CO distributions retrieved from TGO NOMAD SO using multiple orders

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 L_s = 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 L_s ~ 150 to MY 36 L_s ~ 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 L_s 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.