Our knowledge of the temperature structure in the upper atmosphere of Mars (understood here as upper mesosphere/thermosphere, layers between ~80 and 200 km of altitude from the surface) has significantly improved in the last decade thanks to the data provided by the MAVEN/NASA, Mars Express/ESA, and ExoMars-TGO/ESA missions [e.g. 1, 2, 3]. However, aspects such as the variation of temperatures with local time or with latitude are still poorly characterized due to the sampling limitation of the different instruments, and rely mostly on the information provided by Global Climate Models (GCMs). Recent model-data comparisons [4] show that GCMs have problems in reproducing the observed local time variation of the temperatures in the mesopause, but similar comparisons are missing at other regions in the upper atmosphere.Here we analyze infrared spectra measured by the NOMAD instrument on ExoMars-TGO using the solar occultation technique [5] during 3 Mars years to derive CO2 density profiles, from which we build temperature profiles assuming hydrostatic equilibrium. These temperatures allow us to characterize, for example, the seasonal and latitudinal variability of the thermosphere and to study topics such as the effects of dust events on the thermospheric energy balance.We compare the NOMAD temperatures with predictions by the Mars Planetary Climate Model (M-PCM), a state-of-the-art ground-to-exobase GCM for Mars [6, 7]. This comparison helps to alleviate the limited coverage of the NOMAD dataset by complementing the observations with predictions at other locations and times, and is useful to interpret the observed temperature variability and to validate the model.We also compare our derived temperatures with publicly available mesospheric/thermospheric temperatures derived from other instruments and other missions, allowing a more complete local time coverage, and extending the study to a wider altitude range.Our preliminary results show that M-PCM predicts well the temperatures in the thermosphere except in the evening terminator, when observed temperatures are about 20-30 K larger than in the model. Given that the evening terminator is strongly affected by dynamical processes, this result points to deficiencies in the circulation or tidal structure predicted by the model. References[1] Jain, S.K., E. Soto, J.S. Evans, et al., Thermal structure of Mars’ middle and upper atmospheres: Understanding the impacts of dynamics and solar forcing. Icarus, 393, doi:10.1016/j.icarus.2021.114703 (2023)[2] Forget, F., F. Montmessin, J.-L. Bertaux, et al., Density and temperatures of the upper Martian atmosphere measured by stellar occultations with Mars Express SPICAM. JGR, 114, doi:10.1029/2008JE003086 (2009)[3] López-Valverde, M.A., B. Funke, A. Brines, et al., Martian atmospheric temperature and density profiles during the first year of NOMAD/TGO solar occultation measurements. JGR Planets, 128, doi:10.1029/2022JE007278 (2023)[4] Gupta, S., R.V. Yelle, N.M. Schneider, et al., Thermal structure of the Martian upper mesosphere/lower thermosphere from MAVEN/IUVS stellar occultations. JGR Planets, 127, doi:10.1029/2022JE007534 (2022)[5] Vandaele, A.-C., J.J. López-Moreno, M.R. Patel, et al., NOMAD, an integrated suite of three spectrometers for the ExoMars Trace Gas Mission: Technical description, science objectives and expected performance. Space Science Reviews, 214, doi:10.1007/s11214-018-0517-2 (2019)[6] Forget, F., F. Hourdin, R. Fournier, et al., Improved general circulation models of the Martian atmosphere from the surface to above 80 km. JGR, 104, doi:10.1029/1999JE001025 (1999)[7] González-Galindo, F., M.A. López-Valverde, F. Forget, et al., Variability of the Martian thermosphere during eight Martian years as simulated by a ground-to-exosphere global circulation model. JGR Planets, 120, doi:10.1002/2015JE004925 (2015) Acknowledgements:The IAA/CSIC team acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S and by grants PID2022-137579NB-I00, RTI2018-100920-J-I00 and PID2022-141216NB-I00 all funded by MCIN/AEI/10.13039/501100011033. A. Brines acknowledges financial support from the grant PRE2019-088355 funded by MCIN/AEI/10.13039/501100011033 and by ’ESF Investing in your future’. 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 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, ST/Y000234/1 and ST/X006549/1 and Italian Space Agency through grant 2018-2-HH.0. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 101004052.

The ExoMars Trace Gas Orbiter (TGO) mission is a joint venture of the space agencies ESA and ROSCOSMOS which was launched in 2016 and carries onboard instruments dedicated to studying the trace gas compositions of the Martian atmosphere. NOMAD (Nadir and Occultation for MArs Discovery) is one such instrument that housed three observing channels named UVIS (the Ultra Violet and Visible Spectrometer), LNO (Limb Nadir Occultation) and SO (Solar Occultation) to scan the Martian atmosphere in nadir and limb geometries [1]. The SO channel of NOMAD operates in the IR (Infra-Red) region of the solar spectrum in the wavelength range 2.3 – 4.3µm. The SO spectrometer contains an echelle grating which can produce diffraction patterns of multiple orders but only one order is allowed to fall onto the detector selected by an AOTF (Acousto Optical Tunable Filter) filter. Spectral region of diffraction orders from 186 – 191 contains well-separated and strong absorption lines of CO. The NOMAD-SO channel is using diverse diffraction orders to monitor the CO due to its importance in understanding the dynamics and chemistry of the Martian atmosphere. CO is produced in the upper Martian atmosphere by the photolysis of CO2 and destroyed by the hydroxyl (OH) radicals in the lower atmosphere. Hydroxyl radicals thus recycle CO into CO2. The study of the CO vertical distribution is important to understand the photo-chemical stability of the atmosphere. CO not only links the chemistry of the carbon and odd hydrogen families but is a long-lived species which also serves as a dynamical tracer. At IAA-CSIC we have developed a preprocessing scheme to clean the NOMAD calibrated data from a number of systematics and prepare them for inversion of different atmospheric species [2,3,4,5]. Those systematics are spectral shift of the absorption lines and spectral bending which occurs due to thermally induced mechanical stress on the detector [6]. The work presented here is in continuation with our previous work on the retrievals of CO [3] wherein the retrieval scheme has been described in detail. Our previous study reveals two crucial factors that need to be considered for a correct CO retrieval, one is the saturation of spectral lines in diffraction orders 186 and 190, those used in our work to derive CO. The second one is the use of observed temperature and pressure in the retrieval rather than the climatological T/P from GCMs (general circulation model). For order 190, the absorption lines become saturated below 70 km while for orders 186, the lines remain unsaturated for most of the atmospheric region below this altitude. In the altitudes above 70 km, the absorptions in 186 are dominated by random noise but the lines in 190, due to their strength remain clear. Due to this fact, an adequate combination of these two diffraction orders is recommended for performing CO inversions from TGO solar occultation data.In this work, we will present the improved CO vertical densities using this strategy and the impact on the CO distribution.References[1] Vandaele, A. C., Lopez-Moreno, J. J., Patel, M. R., Bellucci, G., Daerden, F., Ristic, B., ... & NOMAD Team. (2018). NOMAD, an integrated suite of three spectrometers for the ExoMars trace gas mission: Technical description, science objectives and expected performance. Space Science Reviews, 214, 1-47.[2] López‐Valverde, M. A., Funke, B., Brines, A., Stolzenbach, A., Modak, A., Hill, B., ... & NOMAD team. (2023). Martian atmospheric temperature and density profiles during the first year of NOMAD/TGO solar occultation measurements. Journal of Geophysical Research: Planets, 128(2), e2022JE007278.[3] Modak, A., López‐Valverde, M. A., Brines, A., Stolzenbach, A., Funke, B., González‐Galindo, F., ... & Vandaele, A. C. (2023). Retrieval of Martian atmospheric CO vertical profiles from NOMAD observations during the first year of TGO operations. Journal of Geophysical Research: Planets, 128(3), e2022JE007282.[4] Stolzenbach, A., López Valverde, M. A., Brines, A., Modak, A., Funke, B., González‐Galindo, F., ... & Vandaele, A. C. (2023). Martian atmospheric aerosols composition and distribution retrievals during the first Martian year of NOMAD/TGO solar occultation measurements: 1. Methodology and application to the MY 34 global dust storm. Journal of Geophysical Research: Planets, 128(11), e2022JE007276.[5] Brines, A., López‐Valverde, M. A., Stolzenbach, A., Modak, A., Funke, B., Galindo, F. G., ... & Vandaele, A. C. (2023). Water vapor vertical distribution on Mars during perihelion season of MY 34 and MY 35 with ExoMars‐TGO/NOMAD observations. Journal of Geophysical Research: Planets, 128(11), e2022JE007273.[6] Liuzzi, G., Villanueva, G. L., Mumma, M. J., Smith, M. D., Daerden, F., Ristic, B., ... & Bellucci, G. (2019). Methane on Mars: New insights into the sensitivity of CH4 with the NOMAD/ExoMars spectrometer through its first in-flight calibration. Icarus, 321, 671-690.Acknowledgements:The IAA/CSIC team acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S and by grants PID2022-137579NB-I00, RTI2018-100920-J-I00 and PID2022-141216NB-I00 all funded by MCIN/AEI/ 10.13039/501100011033. A. Brines acknowledges financial support from the grant PRE2019-088355 funded by MCIN/AEI/10.13039/501100011033 and by ’ESF Investing in your future’. 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).

NOMAD [1] (Nadir and Occultation for MArs Discovery) is a multi-channel spectrometer onboard the ExoMars 2016 Trace Gas Orbiter (TGO), which began its observations in April 2018. The Solar Occultation (SO) channel has a spectral range coverage from 2.3 to 4.3 μm (2320 to 4350 cm-1). An Acousto-Optical Tunable Filter (AOTF) is used to select different spectral windows (with a width that varies from 20 to 35 cm-1) corresponding to the desired diffraction orders to be used during the atmospheric scan. The SO channel has a sample rate of ∼1 s, that allows a vertical sampling of ∼1 km. In order to obtain as much information as possible from the Martian atmosphere, for this study we have analyzed data from diffraction orders 134 (3011-3035 cm-1) and 168 (3775-3805 cm-1), taken simultaneously during different solar occultation scans within the first year of measurements. This combination of diffraction orders allowed us to explore the distribution of the water vapor at atmospheric tangent altitudes from ∼10 km up to about ∼100 km and in different atmospheric conditions, but also to study its spatial and seasonal variability.Here we present the water vapor vertical profiles of a subset of the solar occultations observed during the first year of TGO/ExoMars, including data taken during the 2018 Global Dust Storm (GDS) and during the 2019 local dust storm. Similar studies have been done by [2] and [3], showing that dust storms allow water vapor to reach higher altitudes in the atmosphere. Also, thanks to this phenomenon, escape of atomic hydrogen has a relevant role in the planetary evolution, as [4] shows. The data presented here have been analyzed with pre-processing and cleaning tools developed entirely at the IAA, and then, have been inverted using a state-of-the-art retrieval scheme [5]. Our method allows us to use consistent temperature profiles during the inversion i.e., obtained from inversion of other diffraction orders’ measurements of the same scan (see companion contributions to this conference [6], [7], [8]). This impose the limitation to use a dataset where CO2 orders and H2O orders have been observed simultaneously. Also, synthetic spectra have been generated mimicking the NOMAD SO behavior as a first step of a comprehensive error analysis. Preliminary results will also be presented, showing water vapor vertical profiles and estimations of hydrogen escape. Comparisons with results of other groups in the NOMAD team will be shown [9], [10].AcknowledgmentsThe 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 Astrofísica de Andalucía (SEV- 2017-0709) and funding by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). 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). US investigators were supported by the National Aeronautics and Space Administration.References[1] Vandaele, A. C. et al. NOMAD, an integrated suite of three spectrometers for the ExoMars Trace Gas mission: technical description, science objectives and expected performance. Space Science Reviews 214, 1-47 (2018).[2] Aoki, S. et al. Water vapor vertical profiles on Mars in dust storms observed by TGO/NOMAD. Journal of Geophysical Research: Planets (2019).[3] Fedorova, A. A. et al. Stormy water on Mars: The distribution and saturation of atmospheric water during the dusty season. Science 367, 297{300 (2020).[4] Chaffin, M., Deighan, J., Schneider, N. & Stewart, A. Elevated atmospheric escape of atomic hydrogen from Mars induced by high-altitude water. Nature geoscience 10, 174-178 (2017).[5] Jurado Navarro, A. A. et al. Retrieval of CO2 and collisional parameters from the MIPAS spectra in the earth atmosphere (2016).[6] López-Valverde, M. A. et al. CO2 and Temperature vertical profiles in the Martian atmosphere from solar occultation measurements at 2.7 μm by instruments NOMAD and ACS on board the Exomars Trace Gas Orbiter. EPSC (2021).[7] Modak, A. et al. Retrieval of Martian CO vertical profiles from NOMAD solar occultation measurements. EPSC (2021).[8] Stolzenbach, A. et al. Vertical profiles of Martian aerosols nature and distribution parameters retrievals from NOMAD-SO. EPSC (2021).[9] Aoki, S. et al. Water vapor vertical distributions on Mars: Results from three years of TGO/NOMAD science operations (EPSC2021-153). EPSC (2021).[10] Villanueva, G. L. et al. Water heavily fractionated as it ascends on Mars as revealed by ExoMars/NOMAD. Science Advances 7, eabc8843 (2021).

IntroductionVertical profiles of CO2 and temperature with good vertical resolution are key measurements to characterize the Martian atmosphere, although difficult to obtain from remote observations [1]. For the first time these vertical profiles can be routinely obtained with a solar occultation technique by the instruments NOMAD and ACS on board the Exomars Trace Gas Orbiter [2,3]. A state-of-the-art retrieval scheme designed to derive atmospheric profiles of CO2 and temperature from the bottom to the top of the Martian atmosphere [4] is adapted to solar occulation and applied to exploit the operational sounding of these two instruments. The final goal of this on-going work is to characterize the Martian thermal structure from the troposphere up to the thermosphere with unprecedented vertical resolution and also to cross-validate both TGO instruments as best as possible, with a single retrieval code and entirely consistent data analysis approaches.Retrieval approachThis work is focussed on the solar occultation channels NOMAD-SO and ACS-MIR, in routine operations since April 2018. To exploit these unique datasets, it is of paramount importance to examine the performance of the two instruments and to cross-validate their retrieval results as accurately as possible. For this purpose we apply a flexible and well tested Earth atmosphere retrieval scheme [5,6,7], to both of them, after adaptation to Mars atmospheric conditions [4] and the necessary accomodation of these channels characteristics [8]. The retrievals use calibrated transmittance spectra to tackle three targets, CO2 density, temperature, and dust loading, in a simultaneous global-fit inversion, with updated hydrostatic equilibrium in every iteration, including contaminant species like H2O, and after a pre-processing/data cleaning analysis which is also similar in both instruments. A first error analysis is performed for both instruments with the help of synthetic retrievals and a series of sensitivity tests performed with the same inversion scheme and similar treatment of the key error terms (measurement noise and systematics). Comparison of resultsWe will present data obtained in the 2.7 µm region, dominated by a well known ro-vibrational band of CO2, and sampled by NOMAD-SO in a mixture of diffraction orders that are used routinely in the operational sounding in the vertical. Similarly, we used 3 consecutive orders in one of the ACS-MIR difraction positions, which contain a sufficient number of CO2 lines in the same 2.7 µm band with the capability to sample the whole atmosphere, up to about 180 km, in a single vertical scan. For both instruments the sounding of the lowest troposphere is limited by the amount of atmospheric dust, which is also retrieved simultaneously with CO2 and temperature. We will compare the vertical profiles obtained in a small sample of profiles from each instrument which span different seasons, latitudes and atmopheric dust loadings, during the first year of TGO operations. The comparisons take into account that the two instruments' individual solar occulation scans are non-coincident in time and space. Comparions are also peformed with results from similar efforts by other groups in the NOMAD and ACS teams [9]. Two important applications of the obtained retrievals are : (i) to supply the most appropriate inputs to the retrieval of other atmospheric species from the same instruments and the same scans, without the need to assume a prior or first guesses from global circulation models (GCM), see companion contributions to this conference [10,11], and (ii) to validate predictions from these GCMs, and therefore, to help to improve them, particularly at high altitudes and at the terminator, where these datasets are particularly valuable [1].AcknowledgementsThe 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 Astrofísica de Andalucía (SEV-2017-0709) and funding by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). 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). US investigators were supported by the National Aeronautics and Space Administration. Thanks are extensive to all members of the NOMAD Science Team and the ACS Science Team. References[1] Lopez-Valverde et al., Space Sci Rev, 214, 29 (2018)[2] Vandaele et al., Space Science Reviews 214, 5, 2018[3] Korablev et al., Space. Sci. Rev. 214, 7 (2018).[5] Funke, B., et al. , Atmos. Chem. Phys., 9(7), 2387–2411 (2009).[4] Jimenez-Monferrer et al., Icarus, 353, 113830 (2020), doi.org/10.1016/j.icarus.2020.113830.[6] Stiller et al., JQSRT, 72, 249–280 (2002)[7] von Clarmann et al., J. Geophys. Res. 108, 4746 (2003)[8] Lopez-Valverde et al., EPSC Abstracts, Vol. 14, EPSC2020-924 (2020), doi.org/10.5194/epsc2020-924[9] Trompet et al., "Update on CO2 and temperature profiles retrievals from NOMAD-SO on board ExoMars TGO", contribution to this conference, EPSC 2021.[10] Brines et al., "Martian water vapor vertical profiles with data from solar occultation measurements by NOMAD onboard TGO/ExoMars", contribution to this conference, EPSC 2021.[11] Modak et al., "Retrieval of Martian CO vertical profiles from NOMAD solar occultation measurements", contribution to this conference, EPSC 2021.

*Corresponding author: ashim@iaa.es Abstract NOMAD (Nadir and Occultation for Mars Discovery instrument), is a spectrometer suite onboard Exo Mars Trace Gas Orbiter having within its main scientific objectives the observations of the trace gases in the Martian atmosphere [1]. Here we focus on the retrieval of carbon monoxide (CO) vertical profiles with high vertical resolution. CO is an important trace species which acts as both photochemical and dynamical tracers. We retrieve CO from the solar occultation (SO) observation of the NOMAD orders (186 – 191) using a state-of-the-art retrieval method [2]. The observational dataset covers a wide range of latitudes and seasons. This permits us to study the impact of different conditions such as dust-storm seasons (local and global), southern summer and winter on the CO vertical profiles over different regions. Introduction CO is originated in the upper Martian atmosphere by the photolysis of CO2 and destroyed by the hydroxyl (OH) radicals in the lower atmosphere. Hydroxyl radicals thus recycle CO into CO2 [3]. The study of the CO vertical distribution is important to understand the photo-chemical stability of the atmosphere. CO not only links the chemistry of the carbon and odd hydrogen chemical families but is a long-lived species which also serves as a dynamical tracer. By far the current knowledge of CO vertical profiles is largely unconstrained due to lack of systematic measurements. Though the column density of CO has been measured by instruments like CRISM [4] (Compact Reconnaissance Imaging Spectrometer for Mars) for a wide range of latitudes and seasons for multiple Martian years, the lack of its regular mapping in the vertical, limits a full understanding of its distribution and variability. Very recently, CO density profiles were reported from ACS (Atmospheric Chemistry Suite) observations [5] which found a significant depletion in CO mixing ratio during the 2018 global dust storm. NOMAD is performing routine solar occultation measurements since April 2018. Our aim here is to retrieve CO vertical profiles from these measurements with the best achievable precision and resolution and to investigate its distribution and variability through the different seasons and latitudes. Retrieval of CO from NOMAD SO orders 186 – 190 We present vertical profiles of CO retrieved from a subset of NOMAD solar occultations. The SO channel of NOMAD operates in the 2.3 – 4.3 μm [3] where strong absorption lines for CO lie. In particular, the diffraction orders 186 (4180.32 cm-1 - 4213.88 cm-1) - 191 (4292.69 cm-1 - 4327.16 cm-1) allow for a good quality CO retrieval from 10 to about 100 km tangent altitudes. However, the recorded spectra suffer from calibration issues [6] such as bending and spectral shifts, in addition to variable systematic and random noise components. At IAA we have developed a cleaning procedure which correct the spectra for possible bending and spectral shift and makes it usable for a precise inversion of CO densities. We use the line-by-line radiative transfer model KOPRA (Karlsruhe Optimized and Precise Radiative transfer Algorithm) [2] as forward model, which was adapted to Mars and to the NOMAD instrument characteristics, in conjunction with an interactive solver (RCP) to retrieve CO from the cleaned spectra. Here we present a summary of this on-going work, which builds on a chain of retrievals of atmospheric aerosols, temperatures and density profiles derived from the same NOMAD scan but different diffraction orders, to obtain vertical profiles of CO in a consistent manner. We will also present first comparisons with Mars GCM results. Acknowledgement 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 Astrofísica de Andalucía (SEV-2017-0709). MALV was funded by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). 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). US investigators were supported by the National Aeronautics and Space Administration. References [1] Neefs, Eddy, et al. "NOMAD spectrometer on the ExoMars trace gas orbiter mission: part 1—design, manufacturing and testing of the infrared channels."Applied optics 54.28 (2015): 8494-8520. [2] Jurado Navarro, Ángel Aythami. "Retrieval of CO2 and collisional parameters from the MIPAS spectra in the earth atmosphere." (2016). [3] McElroy, Michael B., and Thomas M. Donahue. "Stability of the Martian atmosphere."Science 177.4053 (1972): 986-988. [4] Smith, Michael D., et al. "The climatology of carbon monoxide and water vapor on Mars as observed by CRISM and modeled by the GEM-Mars general circulation model."Icarus 301 (2018): 117-131. [5] Olsen, K. S., et al. "The vertical structure of CO in the Martian atmosphere from the ExoMars Trace Gas Orbiter."Nature Geoscience14.2 (2021): 67-71. [6] Liuzzi, Giuliano, et al. "Methane on Mars: new insights into the sensitivity of CH4 with the NOMAD/ExoMars spectrometer through its first in-flight calibration." Icarus 321 (2019): 671-690.

. The NOMAD-SO database and retrieval procedure The NOMAD-SO channel [1] is an infrared spectrometer onboard ExoMars TGO working in the 2.2 to 4.3 µm spectral range (2200-4500 cm-1). During a solar occultation measurement, NOMAD-SO scans six diffraction orders each second. These diffraction orders are recorded on four bins leading to a vertical sampling lower than one km. We use vertical profiles of calibrated transmittance and evaluated noise from level 1 data provided by the PI team. These level 1 scans are pre-processed by an in-house algorithm set up to clean the data of possible spectral shifts and/or bending. In order to retrieve the extinction vertical profile due to aerosol we use the model a state-of-the-art line by line radiative transfer forward model code called KOPRA, in conjunction with the iterative non-linear inversion scheme RCP, conceived and developed at the Institute of Meteorology and Climate Research (IMK) of Karlsruhe Institude for Technology and previously used in [2]. A priori and first guess profiles of the atmospheric thermal structure and composition, required by RCP, are taken from specific runs of the LMD Mars-GCM available through the MCD [3]. To minimize vertical error propagation we adopted a global fit inversion instead of a classical onion-peeling method. A first order Tikhonov regularization matrix is used for the aerosol extinction retrieval. For stability, RCP uses Levenberg-Marquardt damping method. Pre-processing and regularization parameters required fine-tuning, a task performed using a sample of orbits. We confirmed that these parameters are valid for all other scans.2. Aerosols characterization and vertical profilesAerosols in Martian atmosphere play a major role in several chemical and radative processes, affecting local to global dynamics and energy budget (see [4] for an exhaustive review of this subject). One crucial parameter is the aerosol composition and distribution, i.e the dust, water ice or CO2 ice content and its variation with with altitude, with a better vertical resolution than previous instruments. TGO offers an excellent opportunity to study it in detail for the first time because of its high vertical resolution. Using the vertical profile of aerosol extinction obtained from NOMAD-SO measurements as described above, we then apply a model/data fitting strategy of the aerosol extinction, as described in [5] in order to evaluate key parameters of the aerosol’s content in the martian atmosphere. We compare the retrieved aerosol extinction to a precomputed look-up table in the same spectral range from a Lorenz-Mie code to compute the scattering properties of an ensemble of polydisperse spherical particles [6] and using refractive indexes for martian dust, water ice and CO2 ice. This procedure allow us to evaluate two other key parameters besides the composition of aerosol present in the martian atmosphere, namely, the effective radius (reff) and the effective variance (νeff). These characteristic parameters of the aerosol’s distribution are of great interest constraining microphysics and dynamics. We will detail this procedure and compare the early results with available data for several chosen scans of NOMAD-SO observation during the first year of TGO operations, which include the GDS 34 [7].AcknowledgementsThe 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 Astrofísica de Andalucía (SEV-2017-0709) and funding by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). 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). US investigators were supported by the National Aeronautics and Space Administration.References[1] Vandaele, A.C., et al.: “Science objectives and performances of NOMAD, a spectrometer suite for the ExoMars TGO mission, Planet”. Space Sci., Vol. 119, pp 233-249, 2015.[2] A. A. Jurado-Navarro. “Retrieval of CO2 and collisional parameters from the MIPAS spectra in the Earth atmosphere”. PhD thesis, Universidad de Granada, 2015.[3] E. Millour, F. Forget, A. Spiga, M. Vals, V. Zakharov, L. Montabone, F. Lefèvre,F. Montmessin, J.-Y. Chaufray3 M. A. López-Valverde, F. González-Galindo, S. R.Lewis, P. L. Read, M.-C. Desjean, F. Cipriani and the MCD development team. “Mars climate database.” From Mars Express to ExoMars, 2018.[4] Kahre, M. A., Murphy, J. R., Newman, C. E., Wilson, R. J., Cantor, B. A., Lemmon, M. T., & Wolff, M. J. (2017). “The Mars dust cycle.” In R.M. Haberle (Ed.), The atmosphere and climate of Mars (pp. 229– 294). Cambridge: Cambridge University Press[5] Luginin, M., A. Fedorova, D. Belyaev, F. Montmessin, V. Wilquet, O. Korablev, J. -L. Bertaux, and A. C. Vandaele. “Aerosol Properties in the Upper Haze of Venus from SPICAV IR Data.” Icarus, 277. 2016.[6] Mishchenko, Michael I., Larry D. Travis, and Andrew A. Lacis. Scattering, absorption, and emission of light by small particles. Cambridge university press, 2002.[7] Liuzzi, G., Villanueva, G. L., Crismani, M. M. J., Smith, M. D., Mumma, M. J., Daerden, F., et al. . Strong variability of Martian water ice clouds during dust storms revealed from ExoMars Trace Gas Orbiter/NOMAD. Journal of Geophysical Research: Planets, 125, 2020.

NOMAD [1] (Nadir and Occultation for MArs Discovery) is a multi-channel spectrometer onboard the ExoMars 2016 Trace Gas Orbiter (TGO), which began its observations in April 2018. Among other two (LNO and UVIS), the Solar Occultation (SO) channel covers the infrared (IR) spectrum from 2.3 to 4.3 µm (2320 to 4350 cm-1). Composed of an echelle grating in Litrow configuration, a total of 6 diffraction orders (with a typical width from 20 to 35 cm-1) are selected during each solar occultation using an Acousto-Optical Tunable Filter (AOTF) with a sample rate of about ~1 s, allowing a vertical resolution of typically 1 km. In order to optimize the information content that can be retrieved from the data, we analyzed Level 1 calibrated transmittances [2, 3] from four diffraction orders: 134 (3011-3035 cm−1), 136 (3056-3081 cm−1), 168 (3775-3805 cm−1) and 169 (3798-3828 cm−1). At the IAA-CSIC we developed processing tools specifically designed to handle and remove some systematics present in the NOMAD data, such as spectral shift and bending on the baseline of the spectra [4]. In addition, we performed an in-house characterization of the measurement noise using covariance matrices, identifying the true random component of the noise. This improvement allowed us to obtain homogeneous vertical profiles from the surface to an altitude about ~120 km. The profiles shown here have been retrieved combining two diffraction orders, this is, performing a global fit using spectra from two orders simultaneously. We combined pairs of orders 134 or 136 with 168 or 169 using them at different altitude ranges. The first two contain relatively weak absorption lines (S~10−21 cm−1/(molecule·cm−2) allowing the sample of the low atmosphere. On the contrary, the second set of orders contain strong lines close to center of the ν3 band(S~10−19 cm−1/(molecule·cm−2) which are useful to sample the upper atmosphere. This methodology is possible only when both orders have been measured during the same solar occultation, and although it limits the number of occultations available, it is necessary in order to avoid optically thick lines. Typically, we used low altitude orders (134/136) below 60 km and high altitude orders (168/169) above 60 km.The content presented here is a follow-up work building upon several previous studies [5–8]. We extended the dataset selecting a total of 6561 occultations taken during Martian Years (MY) 34,35 and 36. We discuss detailed seasonal and latitudinal maps, showing the vertical distribution of the water vapor abundance and its variability thought the year. A summary of this work can be seen in Fig. 1, where we show the seasonal variation of all the retrieved water vapor profiles.Figure 1: Vertical distribution of water vapor during MYs 34, 35 and 36 for the Northern (middle panel) and Southern (bottom panel) hemispheres. Horizontal axis shows the Solar longitude. Top panel indicates the latitude and local time of the observations.Figure 1 clearly shows a repeated pattern in both hemispheres. Water vapor is present in a more vertically extended range during the Southern summer (perihelion season) whereas it is mostly confined to low altitudes (below 20 km) during the aphelion season, which corresponds to the Northern summer. Also, the characteristic Global Dust Storm (GDS) that took place in 2018 can be seen at the beginning of the MY 34 (LS~190º), showing a distinctive peak in water abundance in the northern hemisphere that is not repeated again.In addition, we analyzed in detail the latitudinal distribution of water vapor during the perihelion season. We noticed a strong vertical plume at 60ºS - 50ºS injecting H2O into the mesosphere, reaching abundances of about ~50 ppmv at 100 km. We observed this event repeatedly in the three Martian years analyzed, although with inter-annual variations in both its magnitude and timing. The plume showed a weaker structure with less abundance during MY 34. We suggest that this difference respect to MYs 35 and 36 could possibly due to indirect effects of the MY 34 GDS. A summary of this analysis is presented in Fig. 2 where a storng water vapor injection can be seen in MYs 35 and 36 (panels B and C respectively).Figure 2: Water vapor latitudinal variation during LS = 260º-280º for MYs 34, 35 and 36 (panels A, B and C respectively). Dots in top panels indicate latitude, Solar Longitude and Local Solar Time of the observations.AcknowledgmentsThe IAA/CSIC team acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S and by grants PID2022-137579NB-I00,RTI2018-100920-J-I00 and PID2022-141216NB-I00 all funded by MCIN/AEI/ 10.13039/501100011033. A. Brines acknowledges financial support from PRE2019-088355 funded by MCIN/AEI/10.13039/501100011033. 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 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, ST/Y000234/1 and ST/X006549/1 and Italian Space Agency through grant 2018-2-HH.0. This project has received funding from the European Union’s Horizon 2020 grant No 101004052. US investigators were supported by the NASA. Canadian investigators were supported by the Canadian Space Agency. We want to thank M. Vals, F. Montmessin, F. Lefevre, F. Forget and the broad LMD/IPSL team supporting the Mars PCM.References1. Vandaele, A. C. (2018). Space Science Reviews, 214.2. Thomas, I. R. (2022). Planetary and Space Science, 218.3. Trompet, L. (2023). Journal of Geophysical Research: Planets, 128(3).4. López-Valverde, M. A. (2023). Journal of Geophysical Research: Planets, 128(2).5. Brines, A. (2023). Journal of Geophysical Research: Planets, 128(11).6. Aoki, S. (2019). Journal of Geophysical Research: Planets, 124(12).7. Aoki, S. (2022). Journal of Geophysical Research: Planets, 127(9).8. Villanueva, G. L. (2022). Geophysical Research Letters, 49(12).

IntroductionVertical profiles of CO2 and temperature with good vertical resolution are key measurements to characterize the Martian atmosphere, although difficult to obtain from remote observations [1]. For the first time these vertical profiles can be routinely obtained with a solar occultation technique by the instruments NOMAD and ACS on board the Exomars Trace Gas Orbiter [2,3]. A state-of-the-art retrieval scheme designed to derive atmospheric profiles of CO2 and temperature from the bottom to the top of the Martian atmosphere [4] is adapted to solar occulation and applied to exploit the operational sounding of these two instruments. The final goal of this on-going work is to characterize the Martian thermal structure from the troposphere up to the thermosphere with unprecedented vertical resolution and also to cross-validate both TGO instruments as best as possible, with a single retrieval code and entirely consistent data analysis approaches.Retrieval approachThis work is focussed on the solar occultation channels NOMAD-SO and ACS-MIR, in routine operations since April 2018. To exploit these unique datasets, it is of paramount importance to examine the performance of the two instruments and to cross-validate their retrieval results as accurately as possible. For this purpose we apply a flexible and well tested Earth atmosphere retrieval scheme [5,6,7], to both of them, after adaptation to Mars atmospheric conditions [4] and the necessary accomodation of these channels characteristics [8]. The retrievals use calibrated transmittance spectra to tackle three targets, CO2 density, temperature, and dust loading, in a simultaneous global-fit inversion, with updated hydrostatic equilibrium in every iteration, including contaminant species like H2O, and after a pre-processing/data cleaning analysis which is also similar in both instruments. A first error analysis is performed for both instruments with the help of synthetic retrievals and a series of sensitivity tests performed with the same inversion scheme and similar treatment of the key error terms (measurement noise and systematics). Comparison of resultsWe will present data obtained in the 2.7 µm region, dominated by a well known ro-vibrational band of CO2, and sampled by NOMAD-SO in a mixture of diffraction orders that are used routinely in the operational sounding in the vertical. Similarly, we used 3 consecutive orders in one of the ACS-MIR difraction positions, which contain a sufficient number of CO2 lines in the same 2.7 µm band with the capability to sample the whole atmosphere, up to about 180 km, in a single vertical scan. For both instruments the sounding of the lowest troposphere is limited by the amount of atmospheric dust, which is also retrieved simultaneously with CO2 and temperature. We will compare the vertical profiles obtained in a small sample of profiles from each instrument which span different seasons, latitudes and atmopheric dust loadings, during the first year of TGO operations. The comparisons take into account that the two instruments' individual solar occulation scans are non-coincident in time and space. Comparions are also peformed with results from similar efforts by other groups in the NOMAD and ACS teams [9]. Two important applications of the obtained retrievals are : (i) to supply the most appropriate inputs to the retrieval of other atmospheric species from the same instruments and the same scans, without the need to assume a prior or first guesses from global circulation models (GCM), see companion contributions to this conference [10,11], and (ii) to validate predictions from these GCMs, and therefore, to help to improve them, particularly at high altitudes and at the terminator, where these datasets are particularly valuable [1].AcknowledgementsThe 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 Astrofísica de Andalucía (SEV-2017-0709) and funding by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). 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). US investigators were supported by the National Aeronautics and Space Administration. Thanks are extensive to all members of the NOMAD Science Team and the ACS Science Team. References[1] Lopez-Valverde et al., Space Sci Rev, 214, 29 (2018)[2] Vandaele et al., Space Science Reviews 214, 5, 2018[3] Korablev et al., Space. Sci. Rev. 214, 7 (2018).[5] Funke, B., et al. , Atmos. Chem. Phys., 9(7), 2387–2411 (2009).[4] Jimenez-Monferrer et al., Icarus, 353, 113830 (2020), doi.org/10.1016/j.icarus.2020.113830.[6] Stiller et al., JQSRT, 72, 249–280 (2002)[7] von Clarmann et al., J. Geophys. Res. 108, 4746 (2003)[8] Lopez-Valverde et al., EPSC Abstracts, Vol. 14, EPSC2020-924 (2020), doi.org/10.5194/epsc2020-924[9] Trompet et al., "Update on CO2 and temperature profiles retrievals from NOMAD-SO on board ExoMars TGO", contribution to this conference, EPSC 2021.[10] Brines et al., "Martian water vapor vertical profiles with data from solar occultation measurements by NOMAD onboard TGO/ExoMars", contribution to this conference, EPSC 2021.[11] Modak et al., "Retrieval of Martian CO vertical profiles from NOMAD solar occultation measurements", contribution to this conference, EPSC 2021.

NOMAD [1] (Nadir and Occultation for MArs Discovery) is a multi-channel spectrometer onboard the ExoMars 2016 Trace Gas Orbiter (TGO), which began its observations in April 2018. Among other two (LNO and UVIS), the Solar Occultation (SO) channel covers the infrared (IR) spectrum from 2.3 to 4.3 µm (2320 to 4350 cm-1). Composed of an echelle grating in Litrow configuration, a total of 6 diffraction orders (with a typical width from 20 to 35 cm-1) are selected during each solar occultation using an Acousto-Optical Tunable Filter (AOTF) with a sample rate of about ~1 s, allowing a vertical resolution of typically 1 km. The high spectral resolution (λ/∆λ ~17000) and the relatively low signal to noise ratio of this instrument (~2500) make NOMAD SO suitable for the detection of hydrogen chloride HCl. This trace species, although until now considered to be a negligible compound in the Martian atmosphere [2, 3], it has been detected systematically by two instruments onboard TGO: the Atmospheric Chemistry Suite (ACS) [4] and more recently NOMAD [5]. Several works suggest the surface of Mars to be a source of chloride minerals and perchlorate salts [6], which along with interactions surface-atmosphere could allow for chlorine photochemistry happening on the martian atmosphere. On Earth, one of the main sources of HCl is the volcanic activity [7], so the detection of this species on Mars may be an indicator of active geological processes. Multiple ongoing studies are trying to characterize the climatology of HCl on Mars, currently not completely understood, looking for possible relationships between temperature and other atmospheric species such as dust or water vapor.At the IAA we have carried out a study with the objective of identifying not only sources but seasonal variability of HCl by analyzing NOMAD spectra. This early study [8] using a simplified processing pipeline allowed us to detect HCl during the perihelion season of MYs 34 and 35, confirming previous results from [5]. Here, as a follow-up work of that study, we applied a modified version of our IAA-CSIC NOMAD processing pipeline [9-12] in order to increase the sensitivity required for the detection of weak HCl absorption lines, we have analyzed a total of 2536 solar occultations measured during Martian Years 34, 35 and 36. Among those modifications, we improved the methodology used for the characterization of the spectral continuum, now being able to detect systematic oscillations with amplitudes similar to the measurement noise (10-4 in transmittance). We have performed retrievals using NOMAD spectra from diffraction orders 129 (2899 - 2922 cm-1) and 130 (2921 - 2945 cm-1). In order to obtain robust HCl detections, we used the spectra from three detector bins on each ocucltation, retrieving an independent vertical profile form each bin. We present HCl vertical profiles and the seasonal variability of this species from a climatological view, revealing possible links with water vapor and dust.AcknowledgmentsThe IAA/CSIC team acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S and by grants PID2022-137579NB-I00, RTI2018-100920-J-I00 and PID2022-141216NB-I00 all funded by MCIN/AEI/ 10.13039/501100011033. A. Brines acknowledges financial support from the grant PRE2019-088355 funded by MCIN/AEI/10.13039/501100011033 and by ’ESF Investing in your future’. 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 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, ST/Y000234/1 and ST/X006549/1 and Italian Space Agency through grant 2018-2-HH.0. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 101004052. US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency. We want to thank M. Vals, F. Montmessin, F. Lefevre, F. Forget and the broad LMD/IPSL team supporting the continuous development of the Mars PCM. References1. Vandaele, A. C. (2018). Space Science Reviews, 214, 1-47.2. Hartogh, P. (2010). Astronomy & Astrophysics, 521, L49.3. Villanueva, G. L. (2013). Icarus, 223(1), 11-27.4. Korablev, O. (2021) . Science Advances 7, eabe4386.5. Aoki, S. (2021). Geophysical Research Letters 48, e2021GL092506.6. Glavin, D. P. (2013). Journal of Geophysical Research: Planets 118, 1955–1973.7. Graedel, T. (1995) . Global Biogeochemical Cycles 9, 47–77.8. Belmote-Gimenez, A. (2023) Mater Thesis, University of Granada.

This is the second part of Stolzenbach et al. (2023, https://doi.org/10.1029/2022JE007276 ), named hereafter Paper I, extends the period to the end of MY 34 and the first half of MY 35. This encompasses the end phase of the MY 34 Global Dust Storm (GDS), the MY 34 C‐Storm, the Aphelion Cloud Belt (ACB) season of MY 35, and an unusual early dust event of MY 35 from L S 30° to L S 55°. The end of MY 34 overall aerosol size distribution shows the same parameters for dust and water ice to what was seen during the MY 34 GDS. Interestingly, the layered water ice vertical structure of MY 34 GDS disappears. The MY 34 C‐Storm maintains condition like the MY 34 GDS. A high latitude layer of bigger water ice particles, close to 1 μm, is seen from 50 to 60 km. This layered structure is linked to an enhanced meridional transport characteristic of high intensity dust event which put the MY 34 C‐Storm as particularly intense compared to non‐GDS years C‐Storms as previously suggested by Holmes et al. (2021, https://doi.org/10.1016/j.epsl.2021.117109 ). Surprisingly, MY 35 began with an unusually large dust event (Kass et al., 2020, https://ui.adsabs.harvard.edu/abs/2020AGUFMP039…01K ) found in the Northern hemisphere during L S 35° to L S 50°. During this dust event, the altitude of aerosol first detection is roughly equal to 20 km. This is close to the values encountered during the MY 34 GDS, its decay phase and the C‐Storm of the same year. Nonetheless, no vertical layered structure was observed.

We present CO density profiles up to about 100 km in the Martian atmosphere obtained for the first time from retrievals of solar occultation measurements by the Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). CO is an important trace gas on Mars, as it is controlled by CO 2 photolysis, chemical reaction with the OH radicals, and the global dynamics. However, the measurements of CO vertical profiles have been elusive until the arrival of TGO. We show how the NOMAD CO variations describe very well the Mars general circulation. We observe a depletion of CO in the upper troposphere and mesosphere during the peak period, L S = 190°–200°, more pronounced over the northern latitudes, confirming a similar result recently reported by Atmospheric Chemistry Suite onboard TGO. However, in the lower troposphere around 20 km, and at least at high latitudes of the S. hemisphere, NOMAD CO mixing ratios increase over 1,500 ppmv during the GDS (Global Dust Storm) onset. This might be related to the downwelling branch of the Hadley circulation. A subsequent increase in tropospheric CO is observed during the decay phase of the GDS around L S = 210°–250° when the dust loading is still high. This could be associated with a reduction in the amount of OH radicals in the lower atmosphere due to lack of solar insolation. Once the GDS is over, CO steadily decreases globally during the southern summer season. A couple of distinct CO patterns associated with the Summer solstice and equinox circulation are reported and discussed.

Since the beginning of the Trace Gas Orbiter (TGO) science operations in April 2018, its instrument “Nadir and Occultation for MArs Discovery” (NOMAD) supplies detailed observations of the IR spectrums of the Martian atmosphere. We developed a procedure that allows us to evaluate the composition and distribution's parameters of the atmospheric Martian aerosols. We use a retrieval program (RCP) in conjunction with a radiative forward model (KOPRA) to evaluate the vertical profile of aerosol extinction from NOMAD measurements. We then apply a model/data fitting strategy of the aerosol extinction. In this first article, we describe the method used to evaluate the parameters representing the Martian aerosol composition and size distribution. MY 34 GDS showed a peak intensity from L S 190° to 210°. During this period, the aerosol content rises multiple scale height, reaching altitudes up to 100 km. The lowermost altitude of aerosol's detection during NOMAD observation rises up to 30 km. Dust aerosols r eff were observed to be close to 1 μm and its ν eff lower than 0.2. Water ice aerosols r eff were observed to be submicron with a ν eff lower than 0.2. The vertical aerosol structure can be divided in two parts. The lower layers are represented by higher r eff than the upper layers. The change between the lower and upper layers is very steep, taking only few kilometers. The decaying phase of the GDS, L S 210°–260°, shows a decrease in altitude of the aerosol content but no meaningful difference in the observed aerosol's size distribution parameters.

We present vertical profiles of temperature and density from solar occultation (SO) observations by the “Nadir and Occultation for Mars Discovery” (NOMAD) spectrometer on board the Trace Gas Orbiter during its first operational year, which covered the second half of Mars Year 34. We used calibrated transmittance spectra in 380 scans, and apply an in‐house pre‐processing to clean data systematics. Temperature and CO 2 profiles up to about 90 km, with consistent hydrostatic adjustment, are obtained, after adapting an Earth‐tested retrieval scheme to Mars conditions. Both pre‐processing and retrieval are discussed to illustrate their performance and robustness. Our results reveal the large impact of the MY34 Global Dust Storm (GDS), which warmed the atmosphere at all altitudes. The large GDS aerosols opacity limited the sounding of tropospheric layers. The retrieved temperatures agree well with global climate models (GCM) at tropospheric altitudes, but NOMAD mesospheric temperatures are wavier and globally colder by 10 K in the perihelion season, particularly during the GDS and its decay phase. We observe a warm layer around 80 km during the Southern Spring, especially in the Northern Hemisphere morning terminator, associated to large thermal tides, significantly stronger than in the GCM. Cold mesospheric pockets, close to CO 2 condensation temperatures, are more frequently observed than in the GCM. NOMAD CO 2 densities show oscillations upon a seasonal trend that track well the latitudinal variations expected. Results uncertainties and suggestions to improve future data re‐analysis are briefly discussed.

The water vapor in the Martian atmosphere plays a significant role in the planet's climate, being crucial in most of the chemical and radiative transfer processes. Despite its importance, the vertical distribution of H 2 O in the atmosphere has not still been characterized precisely enough. The recent ExoMars Trace Gas Orbiter mission, with its Nadir and Occultation for MArs Discovery instrument, has allowed us to measure the H 2 O vertical distribution with unprecedented resolution. Recent studies of vertical profiles have shown that high dust concentration in the atmosphere, in particular during dust storms, induces an efficient transport of the H 2 O to higher altitudes, from 40 km up to 80 km. We study the H 2 O vertical distribution in a subset of solar occultations during the perihelion of two Martian years (MYs), including the 2018 Global Dust Storm (GDS), in order to compare the same Martian season under GDS and non‐GDS conditions. We present our state‐of‐the‐art retrieval scheme, and we apply it to a combination of two diffraction orders, which permits sounding up to about 100 km. We confirm recent findings of H 2 O increasing at high altitudes during L s = 190°–205° in MY 34, reaching abundances of about 150 ppmv at 80 km in both hemispheres not found during the same period of MY 35. We found a hygropause's steep rising during the GDS from 30 up to 80 km. Furthermore, strong supersaturation events have been identified at mesospheric altitudes even in presence of water ice layers retrieved by the IAA team.

Introduction The nature, size and content of aerosols in the atmosphere affect the energy budget on all planets, hence the atmospheric dynamic of the planet. Mars exhibits three types of atmospheric aerosol. Mineral dust, water ice and carbon dioxide ice. Martian aerosols nature and size distribution were observed using many different methods and experiments, from rovers to satellites. Exhaustive review scan be found in [1] and in [2]. Usually, dust effective radius, reff, ranges from 1 to 2 μm and its effective variance, νeff, from 0.2 to 0.4. H2O ice reff ranges from 1 to 5 μm and its νeff from 0.1 to 0.4. However, these two parameters and their variability are poorly constraint in the vertical to date. ExoMars TGO mission (ESA/Roscosmos) was primarily designed to study trace gases, thermal structure and aerosol content in Mars atmosphere with unprecedented vertical resolution [3]. NOMAD-SO Data processing NOMAD (Nadir and Occultation for MArs Discovery) is suite of two infrared spectrometers onboard the ExoMars 2016 Trace Gas Orbiter (TGO) orbiter, covering the spectral range of 0.2 to 4.3 μm [4]. An Acousto-Optical Tunable Filter (AOTF) is used to select different spectral windows. The sampling of this channel is approximately of 1 second, allowing a vertical sampling about 1km. the SO channel is able to observe the atmosphere at a given altitude with 6 different diffraction orders. For this study, we selected a configuration of 5 diffraction orders (121,134,149,168,190) effectively spanning the overall spectral range of NOMAD. In order to evaluate the local extinction due to aerosols, we use an inversion program called Retrieval Control Program (RCP). It is a multi-parameter non-linear least squares fitting of measured and modelled spectra [5]. Its forward model, KOPRA, was recently adapted to limb emissions on Mars [6] and for solar occultation data on Mars for the first time. RCP solves iteratively the inverse problem [7] and is described in details in [8]. The regularization matrix is build from Tikhonov-type terms of different orders which can be combined to obtain a custom-tailored regularization for any particular retrieval problem. An example of the retrieved extinction profile is shown in Fig 1. The retrieved extinctions differs from previous work on aerosols using ACS data [9,10] using the Onion-peeling or Abel's transform method since this global fit is less affected by the large error propagation to low altitudes typical of those methods, and the lower Martian atmosphere is precisely where aerosols are particular relevant. Fig 1. Mean extinction cross-section ratio modelling In order to model the optical behavior of the Martian aerosol we chose the log-normal distribution which is widely used in atmospheric sciences. It is a function of two parameters (rg, σg). In optics, we change those parameters to more suitable ones, the effective radius, reff and its corresponding effective variance νeff. For any aerosol size distribution, the extinction k is km-1 is k(λ) = N . σext (λreff,νeff). N is the aerosol number density and σext (λ,reff,νeff) is the mean average extinction cross-section at a wavelength λ, a specific aerosol distribution defined by (reff,νeff). We build a look-up table of dust and water ice σext at the selected NOMAD order's wavelengths for different sets of (reff,νeff). The extinction are evaluated with a Lorenz-Mie code for polydisperse spherical particle from [11]. Aerosol composition and size distribution evaluation We will detail the process of evaluating the aerosol composition and size distribution that consists of a mix of non-linear least square and brute force in order to evaluate the best set of parameters (reff,νeff ,γ) where γ represent a mixture of dust and H2O ice. The NLSQ algorithm is provided by the SciPy Python package [12]. To assess the robustness and limitations of our evaluation procedure, we will present results against synthetic extinction signal. We will discuss our main results, especially for the period covering the Global Dust Storm of MY34 (Fig 2.). Fig 2. Acknowledgments The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the \emph{"Center of Excellence Severo Ochoa"} award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and funding by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). 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). References [1] Robert M. Haberle et al., eds. The Atmosphere and Climate of Mars. Cambridge University Press, 2017. [2] R. Todd Clancy et al. “The distribution, composition, and particle properties of Mars meso-spheric aerosols: An analysis of CRISM visible/near-IR limb spectra with context from near-coincident MCS and MARCI observations”. Icarus 328 (2019). [3] J. Vago et al. “ESA ExoMars program: The next step in exploring Mars”. SSR 49.7 (2015). [4] A. C. Vandaele et al. “NOMAD, an Integrated Suite of Three Spectrometers for the ExoMarsTrace Gas Mission: Technical Description, Science Objectives and Expected Performance”. SSR 214.5 (2018). [5] T. von Clarmann et al. “Retrieval of temperature and tangent altitude pointing from limb emission spectra recorded from space by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)”. JGR: Atmospheres 108.D23 (2003). [6] Sergio Jiménez-Monferrer et al. “CO2 retrievals in the Mars daylight thermosphere from its 4.3μm limb emission measured by OMEGA/MEx”. Icarus 353 (2021). [7] Clive D Rodgers. Inverse Methods for Atmospheric Sounding. WORLD SCIENTIFIC, 2000. [8] Jurado Navarro et al. Retrieval of CO2 and collisional parameters from the MIPAS spectra in the Earth atmosphere. Universidad de Granada, 2016. [9] M. Luginin et al. “Properties of Water Ice and Dust Particles in the Atmosphere of Mars During the 2018 Global Dust Storm as Inferred From the Atmospheric Chemistry Suite”. JGR: Planets 125.11 (2020). [10] A. Stcherbinine et al. “Martian Water Ice Clouds During the 2018 Global Dust Storm as Observed by the ACS-MIR Channel Onboard the Trace Gas Orbiter”. JGR: Planets 125.3 (2020). [11] Michael I Mishchenko et al. Scattering, absorption, and emission of light by small particles. Cambridge university press, 2002. [12] Pauli Virtanen et al. “SciPy 1.0: Fundamental Algorithms for Scientific Computing in Python”. Nature Methods 17 (2020).

Using the Nadir and Occultation for MArs Discovery instrument aboard Trace Gas Orbiter, we derived the CO/CO2 profiles between 75 and 105 km altitude with the equivalent width technique. The derived CO/CO2 profiles showed significant seasonal variations in the southern hemisphere with decreases near perihelion and increases near aphelion. The estimation of the CO/CO2 profiles with a one‐dimensional photochemical model shows that an altitude‐dependent eddy diffusion coefficient better reproduces the observed profiles than a vertically uniform one. Our estimation suggests that the eddy diffusion coefficient in Ls = 240–270 is uniformly larger by a factor of ∼2 than that in Ls = 90–120 in the southern hemisphere, while they are comparable in the northern hemisphere. This fact demonstrates that the eddy diffusion coefficient is variable with season and latitude.

An upward transport of water via the Hadley cell has been suggested as one of the mechanisms to transport water vapor to the upper atmosphere [Shaposhnikov et al., 2019], which would enhance the hydrogen escape on Mars [cf. Chaffin et al., 2017]. Carbon monoxide is one of the tracers which can measure the dynamics in the Martian atmosphere because CO distribution is a combination of photochemistry and dynamics. Above ~60 km altitude, the CO mixing ratio increases with altitude due to the production from photodissociation of CO2 and is further enhanced around the polar regions because of downwelling from the thermosphere [Daerden et al., 2019; Holmes et al., 2019; Olsen et al., 2021; Yoshida et al., accepted]. In the lower atmosphere, CO is recycled to CO2 by the catalytic cycle by odd hydrogen. The photochemical lifetime of CO is too slow, and then its lifetime is ~6 years in the lower atmosphere [Krasnopolsky, 2007]. Thus, seasonal variation of CO in the lower atmosphere is a consequence of CO2 sublimation/condensation at the polar cap [Encrenaz et al., 2006; Smith et al., 2009, 2021]. In addition, transport of rich CO atmosphere from southern to northern hemispheres during Ls = 90 – 180 has been measured by Smith et al. (2008, 2018) as predicted due to the breaking of the polar vortex by the GCM model. Although the vertical distribution of CO VMR is an index to determine the condensation of CO2, photochemistry, and dynamics, there is no direct comparison between measurements and simulations because we did not obtain the CO vertical distribution before the Trace Gas Orbiter (TGO) ExoMars mission. To clarify the vertical and horizontal transport of CO in the Martian atmosphere, we investigate the CO VMR retrieved from the solar occultation (SO) channel of Nadir and Occultation for MArs Discovery (NOMAD) instrument aboard TGO [Vandaele et al., 2018]. The SO channel operates at wavenumbers from 2325.6 to 4347.8 cm-1 with relatively high spectral resolution (R = 17,000). CO (2-0) band spectra features between 3970.7 and 4360.1 cm-1 are measured regularly in orders 186 to 191 of the instrument. We retrieved CO number densities using CO spectra features in orders 186 to 191 and radiative transfer code, ASIMUT [Vandaete et al., 2006], based on the Optimal Estimation Method [Rogers, 2006]. ASIMUT was performed for each spectrum at each tangential altitude independently [e.g., Aoki et al., 2019]. The latest updated instrument calibrations [Villanueva et al., submitted; Thomas et al., 2022] have been applied. We used the GEM-Mars model temperature and CO2 profiles [Neary et al., 2018; Daerden et al., 2019] to derive the CO VMR. The CO spectra were investigated from April 2018 to September 2021, corresponding from MY 34 Ls ~ 150 to MY 36 Ls ~ 105. The total number of dataset is 31,000. Firstly, we found that the retrieved CO VMR in orders 187 to 191 does not correspond to that in order 186 below ~60 km altitude. The CO VMR derived from orders 187 to 191 is underestimated. The strongest 2-0 band of CO is located at 4288.3cm-1, which corresponds to order 190, and it saturates around ~60 km. The saturation of CO lines would be related to the underestimation of CO VMR. When we tested the retrieval sensitivity of saturated lines in order 186, the underestimation of CO VMR also appears below 40 km altitude in the case that we perform the ASIMUT using the entire wavenumber range of order 186 (4179.0 – 4212.2 cm-1) compared with using a partial wavenumber range, 4189.0 – 4198.0 cm-1, of order 186. To avoid the underestimation of CO VMR, the retrieved CO VMR derived from CO spectra in the orders 187 – 191 is limited between 60 and ~110 km altitude, that from CO spectra in order 186 is limited between 40 and ~110 km, and that from CO spectra in 4189.0 – 4198.0 cm-1 is limited between the near-surface to 40 km altitude. The retrieved CO VMR distributes from 300 to ~5000 ppm. In the polar regions, the CO VMR increases above ~40 km and reaches 4000 ppm at 70 km, which is attributed to the production of CO from photodissociation of CO2 and transport of CO-enriched air via meridional circulation [Daerden et al., 2019]. That is consistent with the results measured by the Atmospheric Chemistry Suite aboard TGO [Olsen et al., 2021]. In the lower atmosphere, the enriched CO VMR up to ~3500 ppm appears from 90 to 200 in Ls in the southern hemisphere, which would be attributed to the CO2 condense in the southern winter season. We will report the CO distribution in more detail while distinguishing the dataset into season and latitude along with altitude.