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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.01.001 Byline: Jean-Loup Bertaux (a)(b)(c), Ann Carine Vandaele (d), Valerie Wilquet (d), F. Montmessin (a)(b)(c), R. Dahoo (a)(c), E. Villard (a)(c), O. Korablev (e), A. Fedorova (e) Keywords: Venus; atmosphere; Spectroscopy; Atmospheres; composition; Occultations Abstract: The new ESA Venus Express orbiter is the first mission applying the probing technique of solar and stellar occultation to the atmosphere of Venus, with the SPICAV/SOIR instrument. SOIR is a new type of spectrometer used for solar occultations in the range 2.2-4.3 [mu]m. Thanks to a high spectral resolving power R[approximately equal to]15,000-20,000 (unprecedented in planetary space exploration), a new gaseous absorption band was soon detected in the atmospheric transmission spectra around 2982 cm.sup.-1, showing a structure resembling an unresolved Q branch and a number of isolated lines with a regular wave number pattern. This absorption could not be matched to any species contained in HITRAN or GEISA databases, but was found very similar to an absorption pattern observed by a US team in the spectrum of solar light reflected by the ground of Mars [Villanueva, G.L., Mumma, M.J., Novak, R.E., Hewagama, T., 2008. Icarus 195 (1), 34-44]. This team then suggested to us that the absorption was due to an uncatalogued transition of the.sup.16O.sup.12C.sup.18O molecule. The possible existence of this band was soon confirmed from theoretical considerations by Perevalov and Tashkun. Some SOIR observations of the atmospheric transmission are presented around 2982 cm.sup.-1, and rough calculations of line strengths of the Q branch are produced, based on the isotopic ratio measured earlier in the lower atmosphere of Venus. This discovery emphasizes the role of isotopologues of CO.sub.2 (as well as H.sub.2O and HDO) as important greenhouse gases in the atmosphere of Venus. Author Affiliation: (a) Service d'Aeronomie du CNRS, BP3, 91371, Verrieres-le-Buisson, France (b) Universite Pierre et Marie Curie, Paris, France (c) Institut Pierre Simon Laplace, Universite de Versailles-Saint-Quentin, 78 Saint-Quentin en Yvelines, France (d) Belgian Institute for Space Aeronomy, 3 av. Circulaire, B-1180 Brussels, Belgium (e) Space Research Institute (IKI), 84/32 Profsoyuznaya, 117810 Moscow, Russia Article History: Received 13 September 2007; Revised 9 November 2007

• The "science-softCon UV/Vis+ photochemistry database" contains EUV-VUV-UV–Vis-NIR spectral data. • The database also contains quantum yields and photolysis related data. • The database provides valuable information for astrophysics, earth and planetary sciences, medical and pharmaceutical sciences. The "science-softCon UV/Vis+ Photochemistry Database" (www.photochemistry.org) is a large and comprehensive collection of EUV-VUV-UV–Vis-NIR spectral data and other photochemical information assembled from published peer-reviewed papers. The database contains photochemical data including absorption, fluorescence, photoelectron, and circular and linear dichroism spectra, as well as quantum yields and photolysis related data that are critically needed in many scientific disciplines. This manuscript gives an outline regarding the structure and content of the "science-softCon UV/Vis+ Photochemistry Database". The accurate and reliable molecular level information provided in this database is fundamental in nature and helps in proceeding further to understand photon, electron and ion induced chemistry of molecules of interest not only in spectroscopy, astrochemistry, astrophysics, Earth and planetary sciences, environmental chemistry, plasma physics, combustion chemistry but also in applied fields such as medical diagnostics, pharmaceutical sciences, biochemistry, agriculture, and catalysis. In order to illustrate this, we illustrate the use of the UV/Vis+ Photochemistry Database in four different fields of scientific endeavor. [ABSTRACT FROM AUTHOR]

Nadir and Occultation for MArs Discovery (NOMAD) is a 3-channel spectrometer suite that is currently orbiting Mars onboard ESA's ExoMars Trace Gas Orbiter, measuring the composition of the Martian atmosphere in unprecedented detail. Of the three channels, two operate in the infrared: the Solar Occultation (SO) channel observes gas species in the 2.2–4.3 ​μm spectral region in solar occultation mode, while the Limb, Nadir and Occultation (LNO) channel observes in the 2.2–3.8 ​μm spectral region and can operate in limb-, nadir- and solar occultation-pointing modes. The Ultraviolet–VISible (UVIS) channel operates in the UV–visible region, from 200 to 650 ​nm. Both infrared channels have a spectral resolution typically an order of magnitude better than previous instruments orbiting Mars, to measure molecular absorption lines and therefore determine the abundances of constituents of the Martian atmosphere and the processes that govern their distribution and transport. To maximise the full potential of the instrument, a wide range of calibration measurements were made prior to launch and continue to be made in-flight. This work, part 1, addresses the aspects of the SO channel calibration that are not covered elsewhere, namely: the SO channel ground calibration setup, boresight pointing vector determination, detector characterisation, detector illumination pattern and saturation levels, and an investigation of the instrument line shape. An accompanying paper, part 2, addresses similar aspects for LNO, the other infrared channel in NOMAD (Thomas et al., 2021, this issue). [ABSTRACT FROM AUTHOR]

The Nadir and Occultation for MArs Discovery (NOMAD) instrument is a 3-channel spectrometer suite on the ESA ExoMars Trace Gas Orbiter. Since April 2018, when the nominal science mission began, it has been measuring the constituents of the Martian atmosphere. NOMAD contains three separate spectrometers, two of which operate in the infrared: the Solar Occultation (SO) channel makes only solar occultation observations, and therefore has the best resolving power (∼20,000) and a wider spectral region covering 2.2–4.3 ​μm. The Limb, Nadir and Occultation (LNO) channel covers the 2.2–3.8 ​μm spectral region and can operate in limb, nadir or solar occultation pointing modes. The Ultraviolet–VISible (UVIS) channel operates in the UV–visible region, from 200 to 650 ​nm, and can measure in limb, nadir or solar occultation modes like LNO. The LNO channel has a lower resolving power (∼10,000) than the SO channel, but is still typically an order of magnitude better than previous instruments orbiting Mars. The channel primarily operates in nadir-viewing mode, pointing directly down to the surface to measure the narrow atmospheric molecular absorption lines, clouds and surface features in the reflected sunlight. From the depth and position of the observed atmospheric absorption lines, the constituents of the Martian atmosphere and their column densities can be derived, leading to new insights into the processes that govern their distribution and transport. Surface properties can also be derived from nadir observations by observing the shape of the spectral continuum. Many calibration measurements were made prior to launch, on the voyage to Mars, and continue to be made in-flight during the science phase of the mission. This work, part 2, addresses the aspects of the LNO channel calibration that are not covered elsewhere, namely: the LNO ground calibration setup, the LNO occultation and nadir boresight pointing vectors, LNO detector characterisation and nadir/limb illumination pattern, instrument temperature effects, and finally the radiometric calibration of the LNO channel. An accompanying paper, part 1 (Thomas et al., 2021, this issue), addresses similar aspects for SO, the other infrared channel in NOMAD. A further accompanying paper (Cruz-Mermy et al., 2021, this issue) investigated the LNO radiometric calibration in more detail, approaching the work from a theoretical perspective. The two calibrations agree with each other to within 3%, validating each calibration method. • The LNO channel is 1 of 3 spectrometers in NOMAD on ExoMars Trace Gas Orbiter. • The LNO channel observes Mars in limb, nadir and solar occultation modes. • This article describes the operational modes and calibration of the LNO channel. • The boresight, detector illumination, slant, bad pixels and saturation were found. • The procedure for radiometrically calibrating nadir observations is described. [ABSTRACT FROM AUTHOR]

We present an in-flight straylight removal method for the Ultraviolet and Visible Spectrometer (UVIS) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument aboard the ExoMars Trace Gas Orbiter (TGO). The presence of a 'red-leak' straylight signal in the UVIS instrument was discovered post-launch in ground calibration measurements of spectral lamps; UVIS observations of lamps with negligible UV light emission (RS12) showed a significant signal at UV wavelengths. Subsequent analyses of nadir observations of the martian atmosphere revealed that at UV wavelengths the red-leak straylight was in excess of 300% of the true UV signal, jeopardising the primary science observations of the instrument (retrievals of atmospheric ozone). By modifying the UVIS readout method to obtain a region of interest around the illuminated region on the Charge-Coupled Device (CCD) detector, instead of a binned one-dimensional spectrum, and utilising straylight profiles derived from measurements of the RS12 calibration lamp we show that the majority of the straylight at UV wavelengths can be successfully removed for the nadir channel in a self-consistent manner. The corrected UVIS radiances are compared to coincident Mars Color Imager (MARCI) instrument observations with residuals between the two instruments generally remaining within 15%. • Retaining the full CCD array can be a powerful tool for the removal of straylight. • Essential to measure straylight profiles using different light sources and filters. • Accurate removal of straylight and recovery of measured ultraviolet radiances. [ABSTRACT FROM AUTHOR]

The Ultraviolet and VIsible Spectrometer (UVIS), covering the 200–650 ​nm range, is one of three spectrometers that comprise the NOMAD instrument on the ExoMars 2016 Trace Gas Orbiter (TGO). UVIS can operate in solar occultation, nadir and limb viewing mode and was designed to monitor ozone and aerosols in the Martian atmosphere. Here, we describe the calibration procedure to convert the UVIS raw data into a calibrated data product ready for scientific exploitation. The calibration includes the CCD offset and dark current subtraction, the wavelength assignment, the noise identification and removal, the smearing removal, and the radiance or transmittance conversion. A straylight correction, critical for some parts of the UVIS spectral range, is also applied during the data reduction process, which is described in more detail in two companion papers [Mason et al., 2022; Depiesse et al., In prep] corresponding to two different and independent methods giving consistent results. The solar occultation observations are converted into transmittance and are therefore self-calibrating, while nadir and limb measurements require an absolute radiometric calibration. A comparison with coincident nadir MRO/MARCI measurements is provided as a final validation and generally shows a ±10% agreement on the radiances measured by both instruments. • Description of the calibration of UVIS measurements to produce level 1 data from raw data • CCD non-linearity correction, offset removal, dark current removal, Bad pixel removal, Smearing removal. • A straylight correction is also applied but described in companion papers. • Conversions: Pixel-wavelength assignment, Radiance (nadir) or Transmittance (occultation) conversion. • Radiometric comparison with coincident MRO/MARCI measurements, serving as validation for the radiance calibrated data. [ABSTRACT FROM AUTHOR]

Observations from Pioneer Venus and from SPICAV/SOIR aboard Venus Express (VEx) have shown the upper haze (UH) of Venus to be highly spatially and temporally variable, and populated by multiple particle size modes. Previous models of this system (e.g., Gao et al., 2014. Icarus 231, 83–98), using a typical temperature profile representative of the atmosphere (viz., equatorial VIRA profile), did not investigate the effect of temperature on the UH particle distributions. We show that the inclusion of latitude-dependent temperature profiles for both the morning and evening terminators of Venus helps to explain how the atmospheric aerosol distributions vary spatially. In this work we use temperature profiles obtained by two instruments onboard VEx, VeRa and SPICAV/SOIR, to represent the latitudinal temperature dependence. We find that there are no significant differences between results for the morning and evening terminators at any latitude and that the cloud base moves downwards as the latitude increases due to decreasing temperatures. The UH is not affected much by varying the temperature profiles; however, the haze does show some periodic differences, and is slightly thicker at the poles than at the equator. We also find that the sulphuric acid “rain” seen in previous models may be restricted to the equatorial regions of Venus, such that the particle size distribution is relatively stable at higher latitudes and at the poles. [ABSTRACT FROM AUTHOR]

We investigate water vapor saturation in the martian atmosphere in the presence or proximity of water ice clouds. We evaluate mixing ratio profiles of ambient water vapor derived from measurements by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and the Nadir and Occultation for MArs Discovery (NOMAD) instrument in comparison with saturated vapor mixing ratio profiles derived from co-located temperature measurements by the Mars Climate Sounder (MCS). We find that during the aphelion season the average saturation state of the atmosphere is close to saturation in the presence of clouds, with supersaturation ratios reaching values of no more than two to three towards the top of the cloud layer. During the perihelion season, subseasonally averaged water vapor is close to saturation or somewhat subsaturated in the presence of clouds, with some supersaturation at southern high latitudes towards the top of the clouds. Measurements at northern mid- to high latitudes during a Global Dust Storm suggest a shift of the cloud occurrence to higher altitudes but exhibit a similar saturation structure, with supersaturations not exceeding a ratio of five. The atmosphere above cloud layers is largely subsaturated, suggesting that cloud formation is fairly instantaneous upon the temperature dropping below the frost point and high levels of supersaturation are not required to form water ice clouds. We propose a schematic model of cloud evolution in which small cloud particles are formed rapidly in slightly supersaturated regions, and then fall and grow such that ice opacity is still observed even in the subsaturated regions below. [Display omitted] • Water vapor saturation close to clouds in the martian atmosphere is investigated. • The average state of the atmosphere is close to saturation. • The atmosphere above and below cloud layers appears largely subsaturated. • High levels of supersaturation are not required to form water ice clouds. [ABSTRACT FROM AUTHOR]

Abstract: This paper is the fourth of a series of papers reporting critically evaluated rotational–vibrational line positions, transition intensities, pressure dependences, and energy levels, with associated critically reviewed assignments and uncertainties, for all the main isotopologues of water. This paper presents energy level and transition data for the following doubly and triply substituted isotopologues of water: D2 16O, D2 17O, and D2 18O. The MARVEL (Measured Active Rotational–Vibrational Energy Levels) procedure is used to determine the levels, the lines, and their self-consistent uncertainties for the spectral regions 0–14016, 0–7969, and 0–9108cm−1 for D2 16O, D2 17O, and D2 18O, respectively. For D2 16O, D2 17O, and D2 18O, 53534, 600, and 12167 lines are considered, respectively, from spectra recorded in absorption at room temperature and in emission at elevated temperatures. The number of validated energy levels is 12269, 338, and 3351 for D2 16O, D2 17O, and D2 18O, respectively. The energy levels have been checked against the ones determined, with an average accuracy of about 0.03cm−1, from variational rovibrational computations employing exact kinetic energy operators and an accurate potential energy surface. Furthermore, the rovibrational labels of the energy levels have been validated by an analysis of the computed wavefunctions using the rigid-rotor decomposition (RRD) scheme. The extensive list of MARVEL lines and levels obtained is deposited in the Supplementary Material of this paper, in a distributed information system applied to water, W@DIS, and on the official MARVEL website, where they can easily be retrieved. [Copyright &y& Elsevier]

Abstract: This is the third of a series of articles reporting critically evaluated rotational–vibrational line positions, transition intensities, and energy levels, with associated critically reviewed labels and uncertainties, for all the main isotopologues of water. This paper presents experimental line positions, experimental-quality energy levels, and validated labels for rotational–vibrational transitions of the most abundant isotopologue of water, H2 16O. The latest version of the MARVEL (Measured Active Rotational–Vibrational Energy Levels) line-inversion procedure is used to determine the rovibrational energy levels of the electronic ground state of H2 16O from experimentally measured lines, together with their self-consistent uncertainties, for the spectral region up to the first dissociation limit. The spectroscopic network of H2 16O containstwo components, an ortho (o) and a para (p) one. For o-H2 16O and p-H2 16O, experimentally measured, assigned, and labeled transitions were analyzed from more than 100 sources. The measured lines come from one-photon spectra recorded at room temperature in absorption, from hot samples with temperatures up to 3000K recorded in emission, and from multiresonance excitation spectra which sample levels up to dissociation. The total number of transitions considered is 184667 of which 182156 are validated: 68027 between para states and 114129 ortho ones. These transitions give rise to 18486 validated energy levels, of which 10446 and 8040 belong to o-H2 16O and p-H2 16O, respectively. The energy levels, including their labeling with approximate normal-mode and rigid-rotor quantum numbers, have been checked against ones determined from accurate variational nuclear motion computations employing exact kinetic energy operators as well as against previous compilations of energy levels. The extensive list of MARVEL lines and levels obtained are deposited in the supplementary data of this paper, as well as in a distributed information system applied to water, W@DIS, where they can easily be retrieved. [Copyright &y& Elsevier]

Abstract: High-resolution water vapor absorption spectra have been measured at room temperature in the 8800–11,600cm−1 spectral region. They were obtained using the mobile BRUKER IFS 120M Fourier transform spectrometer (FTS) from ULB-SCQP coupled to the 50m base long multiple reflection White type cell in GSMA laboratory. The absorption path was 600m and different H2O/HDO/D2O mixtures were used. Measurements of line positions, intensities and self-broadening coefficients were performed for the HD16O isotopologue. 6464 rovibrational assignment of the observed lines was made on the basis of global variational predictions and allowed the identification of new energy levels. 3ν 3, 2ν 1+ν 3, 3ν 1+ν 2, ν 1+2ν 3 and 2ν 2+2ν 3 are the five strongest bands. The present paper provides a complementary data set on water vapor for atmospheric and astrophysical applications. [Copyright &y& Elsevier]

Abstract: This is the second of a series of articles reporting critically evaluated rotational–vibrational line positions, transition intensities, pressure dependences, and energy levels, with associated critically reviewed assignments and uncertainties, for all the main isotopologues of water. This article presents energy levels and line positions of the following singly deuterated isotopologues of water: HD16O, HD17O, and HD18O. The MARVEL (measured active rotational–vibrational energy levels) procedure is used to determine the levels, the lines, and their self-consistent uncertainties for the spectral regions 0–22708, 0–1674, and 0–12105cm−1 for HD16O, HD17O, and HD18O, respectively. For HD16O, 54740 transitions were analyzed from 76 sources, the lines come from spectra recorded both at room temperature and from hot samples. These lines correspond to 36690 distinct assignments and 8818 energy levels. For HD17O, only 485 transitions could be analyzed from three sources; the lines correspond to 162 MARVEL energy levels. For HD18O, 8729 transitions were analyzed from 11 sources and these lines correspond to 1864 energy levels. The energy levels are checked against ones determined from accurate variational nuclear motion computations employing exact kinetic energy operators. This comparison shows that the measured transitions account for about 86% of the anticipated absorbance of HD16O at 296K and that the transitions predicted by the MARVEL energy levels account for essentially all the remaining absorbance. The extensive list of MARVEL lines and levels obtained are given in the Supplementary Material of this article, as well as in a distributed information system applied to water, W@DIS, where they can easily be retrieved. In addition, the transition and energy level information for H2 17O and H2 18O, given in the first paper of this series [Tennyson, et al. J Quant Spectr Rad Transfer 2009;110:573–96], has been updated. [Copyright &y& Elsevier]

Abstract: This is the first part of a series of articles reporting critically evaluated rotational–vibrational line positions, transition intensities, pressure dependence and energy levels, with associated critically reviewed assignments and uncertainties, for all the main isotopologues of water. The present article contains energy levels and data for line positions of the singly substituted isotopologues H2 17Oand H2 18O. The procedure and code MARVEL, standing for measured active rotational–vibrational energy levels, is used extensively in all stages of determining the validated levels and lines and their self-consistent uncertainties. The spectral regions covered for both isotopologues H2 17O and H2 18O are . The energy levels are checked against ones determined from accurate variational calculations. The number of critically evaluated and recommended levels and lines are, respectively, 2687 and 8614 for H2 17O, and 4839 and 29364 for H2 18O. The extensive lists of MARVEL lines and levels obtained are deposited in the Supplementary Material, as well as in a distributed information system applied to water, W@DIS, where they can easily be retrieved. A distinguishing feature of the present evaluation of water spectroscopic data is the systematic use of all available experimental data and validation by first-principles theoretical calculations. [Copyright &y& Elsevier]

Abstract: New high-resolution water vapor absorption spectra were obtained at room temperature in the 4200–6600cm−1 spectral region by combining Fourier transform spectrometers (FTS) with single and multiple reflection cells. With absorption paths from 0.3 to 1800 m in pure and air diluted water vapor, accurate measurements of about 10400 lines in an intensity range from 10−29 to 10−19 cm/molecule have been performed. Positions, intensities, self- and air-broadening coefficients and air-induced shifts were determined for the H2 16O, H2 17O, H2 18O and HDO isotopologues. The rovibrational assignment of the observed lines was performed with the use of global variational predictions and allowed the identification of several new energy levels. One major contribution of this work consists of the identification of 3280 new weak lines. A very close agreement between the new measured parameters and those listed in the database is reported as well as between the observations and the most recent variational calculations for the positions and the intensities. The present parameters provide an extended and homogeneous data set for water vapor, which is shown to significantly improve the databases for atmospheric applications, especially in the transmission windows on both sides of the band centered at 5400cm−1. [Copyright &y& Elsevier]

The absorption of solar light by atmospheric water vapor in the visible spectral region is analyzed by means of ground-based absorption Fourier transform spectroscopy, performed at high resolution in Brussels during summer 2001. Several microwindows between 14,000 and 18,000 cm−1, in which water vapor lines are well isolated from solar lines and other atmospheric trace gases absorptions, are examined. They are demonstrated to be adequate for the retrieval of the total water vapor column. Based on the retrievals, a detailed analysis of the water vapor line parameters published in the HITRAN database and recently reinvestigated by different groups is performed. The analysis focuses on the one hand on the comparison of the retrieved water vapor columns with in situ measurements, performed at the same time as the spectroscopic measurements and at the same location, and on the other hand on the quality of the spectral fits. It is shown that the discrepancies between the line lists affect significantly the results. In particular it is shown that the weaker lines, not measured in earlier laboratory experiments, do contribute at large zenith angles and need to be taken into account in order to better simulate the atmospheric spectra. The importance of the pressure broadening parameters is also highlighted. [Copyright &y& Elsevier]

Considering the unique role that water in its vapor phase plays in atmospheric physical and chemical processes, there is a need for accurate spectroscopic parameters for this molecule. Long-pathlength Fourier transform spectra of water vapor with synthetic air as the perturbing gas were recorded and analyzed in the 26,000–13,000 cm−1 spectral region. New measurements of air-broadening and air-shifting parameters with associated uncertainties are presented for about 5000 lines. These data complement our existing database, providing a homogeneous and extensive dataset extending from 26,000 to 9250 cm−1. Comparisons with the two most frequently used HITRAN and ESA databases as well as with other literature data available are made. Agreements and discrepancies are underlined and briefly discussed. [Copyright &y& Elsevier]

Line parameters of water vapor have been obtained in the 13 000–9250 cm−1 spectral region. The measurements were performed with a high resolution Fourier transform spectrometer coupled to long-path multiple reflection cells. Emphasis was put on the determination of accurate parameters for the weak lines, most of which have not been previously reported in the literature. A total of 7061 lines have been studied. Their positions and intensities have been determined, and for many of analyzed lines, self- and air-broadening coefficients, as well as pressure-induced shifts were obtained. A preliminary analysis of the dependence of the broadenings and shifts, with vibrational and rotational quantum numbers is given. The discussion is focused on the comparison between our measured parameters and those listed in the HITRAN database. A very good agreement is reported for the positions and intensities. It is suggested that the construction of a comprehensive and complete dataset for water vapor in this spectral region requires the merging of independent measurements, and of calculated values of broadenings and shifts for weak and blended lines. [Copyright &y& Elsevier]

The radiative properties of water vapor play an important role in the physical and chemical processes occurring in the atmosphere. Accurate knowledge of the line parameters for this species is therefore needed. This work presents new measurements of water vapor line parameters in the 26 000–13 000 cm−1 spectral region. The measurements were obtained by combining a high-resolution Fourier transform spectrometer with a long-path absorption cell, thus allowing the observation of very weak, previously unobserved, lines. A total of more than 9000 lines have been identified and their position, integrated cross section and self-broadening parameter have been determined. The dependence of the line parameters on nitrogen buffer gas pressure (0–800 hPa) has also been studied. The complete line list presented here is primarily compared to the HITRAN spectroscopic database, most frequently used in atmospheric calculations. [Copyright &y& Elsevier]

Because of its important role in the Martian carbon cycle, carbon monoxide (CO) has been the subject of many measurements from ground and from space. Daytime measurements have been mostly exploited to measure the CO abundance because of their good signal-to-noise ratio, but night-time observations have not been documented yet. We demonstrate here the possibility of using PFS (Planetary Fourier Spectrometer) night-time observations to measure the Martian CO abundance. More particularly in this paper, we show that measuring CO during night with PFS can be achieved by averaging a large number of spectra to reach sufficient signal-to-noise. Furthermore, we show that the number of averaged spectra is not the only driving parameter for the detection of CO. High surface temperatures and high thermal contrast (negative in our case) are the two other conditions required for the measurement of the night-time CO abundance. Because of this, the retrievals are especially successful in the Southern Hemisphere during spring and summer when and where these two conditions are met. For night-time spectra with a positive detection, CO vertical profiles are successfully retrieved using the Optimal Estimation method and are characterized in terms of vertical sensitivity. Successful retrievals imply the use of accurate temperature profiles, and in particular an appropriate representation of the thermal inversion in the lower atmosphere. The temperature was obtained by using the relevant information from the CO 2 ν 3 band (Bauduin et al., 2020, this issue). A complete error budget of the retrieved CO profiles is also performed and includes different sources of uncertainty. Although the retrieved profiles are not resolved vertically, we show in particular that night-time PFS observations carry information mostly on the CO abundance for the 0–10 ​km altitude region, and thus provide a stronger constraint on the near-surface CO abundance compared to daytime observations. [Display omitted] • CO night-time abundance is retrieved from averaged night-time nadir PFS/SWC spectra. • Averages of large ensemble of spectra required to detect CO from night-time spectra. • A detection method is developed to select spectra with detectable CO lines. • Detection of CO driven by the surface temperature and thermal contrast values. • Night-time CO is best measured in the Southern Hemisphere during spring and summer. [ABSTRACT FROM AUTHOR]

Nadir remote sensing of the night side of Mars is challenging, mainly due to the low signal-to-noise ratio of such observations. We show in a companion paper that the abundance of carbon monoxide (CO) during night can be retrieved from the observations of the Planetary Fourier Spectrometer (PFS). This requires, however, an accurate knowledge of the temperature profile, and especially of the night-time thermal inversions, to properly model the atmospheric emission. While the temperature profile is usually retrieved from the ν 2 band of CO 2 (centered at 667 ​cm−1), this work shows that, for averaged night-time PFS observations built from a large ensemble of spectra, the temperature profile can be retrieved from the more saturated ν 3 band of CO 2 (centered at 2349 ​cm−1). We show especially that, due to IFOV (instantaneous field-of-view) size differences and boresight offset between the longwave and shortwave channels of PFS, the temperature profile retrieved from the ν 3 band is more consistent with the emission observed in the 1-0 band of CO (centered at 2143 ​cm−1), which is used in the second part paper. We provide a complete characterization of the retrieved temperature profiles in terms of error and vertical sensitivity. Using this, we show that using the ν 3 CO 2 band allows to properly constrain and characterize the thermal inversions encountered near the surface for most night-time observations. The resulting set of temperature profiles is essential for the retrieval of the night-time CO abundance that is presented in the companion paper. Beyond their usefulness for the night-time CO retrieval, we suggest with a last example that temperature profiles retrieved from the ν 3 band of CO 2 could be use more generally to study surface thermal inversions encountered at night. [Display omitted] • Retrieval of temperature profiles from averaged night-time nadir PFS/SWC spectra. • The wings of the saturated CO 2 ν 3 band can be exploited for the temperature retrieval. • Full characterization of the retrieved temperature profiles. • The night-time thermal inversions are well constrained when using the CO 2 ν 3 band. [ABSTRACT FROM AUTHOR]

• Fully blind machine learning approach to produce a quicklook and discover new lines on large spectroscopic datasets. • Validation on ESA/ExoMars TGO NOMAD-SO dataset. • Absence of methane in 100–500 ppt from this analysis. • Exhibition of unexpected lines. One of the main difficulties to analyze modern spectroscopic datasets is due to the large amount of data. For example, in atmospheric transmittance spectroscopy, the solar occultation channel (SO) of the NOMAD instrument onboard the ESA ExoMars2016 satellite called Trace Gas Orbiter (TGO) had produced ~ 10 millions of spectra in ~ 20000 acquisition sequences since the beginning of the mission in April 2018 until 15 January 2020. Other datasets are even larger with ~ billions of spectra for OMEGA onboard Mars Express or CRISM onboard Mars Reconnaissance Orbiter. Usually, new lines are discovered after a long iterative process of model fitting and manual residual analysis. Here we propose a new method based on unsupervised machine learning, to automatically detect new minor species. Although precise quantification is out of scope, this tool can also be used to quickly summarize the dataset, by giving few endmembers ("source") and their abundances. The methodology is the following: we proposed a way to approximate the dataset non-linearity by a linear mixture of abundance and source spectra (endmembers). We used unsupervised source separation in form of non-negative matrix factorization to estimate those quantities. Several methods are tested on synthetic and simulation data. Our approach is dedicated to detect minor species spectra rather than precisely quantifying them. On synthetic example, this approach is able to detect chemical compounds present in form of 100 hidden spectra out of 104, at 1.5 times the noise level. Results on simulated spectra of NOMAD-SO targeting CH 4 show that detection limits goes in the range of 100–500 ppt in favorable conditions. Results on real martian data from NOMAD-SO show that CO 2 and H 2 O are present, as expected, but CH 4 is absent. Nevertheless, we confirm a set of new unexpected lines in the database, attributed by ACS instrument Team to the CO 2 magnetic dipole. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• CO total columns obtained from constrained profile retrievals on nadir measurements. • Retrieved profiles fully characterized in terms of error and vertical sensitivity. • Potential use of single PFS spectra to monitor CO with good accuracy demonstrated. The knowledge of the carbon monoxide (CO) abundance on Mars is essential in order to assess the processes driving the carbon cycle on the planet. Solar occultation measurements provide vertically-resolved measurements of CO from a few kilometers to higher altitudes and can be complemented by nadir measurements to enhance the spatial coverage of observations and the monitoring of the near-surface layer. Up to now, in the shortwave, CO retrievals from nadir observations have, however, mostly been performed on mean spectra and only total column abundances have been obtained. In this work we explore the possibility of exploiting nadir measurements from the Planetary Fourier Spectrometer (PFS) in the 1–0 band of CO (centered at 2143 cm − 1) to retrieve vertical profiles of that species on individual measurement. The retrievals are performed for a set of 16 selected PFS spectra with reasonable signal-to-noise ratios by applying the Optimal Estimation Method (OEM) with appropriate constraints, built from model simulations of the Martian atmosphere. The retrieved profiles are characterized in terms of vertical sensitivity and errors. We demonstrate, in particular, that the PFS nadir measurements carry information mostly on the CO column below 15 km, with a maximum sensitivity to the near-surface atmosphere. These measurements allow to substantially reduce the prior uncertainty on the CO abundance in this altitude range, with an estimated total retrieval error on the column-averaged volume mixing ratio (VMR) around 10%. We show that the set of retrieved VMRs are in the range of values reported from other instruments. The retrieved VMRs also capture well the known spatial and seasonal CO variability, which is promising in the perspective of better exploiting the exceptional set of PFS observations on Mars. [ABSTRACT FROM AUTHOR]