozone variations. It is shown that with the MicroCarb spectral resolution power (25 000) and signal-to-noise ratio (SNR), the contribution of the O2 * emission at 1.27 µ m to the observed spectral radiance in nadir viewing may be disentangled from the lower atmosphere/ground absorption signature with a great accuracy. Indeed, simulations with 4ARCTIC radiative transfer inversion tool have shown that the CO2 mixing ratio may be retrieved with the accuracy required for quantifying the CO2 natural sources and sinks (pressure-level error ≤1 hPa; X CO 2 accuracy better than 0.4 ppmv) with the O2 1.27 µ m band only as the air proxy (without the A band). As a result of these studies (at an intermediate phase), it was decided to include this band (B4) in the MicroCarb design, while keeping the O2 A band for reference (B1). Our approach is consistent with the approach of Sun et al. (2018), who also analysed the potential of the O2 1.27 µ m band and concluded favourably for GHG monitoring from space. We advocate for the inclusion of this O2 band on other GHG monitoring future space missions, such as GOSAT-3 and EU/European Space Agency (ESA) CO2 -M missions, for a better GHG retrieval.

Measuring vertical variations in the deuterium to hydrogen ratio with altitude is essential in understanding the processes that lead to the escape of water vapour from the Martian atmosphere. We retrieve vertical profiles of HDO and H2O from the ACS instrument, monitoring seasonal changes particularly above the water condensation level. We discuss these results in relation to previously observed seasonal variations in D/H together with the expected variations from theoretical models. Introduction The ratio of deuterium to hydrogen (D/H) is a sensitive tracer of the rate of escape of water from the Martian atmosphere over its history. Hydrogen preferentially escapes from the atmosphere over its heavier isotope (e.g. [1]), and so the greater the amount of historical escape of water vapour, the larger the average D/H ratio. On Mars, this value is measured to be around 4-6 times that of terrestrial distilled ocean water [2,3], showing that the early atmosphere of Mars contained significantly more water than it does today. In addition, a number of processes in the lower and middle atmosphere can cause relative changes in the concentrations of semi-heavy water (HDO) with respect to water vapour, notably due to differences in rates of cloud deposition (e.g. [4,5]) and photolysis (e.g. [6,7]). We therefore wish to look at spatial and temporal changes in the vertical profile of D/H, particularly above the level of water condensation, in order to better characterise the processes that influence the escape of water vapour from the lower atmosphere into space. Method The mid-infrared channel of the Atmospheric Chemistry Suite Instrument (ACS MIR, [8]) on board the ExoMars Trace Gas Orbiter obtains transmission spectra of the Martian atmosphere in solar occultation geometry, which is sensitive to trace gases at very low abundance and at high vertical resolution. We make use of observations in grating position 11, which is sensitive to a wavenumber regime in which a large number of resolvable HDO lines are present that in the best cases provide sensitivity to HDO abundance up to around 70 km. These are then inverted using the RISOTTO radiative transfer and retrieval pipeline [9,10] to give vertical profiles of HDO volume mixing ratio. Concurrent vertical profiles of H2O are obtained using the near-infrared (NIR) channel of the same instrument [11], and then the D/H ratio computed assuming that H2O and HDO are the main carriers of the two isotopes of hydrogen. Accurate quality control of the data is performed using probability-sparse Non-negative Matrix Factorisation (psNMF [12,13]). Results and Perspective We report seasonal changes in the fractionation of D/H in the middle atmosphere throughout the temporal range of the data starting from the autumn equinox in MY34 to the end of MY 35. These will be discussed in relation to the findings of seasonal changes in the vertical profiles of D/H reported by the NOMAD instrument [14], and the results interpreted in relation to predictions from Global Climate Models (GCMs) of HDO ([5,15,16]). Fig. 1: Retrieved vertical profiles of (left) water vapour from (right) D/H from the near- and mid-infrared channels respectively of the ACS instrument. References [1] Krasnopolsky, V. A., Mumma, M. J., & Randall Gladstone, G. 1998, Science, 280, 1576 [2] Owen, T., Maillard, J. P., de Bergh, C., et al. 1988, Science, 240, 1767-1770 [3] Webster, C. R.; Mahaffy, P. R.; Flesch, G. J., et al. 2013, Science, 341, 260-263 [4] Fouchet, T. & Lellouch, E. 2000, Icarus, 144, 114-123 [5] Montmessin, F., Fouchet, T. & Forget, F. 2005, J. Geophys. Res. Plan., 110 [6] Yung, Y. L., Wen, J.-S., Pinto, J. P., et al. 1988, Icarus 76, 146-159 [7] Alday, J., Trokhimovskiy, A., Irwin, P. G. J., et al. 2021, Nat. As. 5, 943-950 [8] Korablev, O., Montmessin, F., Trokhimovskiy, A., et al. 2018, Space Sci. Rev.,214,7 [9] Braude, A. S., Ferron, S., & Montmessin, F. 2021, J. Quant. Spec. Rad. Transf.,274,107848 [10] Braude, A. S., Montmessin, F., Olsen, K. S., et al. 2022, A&A,658,A86 [11] Fedorova, A. A., Montmessin, F., Korablev, O., et al. 2020, Science 367, 297-300 [12] Hinrich, J.L. & Mørup, M. 2018, Latent Variable Analysis and Signal Separation. LVA/ICA 2018. Lecture Notes in Computer Science(), 10891, 488–498 [13] Schmidt, F.; Mermy, G. C.; Erwin, J., et al. 2021, JQSRT, 259, 107361 [14] Villanueva, G. L.; Liuzzi, G.; Crismani, M. M. J., et al. 2021, Sci. Adv. 7, eabc8843 [15] Rossi, L., Vals, M., Montmessin, F., et al., J. Geophys. Res. Plan., in review. [16] Vals, M., Rossi, L., Montmessin, F., et al. 2022, J. Geophys. Res. Plan., in press.

The geological record provides convincing evidence for very recent volcanic activity on Mars (e.g. Horvath et al. 2021, Icarus 365, 114499). In this talk we present the results of Braude et al. (2022, A&A 658, A86), in which a comprehensive analysis was performed of solar occultation observations from the ACS MIR instrument from MY 34 to 35, in search of three tracers of residual volcanic outgassing from the surface of Mars: SO2, H2­S and OCS. We found no statistically significant detections of either of these three molecules, instead imposing upper limits of 20 ppbv of SO2, 15 ppbv of H2S, and 0.4 ppbv of OCS. We thereby estimated that volcanic outgassing from the surface of Mars must be below 2 ktons day­-1. Introduction Several pieces of geological evidence indicate the presence of residual volcanic activity in the Tharsis and Elysium regions dating back only a few million [1] or even hundreds of thousands [2] of years, with the InSight Lander recently confirming present-day seismic activity in the Cerberus Fossae region [3]. On Earth, volcanic emission is dominated by carbon dioxide, water vapour and sulphur species, notably sulphur dioxide (SO­2) but with smaller amounts of hydrogen sulphide (H2S) and the occasional detection of carbonyl sulphide (OCS). Previous ground-based observations of sulphur species on Mars, however, have not resulted in any confirmed detections, instead imposing lowest disc-integrated upper limits of 0.3 ppbv of SO2 [4,5], 1.5 ppbv of H2S [6] and 1.1 ppbv of OCS [7]. In this talk we summarise the results of [8], in which we used solar occultation measurements from the mid-infrared channel of the Atmospheric Chemistry Suite (ACS MIR, [9]) instrument from the start of the ACS science phase in MY 34 to the end of MY 35, in search of SO2, H2S and OCS in Mars’ lower atmosphere. Fig. 1: Retrieved vertical profiles of SO2 upper limit values from each of the 190 processed solar occultations between Ls = 165 - 280° of MY 34 and Ls = 140 - 350° of MY 35. Yellower colours indicate an increased density of retrieved values. (Reproduced with permission from Fig. 4 of [8]) Method We made use of three sets of ACS MIR data each obtained using different diffraction grating positions that are sensitive to different wavenumber ranges. 190 individual occultations carried out using grating position 9 were analysed, covering a time period of Ls = 165 - 280° of MY 34 and Ls = 140 - 350° of MY 35. This grating position covers the strongest SO2 absorption features in the wavenumber range covered by ACS MIR. Using the RISOTTO radiative transfer and retrieval code [10], we performed retrievals of this data within a wavenumber range of 2481 and 2492 cm-1, which is sensitive to a series of overlapping SO2 and CO2 absorption lines, and imposed upper limits by deriving the volume mixing ratio (VMR) of SO2 that would result in a change in goodness of fit (χ2/n) to the observed spectra above a given threshold. Equivalently, we searched for H2S in two spectral windows between 3827 and 3833 cm-1 and around a single absorption line at 3839.2 cm-1 covered by grating position 5, of which we analysed 86 occultations between Ls = 164 - 218° and Ls = 315 - 354° of MY 34. Finally, we searched for OCS in two spectral windows between 2908 - 2912 and 2923 - 2927 cm-1, corresponding to the P and R branches respectively of an OCS absorption band in a wavenumber region that is mostly devoid of other gas absorption lines. This region is covered by grating position 11, for which there is almost complete coverage between Ls = 163° of MY 34 and Ls = 355° of MY 35. Upper limits for these two molecules were imposed by averaging retrieved vertical VMR profiles from adjacent rows on the MIR detector. Results and Perspective We found no significant detections (>3σ) of any of these three molecules, instead deriving our best upper limits of 20 ppbv of SO2, 15 ppbv of H2­S and 0.4 ppbv of OCS. The latter value is the lowest reported upper limit of OCS in the Martian atmosphere to date, as opposed to the previous value of 1.1 ppbv reported by [7]. However, SO2 is expected to dominate in concentration relative to H2S and OCS in volcanic emission. We estimate that during the period of observation, no more than 750 ktons of SO2 could be present in the entire Martian atmosphere, implying surface outgassing of SO2 below 2 ktons/day. Future targeted measurements of sulphur species in regions associated with historic volcanism (e.g. the Tharsis region, Elysium Planitia) could shed greater light on the probability of residual present-day volcanism. References [1] Neukum, G., Jaumann, R., Hoffmann, H., et al. 2004, Nature, 432, 971 [2] Horvath, D. G., Moitra, P., Hamilton, C.W., Craddock, R. A, & Andrews-Hanna,J. C. 2021, Icarus, 365, 114499 [3] Giardini, D., Lognonné, P., Banerdt, W. B., et al. 2020, Nat. Geosci.,13,205 [4] Encrenaz, T., Greathouse, T. K., Richter, M. J., et al. 2011, A&A,530,A37 [5] Krasnopolsky, V. A. 2012, Icarus,217,144 [6] Khayat, A. S., Villanueva, G. L., Mumma, M. J., & Tokunaga, A. T. 2015, Icarus,253,130 [7] Khayat, A., Villanueva, G. L., Mumma, M. J., & Tokunaga, A. T. 2017, Icarus,296,1 [8] Braude, A. S., Montmessin, F., Olsen, K. S., et al. 2022, A&A,658,A86 [9] Korablev, O., Montmessin, F., Trokhimovskiy, A., et al. 2018, Space Sci. Rev.,214,7 [10] Braude, A. S., Ferron, S., & Montmessin, F. 2021, J. Quant. Spec. Rad. Transf.,274,107848

Introduction The D/H ratio observed in a planetary atmosphere is a proxy for the ratio of the current water reservoir over the initial water reservoir of the planet. The current D/H ratio measured in the Martian atmosphere is at least five that of the Vienna Standard Mean Ocean Water (SMOW) (Owen et al. 1988, Encrenaz et al. 2018, Krasnopolsky 2015, Villanueva et al. 2015). This high value of the martian D/H ratio, derived from the HDO/H2O abundance ratio, is a precious indicator of the large escape of water from the martian atmosphere over time. Apart from the mass difference between both isotopes, the differential escape of H and D comes from the preferential photolysis of H2O over HDO (Cheng et al. 1999) and the Vapor Pressure Isotope Effect (VPIE) that produces an isotopic fractionation at condensation (Krasnopolsky, 2000, Bertaux et al. 2001, Fouchet et al. 2000). Modelling the HDO cycle Rossi et al. 2021 have implemented the HDO cycle, adapted from Montmessin et al. 2005, within the current version of the LMD Martian GCM (Forget et al. 1999). The fractionation effect caused by the condensation is taken into account with the calculation of a fractionation factor depending on the temperature, according to the empirical formulation of Lamb et al. 2017. In Rossi et al. (2021)’s study, clouds are produced and evolve according to simplified physics consisting in turning water vapour into ice if any supersaturation of water. In the study presented here, the HDO cycle is upgraded to the last version of the water cycle of the model, including the activation of the radiative effect of clouds and their microphysics (referring to the parametrization of the different processes of formation of the clouds, as nucleation of the ice particles on dust particles, water ice growth, dust scavenging and supersaturation), implemented in the model by Navarro et al. 2014. The calculation of the fractionation has also been improved by implementing the effect of kinetics impacting the condensation process. Indeed, the slower diffusion of the heavier HDO molecule in comparison to that of H2O reduces condensation flux, and an expression has been established by Jouzel and Merlivat 1984 to include this effect in the overall fractionation of water isotopologues during condensation into ice. Results and perspectives Our results, on the basis of these recent improvements, will be presented at the conference. Reinforcement of the Hadley circulation by the radiative effect of clouds which is now taken into account, as well as the role of nuclei played by dust in the cloud formation, do affect the transport of HDO and its vertical distribution, which directly impacts the HDO/H2O ratio in the low-to-mid atmosphere (see Figure 1). Kinetics of condensation reduces the fractionation effect whenever saturation occurs and increases with supersaturation, which means mainly at the poles (see Figure 2). Further aspects of this upgraded modelling of the HDO cycle and its sensitivity to different parameters will be explored and presented at the conference. The comparison of the model results with observations from TGO/ACS are presented by Loïc Rossi in session TP16. For now, the here-presented simulations extend to an altitude of around 120 km, and do not include the parametrizations of the thermosphere (Angelats i Coll et al. 2005, Gonzalez-Galindo et al. 2009), the photochemistry (Lefèvre et al. 2004, 2008) and the escape (Chaufray et al. 2015a), which have been recently extended to deuterated species. The ultimate goal of this work is the development of a complete representation of the deuterium cycle, from its source as HDO in the lower atmosphere, to its photodissociation and escape from the upper atmosphere. Figure 1: Meteorological variables as output of the model zonally averaged at LT=14:00 around Ls=90°. (Up) Results for the GCM simulation run with the simple physics of clouds (1). (Middle) Results for the GCM simulation run with the microphysics and the radiative effect of clouds (2). (Down) Difference between the two simulations (2) - (1). (Left) Temperature (K). (Middle) H2O ice mixing ratio (ppm). (Right) D/H ratio of the vapour phase. Figure 2: (Left) Zonal average of the integrated column of the D/H ratio of the vapour phase for a GCM simulation run with microphysics and radiative effect of clouds, and taking into account the effect of kinetics in the fractionation process. (Right) Difference between simulation with and without taking into account the effect of kinetics. Acknowledgements ExoMars is the space mission of ESA and Roscosmos. The ACS experiment is led by IKI Space Research Institute in Moscow. The project acknowledges funding by Roscosmos and CNES. Science operations of ACS are funded by Roscosmos and ESA. Science support in IKI is funded by Federal agency of science organizations (FANO). M.V. acknowledges support from the DIM ACAV labelled by the Ile-de-France region in support for the research (Domaine d’Intérêt Majeur, Astrophysique et Conditions d’Apparition de la Vie)".

1. IntroductionVenus is hosting a global sulfuric acid cloud layer between 45 and 70 km which has been investi- gated by the Venus Express and Akatsuki mission as well as its coupling with the surface. One of the main questions that remains unclear about the dynamics of the Venusian atmosphere is how this convective cloud layer mixes momentum, heat, and chemical species and generates gravity waves. Several models have been developed to study these phenomenons. We proposed to use these models to study the impact of this turbulence on the chemical species, focusing on water and sulfuric dioxide. 2. ModelTo study the convective layer, a Large Eddy Simulations (LES) model [1] has been developed using the Weather-Research Forecast (WRF) non-hydrostatic dynamical core [2] coupled with the IPSL Venus GCM physics package [3]. The model is able to resolve a realistic convective layer between 47 and 55 km as well as one convective layer at cloud top altitudes (70 km) at the substellar point (Fig 1). Figure 1: Vertical cross-section of the vertical wind (m/s) at the Equator at noon. Between 47 and 55 km is the main convective layer, between 55 and 67 km are the gravity waves induced by convection and between 67 and 73 is the cloud top convective layer presents only at the substellar point. Tracers has been included in the model representing H2O and SO2, the chemistry and photodissociation sources and sinks are modeled by a linear relaxation of the tracer abundance toward a prescribed vertical profile with a characteristic time. The relaxation time ranges from 102 to 106 s. The prescribed vertical tracer profiles are constructed using observed abundance visible in Fig 2. Figure 2: Vertical profile of the tracer abudance relaxation profile. The black represents the value for the deep atmosphre [4], the star is the SO2 ground based observations at 65 km [5] and the circle is the cloud top H2O Venus Express value [6, 7]. 3. ResultsThis simple model is able to determine the vertical mixing for SO2 and H2O in the cloud layer, and for which chemical timescale the convection plays an important role. The resolution of 500 m allow an estimate of the horizontal turbulent spatial features, induced by the convection and gravity waves, for SO2 and H2O. References[1] Lefèvre et al., JGR : Planets, 123, 2773-2789, 2018.[2] Skamarock, W. C. and J. B. Klemp, J., Comput. Phys., 227, 3465-3485, 2008[3] Garate-Lopez, I. and Lebonnois., S., Icarus, 314,1-11, 2018.[4] Bézard, B. and De Bergh, C., JGR : Planets, 112, 2007.[5] Encrenaz, T. et al., A. & A., 595, 2016.[6] Fedorova, A. et al., Icarus, 275, 143-162, 2016.[7] Cottini., V. et al., Icarus, 217, 561-569, 2012.

The thermal escape of hydrogen from Mars is recognized as one of the major drivers of the long-term climatic evolution of the planet. Recent works have shown that, contrary to what was previously believed, water is not trapped in the lower atmosphere of Mars. Instead, it can be transported to the middle/upper atmosphere, producing layers of supersaturated water (Fedorova et al., 2018, 2021). Upper atmospheric water can then be converted to hydrogen by photolysis or chemical reactions with ions, boosting the rate of hydrogen escape (Chaffin et al., 2017; Stone et al., 2020). Strong seasonal variations in the escape rate, and significant increases of both the water abundance in the mesosphere and the hydrogen escape rate during dust storms, evidence the strong coupling between the hydrogen escape and the water cycle (Chaffin et al., 2014; Fedorova et al., 2018, 2020). A global model able to simulate all the processes related to water, from the ice sublimation to the transport to the upper atmosphere and its atmospheric escape, is needed in order to help interpreting the observations. This model can also be used to explore also the water cycle and hydrogen escape on past Mars conditions characterized by different orbital parameters, allowing for a better estimation of the accumulated escape rate. Previous simulations with the LMD-Mars Global Climate Model (LMD-MGCM), and their comparison with observational results by SPICAM/Mars Express showed that the simulated escape rate was underestimated, in particular during the second half of the Martian year (Chaufray et al., 2021). However, those simulations did not take into account the microphysical processes producing water supersaturation, and thus underestimated the role of water transport in the escape rate. In addition, the model did not include the photochemistry of water-derived ions, which can play an important role in converting water into hydrogen (Stone et al., 2020). New simulations with an improved version of the LMD-MGCM have been produced that overcome those previous limitations. The water cloud microphysics has now been fully considered in the simulations, using the model by Navarro et al. (2014). The photochemical model has been updated to include water-derived ions (H2O+, H3O+, OH+). Also, the deuterium fractionation model has been improved (Rossi et al., 2021), and deuterated species have been included in the photochemical model. While this last modification is not expected to modify the hydrogen escape rate, the inclusion of deuterated species can provide important diagnostics on the hydrogen escape and its accumulation over Mars history. In this presentation we will show the results of the improved version of the LMD-MGCM, comparing with available observations. The focus will be on the predicted hydrogen escape rate, and how it is affected by the inclusion of different physical processes. We find that including the possibility of water supersaturation increases the Hydrogen escape rate in more than one order of magnitude at most seasons, taking the simulated rate to better agreement with SPICAM observations during the second half of the year. This confirms previous observational results indicating the importance of water supersaturation (Fedorova et al. 2020). We also find that the inclusion of water-derived ions in the photochemistry also increases the escape rate, in particular during the first part of the year. We will also compare the predicted water abundance in the mesosphere with Mars Express and ExoMars TGO observations, and the abundances of water-derived ions with NGIMS/MAVEN measurements. References: Chaffin, M. et al., Unexpected variability of Martian hydrogen escape, Geophysical Research Letters, Volume 41, pp. 314-320 (2014) Chaffin, M. et al., Elevated atmospheric escape of atomic hydrogen from Mars induced by high-altitude water, Nature Geoscience, 10, pp. 174-178 (2017) Fedorova, A. et al., Water vapor in the middle atmosphere of Mars during the 2007 global dust storm, Icarus, 300, pp. 440-457 (2018) Fedorova, A. et al., Stormy water on Mars: The distribution and saturation of atmospheric water during the dusty season, Science, 367, pp. 297-300 (2020) Fedorova, A. et al., Multi-Annual Monitoring of the Water Vapor Vertical Distribution on Mars by SPICAM on Mars Express, Journal of Geophysical Research: Planets, 126, e06616 (2021) Navarro, T. et al., Global climate modeling of the Martian water cycle with improved microphysics and radiatively active water ice clouds, Journal of Geophysical Research: Planets, 119, pp. 1479-1495 (2014) Rossi, L. et al., The Effect of the Martian 2018 Global Dust Storm on HDO as Predicted by a Mars Global Climate Model, Geophysical Research Letters, 48, e90962 (2021) Stone, S. et al., Hydrogen escape from Mars is driven by seasonal and dust storm transport of water,Science, 370, pp. 824-831 (2020)

By the utilisation of a novel synergistic approach, we constrain the vertical distribution of water vapor on Mars with nadir-pointing instruments. Water vapor column abundances were retrieved simultaneously from PFS (sensing the thermal infrared range) and SPICAM (sensing the near-infrared range) on Mars Express, yielding distinct yet complementary sensitivity to different parts of the atmospheric column. This approach offers a way to study the vertical partitioning of water, which has remained out of the scope of nadir observations made by single instruments covering a specific spectral range. Here we present a global dataset covering all seasons and latitudes, with measurements taken over 8 Martian years. Special focus is given to the high-latitude regions in spring and summer during the sublimation of the seasonal polar cap. A significant deviance was discovered between the retrieved vertical distribution water vapor, and the predicted profile from the Mars Climate Database, which is used as prior. We also show that by exploiting a synergistic retrieval approach, we obtain more accurate water vapor column abundances compared to when only one instrument is used. The synergistic retrieval has a tendency to obtain a lower total column abundance compared to the prior, albeit with a stronger vertical partitioning. This indicates a more prominent low-altitude layer, with more water vapor contained close to the surface than predicted by models.

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".

International audience; Carbon monoxide (CO) is the main product of CO2 photolysis in the Martian atmosphere. Production of CO is balanced by its loss reaction with OH, which recycles CO into CO2. CO is therefore a sensitive tracer of the OH-catalysed chemistry that contributes to the stability of CO2 in the atmosphere of Mars. To date, CO has been measured only in terms of vertically integrated column abundances, and the upper atmosphere, where CO is produced, is largely unconstrained by observations. Here we report verti- cal profiles of CO from 10 to 120 km, and from a broad range of latitudes, inferred from the Atmospheric Chemistry Suite on board the ExoMars Trace Gas Orbiter. At solar longitudes 164–190°, we observe an equatorial CO mixing ratio of ~1,000 ppmv (10–80 km), increasing towards the polar regions to more than 3,000 ppmv under the influence of downward transport of CO from the upper atmosphere, providing a view of the Hadley cell circulation at Mars’s equinox. Observations also cover the 2018 global dust storm, during which we observe a prominent depletion in the CO mixing ratio up to 100 km. This is indicative of increased CO oxidation in a context of unusually large high-altitude water vapour, boosting OH abundance.

International audience; We present the first water vapor profiles encompassing the upper mesosphere of Mars, 100–120 km, far exceeding the maximum altitudes where remote sensing has been able to observe water to date. Our results are based on solar occultation measurements by Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter (TGO). The observed wavelength range around 2.7 μm possesses strong CO2 and H2O absorption lines allowing sensitive temperature and density retrievals. We report a maximum H2O mixing ratio varying from 10 to 50 ppmv at 100–120 km during the global dust storm (GDS) of Martian Year (MY) 34 and around southern summer solstice of MY 34 and 35. During other seasons water remains persistently below ∼2 ppmv. We claim that contributions of the MY34 GDS and perihelion periods into the projected hydrogen escape from Mars are nearly equivalent.

Ozone depletion over Polar Regions is monitored each year by satellite and ground-based instruments. The first signs of healing of the ozone layer linked to the decrease of ozone destructive substances (ODSs) were observed in Antarctica using different metrics (ozone mean values, ozone mass deficit, area of the ozone hole) and simple or sophisticated models. Chemistry climate models predict that climate change will not affect expected ozone recovery over Antarctica but will accelerate recovery in the Arctic due to the possible enhancement of the Brewer Dobson circulation. However, ozone loss observations by SAOZ UV-Vis spectrometers do not show a clear sign of recovery in the latter region. In addition, a record of 38% ozone loss in 2010/2011 and 2019/2020 was estimated.In this study, the vortex-averaged ozone loss in the last three decades will be evaluated for both Polar Regions using the passive ozone tracer of two chemical transport models (REPROBUS and SLIMCAT CTMs) and total ozone observations from SAOZ and satellite observations (IASI/METOP and Multi-Sensor Reanalysis (MSR-2)).The tracer method allows us to determine the evolution of the daily rate of ozone destruction, and the amplitude of the cumulative loss at the end of the winter. The cumulative ozone destruction in the Artic varies between 0-10% in relatively warm winters with short vortex duration to up to 25-38% in colder winters with longer vortex persistence, while in Antarctica it is mostly stable, around 50%.Interannual variability of 10-days average rate will be analyzed and compared between both hemispheres as well as the timing to reach different thresholds of absolute ozone loss values. Finally, linear trend of ozone loss and temperature since 2000 will be estimated in both Polar Regions in order to evaluate possible ozone recovery.

The molecular oxygen (O2) and carbon oxide (CO) are minor constituents of the Martian atmosphere with the annual mean mixing ratio of (1560 ± 60 ppm) and (673 ± 2.6 ppm), respectively (Krasnopolsky, 2017). Both are non-condensable species and their latitudinal variations are induced by condensation and sublimation of CO2 from the polar caps that result in enrichment and depletion and seasonal variations are following the total CO2 amount in the atmosphere.The Atmospheric Chemistry Suite (ACS) is a set of three spectrometers (-NIR, -MIR, and -TIRVIM) intended to observe Mars atmosphere onboard the ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission (Korablev et al., 2018). The near infrared channel (NIR) is a compact spectrometer operating in the range of 0.7–1.7 µm with a resolving power of λ/Δλ ~ 25,000. It is designed to operate in nadir and in solar occultation modes. The simultaneous vertical profiling of the O2 and CO density at altitudes of 10-60 km based on 0.76 µm and 1.57 µm bands, respectively, is a unique feature of the ACS NIR science in occultation. In this work we present the seasonal and latitudional distribution of the O2 and CO mixing ratios obtained for period of 2018-2020 (MY34 and35) and the comparison with the LMD General Circulation model. We report the averaged mixing ratio for CO of ~950 ppm and for O2 of~1800 ppm at low altitudes (~20 km). Also, we detected extremely enriched CO layer at 10-15 km in the southern polar region at Ls=100-200° both for MY34 and MY35.

International audience; A numerical simulation of the upper atmosphere of Venus is carried out with an improved version of the Institut Pierre-Simon Laplace (IPSL) full-physics Venus General Circulation Model (GCM). This simulation reveals the organization of the atmospheric circulation at an altitude above 80 km in unprecedented detail. Converging flow towards the antisolar point results in supersonic wind speeds and generates a shock-like feature past the terminator at altitudes above 110 km. This shock-like feature greatly decreases nightside thermospheric wind speeds, favoring atmospheric variability on a hourly timescale in the nightside of the thermosphere. A 5-day period Kelvin wave originating in the cloud deck is found to substantially impact the Venusian upper atmosphere circulation. As the Kelvin wave impacts the nightside, the poleward meridional circulation is enhanced. Consequently, recombined molecular oxygen is periodically ejected to high latitudes, explaining the characteristics of the various observations of oxygen nightglow at 1,27 µm. An analysis of the simulated 1,27µm oxygen nightglow shows that it is not necessarily a good tracer of the upper atmospheric dynamics, since contributions from chemical processes and vertical transport often prevail over horizontal transport. Moreover, dayside atomic oxygen abundances also vary periodically as the Kelvin wave momentarily decreases horizontal wind speeds and enhances atomic oxygen abundances, explaining the observations of EUV oxygen dayglow. Despite the nitrogen chemistry not being currently included in the IPSL Venus GCM, the apparent maximum NO nightglow shifted towards the morning terminator might be explained by the simulated structure of winds.

The current Martian atmosphere is about five times more enriched in deuterium than Earth's, providing a direct testimony that Mars hosted several times more water in its early youth than nowadays. Estimates of the total amount of water lost to space from the current mean D/H value depend on a rigorous appraisal of the relative escape between deuterated and non-deuterated water. The transport of water to the upper atmosphere, from whence it may escape to space, has been assumed to be controlled by water condensation and photolysis, both of which affect the isotope composition of the escaping hydrogen. Their respective role in influencing the relative proportion of escaping D and H atom populations in the upper atmosphere has remained speculative. Here we report HDO and HO profiles observed by the Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) in orbit around Mars that, once combined with expected photolysis rates, reveal that the ultraviolet dissociation of water not only governs the production of atomic hydrogen, prevailing over the ion chemistry mechanism, but also dominates the production of H relative to D atoms, disrupting the old paradigm of atmospheric condensation being the main process differentiating D and H in the upper atmosphere.

Phosphine (PH3) is proposed to be a possible biomarker in planetary atmospheres and has been claimed to have been observed in the atmosphere of Venus, sparking interest in the habitability of Venus’s atmosphere. Observations of another biomarker, methane (CH4), have been reported several times in the atmosphere of Mars, hinting at the possibility of a past or present biosphere. The Atmospheric Chemistry Suite on the ExoMars Trace Gas Orbiter has a spectral range that includes several absorption lines of PH3 with line strengths comparable to previously observed CH4 lines. The signature of PH3 was not observed in the 192 observations made over a full Martian year of observations, and here we report upper limits of 0.1–0.6 ppbv.

Context. Reports on the detection of methane in the Martian atmosphere have motivated numerous studies aiming to confirm or explain its presence on a planet where it might imply a biogenic or more likely a geophysical origin. Aims. Our intent is to complement and improve on the previously reported detection attempts by the Atmospheric Chemistry Suite (ACS) on board the ExoMars Trace Gas Orbiter (TGO). This latter study reported the results of a campaign that was a few months in length, and was significantly hindered by a dusty period that impaired detection performances. Methods. We unveil 640 solar occultation measurements gathering 1.44 Martian years worth of data produced by the ACS. Results. No methane was detected. Probing the clear northern summer season allowed us to reach 1σ upper limits of around 10 pptv (20 pptv at 2σ), with an annual mean of the smallest upper limits of 20 pptv. Upper limits are controlled by the amount of dust in the atmosphere, which impairs detection performance around the equator and during the southern spring and summer seasons. Observations performed near Gale crater yielded 1σ upper limits of up to four times less than the background values measured by the Curiosity rover during the corresponding seasons. Conclusions. Reconciliation of the absence of methane in the TGO spectra with the positive detections by Curiosity is even more difficult in light of this annual survey performed by ACS. Stronger constraints are placed on the physical and chemical mechanism capable of explaining why the mean of the best overall upper limits of ACS is ten times below the smallest methane abundances measured by Curiosity. The ACS investigation was developed by the Space Research Institute (IKI) in Moscow, and the Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS) in Guyancourt. The investigation was funded by Roscosmos, the National Centre for Space Studies of France (CNES) and RSF (Russian Science Foundation 20-42-0903). This work was funded by CNES, the Agence Nationale de la Recherche (ANR, PRCI, CE31 AAPG2019, MCUBE project), the Natural Sciences and Engineering Research Council of Canada (NSERC) (PDF–516895–2018), the UK Space Agency (ST/T002069/1), the UK Science and Technology Facilities Council (ST/R001502/1, ST/P001572/1). All spectral fitting was performed by F.M. The interpretation of the results was done by F.M. and O.K. The preparation of ACS spectra is done at LATMOS by L.B. and at IKI by A.T. Ancillary data are produced in LATMOS by G.L. and in IKI by A.P. Input and aid on spectral fitting were given by K.O. and A.T. The ACS instrument was designed, developed, and operated by A.P., A.S., A.T., F.M., and O.K. Peerreview

An improved high-resolution ground-to-thermosphere version of the Institut Pierre-Simon Laplace (IPSL) Venus General Circulation Model (VGCM), including non-orographic gravity waves (GW) parameterization and fine-tuned non-LTE parameters, is presented here. We focus on the validation of the model built from a collection of data mostly from Venus Express experiments and coordinated ground-based telescope campaigns, in the upper mesosphere/lower thermosphere of Venus. These simulations result in an overall better agreement with temperature observations above 90 km, compared with previous versions of the VGCM. Density of CO2 and light species (CO and O) are also comparable with observations in terms of trend and order of magnitude. Systematic biases in the temperature structure are found at about 80-100 km and above 130 km at the terminator, possibly due to assumptions on the CO2 mixing ratio made for stellar/solar occultation retrievals and uncertainties on the collisional rate coefficients used in the non-LTE parameterization, respectively. Diurnal and latitudinal distribution of dynamical tracers are also analyzed. Overall, our simulations indicate that a weak westward retrograde wind is present up to about 120 km, producing the CO bulge displacement toward 2h-3h in the morning, instead of piling up at the anti-solar point, as for an idealised SS-AS circulation. This retrograde imbalance is suggested to be produced by perturbations of about 5 days Kelvin wave impacting the mesosphere up to 110 km (see the companion paper Navarro et al. 2021), combined with GW westward acceleration mostly above 110 km. On the whole, these model developments point to the importance of the inclusion of the lower atmosphere, higher resolution and finely tuned parameterizations in GCM of the Venusian upper atmosphere, in order to shed light on existing observations., Comment: Icarus (2021), in press, 24 pages, 17 Figures