CO density in the Mars atmosphere retrieved from NOMAD solar occultation observations

CO is produced in the Mars upper atmosphere by the photolysis of CO2 and destroyed in low altitudes mostly via reaction with hydroxyl radicals (OH)[1]. Thus CO is closely linked to the stability of this CO2 atmosphere and with key photochemical processes including the H2O cycle. Although, columnar densities of CO have been measured by instruments like CRISM [2] and more recently by NOMAD nadir channel[3], the lack of systematic mapping of CO density, limits our understanding of its distribution and variability. Of late few CO density profiles were reported from ACS observations[4] which found a significant depletion in CO mixing ratio during the 2018 global dust storm. The NOMAD-SO channel operates in the spectral range 2.3 4.3 m [5] which includes the diffraction orders 186 (4180.32 cm-1 - 4213.88 cm-1) - 191 (4292.69 cm-1 - 4327.16 cm-1). In these diffraction orders lie strong CO absorption lines which allows us for a good quality CO retrieval from 10 km with a vertical resolution better than 4km. The NOMAD observations suffer from a number of known calibration issues [6] such as bending and spectral shifts, in addition to variable systematic and random noise components. We have developed a cleaning procedure at IAA to correct the spectra for possible bending and spectral shift to make it usable for a precise retrieval of CO densities. We use the line-by-line radiative transfer model KOPRA 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 CO retrieval, built on a chain of retrievals of atmospheric aerosols, temperatures and density profiles derived from the same NOMAD scan but different diffraction orders, to obtain consistent CO profiles in a subset of observation spanning clear/dusty condition and seasonal and latitudinal variations. We will also present first comparisons with Mars GCM results. References: [1] McElroy, et al. (1972). Science 177.4053: 986-988. [2] Smith, Michael D., et al. (2018). Icarus 301: 117-131. [3]Smith, Michael D., et al. (2021). Icarus 362: 114404. [4] Olsen, K. S., et al. (2021). Nature Geoscience14.2: 67-71. [5] Neefs, Eddy, et al. (2015). Applied optics 54.28: 8494-8520. [6] Liuzzi, Giuliano, et al. (2019). Icarus 321: 671-690.