Update on CO2 and temperature profiles from NOMAD-SO on board ExoMars TGO
Authors: Trompet Loïc , Vandaele Ann Carine , Aoki Shohei , Erwin Justin , Thomas Ian , Villanueva Geronimo , Liuzzi Giulliano , Crismani Matteo , Lopez-Valverde Miguel Angel , Hill Brittany , Piccialli Arianna , Daerden Frank , Ristic Bojan , Lopez-Moreno Juan Jose , Bellucci Giancarlo , Patel Manish 
Published: October 2020

The NOMAD-SO channel [1, 2] is an infrared spectrometer working in the 2.2 to 4.3 &#181;m spectral range (2325-4545 cm-1). The instrument is composed of an echelle grating coupled to an Acousto-Optical Tunable Filter for the diffraction order selection [3]. NOMAD started to perform solar occultation measurement on April 21, 2018. As TGO is on a quasi-circular orbit at around 400&#160;km of altitude, it performs one orbit every two hours. During a solar occultation measurement, SO scans six diffraction orders each second. These diffraction orders are recorded on four bins leading to a vertical sampling below 1 km. The calibration of the SO channel is described in [4] and is being refined.NOMAD-SO regularly scans different diffraction orders containing CO2 lines to allow CO2 retrievals from low to high altitudes. For each solar occultation measurement, we derive a slant column profile of CO2 using ASIMUT-ALVL [4]. ASIMUT is a radiative transfer program developed at BIRA-IASB and based on the Optimal Estimation Method [5]. The GEM-Mars GCM provides the a priori profiles of CO2 local density, pressure and temperature. We then apply Tikhonov linear regularization on the slant column to derive a smoothed local density. We finally apply the hydrostatic equilibrium equation and the ideal gas law to derive the temperature profiles [6-8]. That derived temperature profile serves then in a new loop where we perform again the previous steps until the profiles converge [8]. Several comparisons are ongoing with joint or co-located measurements from MAVEN-EUVM, Maven-NGIMS, and TGO-ACS-NIR as well as with GCM derived profiles from GEM-Mars and LMD-MGCM. We derived the NOMAD-SO CO2 and temperature profiles for MY34 with solar longitudes (Ls) extending from 298&#176; to 326&#176;. That time range contains the regional dust storm of MY34 that started at Ls 317&#176;. We will present the updated CO2 and temperature profiles from NOMAD-SO measurementsAcknowledgementsExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by the Spanish 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 grant ST/R005761/1 and Italian Space Agency through grant 2018-2-HH.0. The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the &#8216;Center of Excellence Severo Ochoa&#8217; award for the Instituto de Astrof&#237;sica de Andaluc&#237;a (SEV-2017-0709). This work was supported by the Belgian Fonds de la Recherche Scientifique &#8211; FNRS under grant numbers 30442502 (ET_HOME) and T.0171.16 (CRAMIC) and BELSPO BrainBe SCOOP Project. US investigators were supported by the National Aeronautics and Space Administration.&#160;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] Neefs, E., et al.: NOMAD spectrometer on the ExoMars trace gas orbiter mission: part1 &#8211; design, manufacturing and testing of the infrared channels, Appl. Opt., Vol. 54(28), pp 8494-8520, 2015.[3] Thomas, I.R., et al.: Optical and radiometric models of the NOMAD instrument part II: the infrared channels &#8211; SO and LNO, Opt. Express, Vol. 24(4), pp 3790-3805, 2016.[4] Liuzzi, G. 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, 2018.[4] Vandaele, A.C., Kruglanski, M., De Mazi&#232;re, M., Modeling and retrieval of atmospheric spectra using ASIMUT. Proceedings of the First Atmospheric Science Conference, ESRIN, Frascati, Italy, 2006.[5] Rodgers, C.D., Inverse method for atmospheric sounding: theory and practice. Hackensack, N.J. (Ed.), World Scientific University of Oxford, Oxford, 2000.[6] Gr&#246;ller, H., et al.: MAVEN/IUVS stellar occultation measurements of Mars atmospheric structure and composition. Journal of Geophysical Research: Planets, 123, 1449&#8211;1483, 2018.[7] Snowden D., et al. :The thermal structure of titan's upper atmosphere, I: temperature profiles from Cassini INMS observations, Icarus, Vol. 226, pp. 552-582, 2013.[8] Mahieux, A. et al.: Densities and temperatures in the Venus mesosphere and lower thermosphere retrieved from SOIR on board Venus Express: Carbon dioxide measurements at the Venus terminator, J. Geophys. Res., Vol 117, E07001, 2012.&#160;