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Using the Nadir and Occultation for MArs Discovery instrument aboard Trace Gas Orbiter, we derived the CO/CO2profiles between 75 and 105 km altitude with the equivalent width technique. The derived CO/CO2profiles showed significant seasonal variations in the southern hemisphere with decreases near perihelion and increases near aphelion. The estimation of the CO/CO2profiles 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. The eddy diffusion coefficient is widely used to parameterize the efficiency of vertical diffusion in the planetary atmosphere, whose variation characterizes the transportation of trace gas species. Additionally, it could vary their vertical distributions in the middle and upper atmosphere, which might cause an impact on the species escaping to space. However, the variability of the eddy diffusion coefficient in those altitude regions have been poorly understood. In this study, we focus on the estimation of variation in the eddy diffusion coefficient by analyzing the CO and CO2measurements made by the ExoMars Trace Gas Orbiter. The observed CO/CO2ratio between altitudes of 75 and 105 km shows a significant seasonal variation in the southern hemisphere. The observed CO/CO2profiles are compared with the simulated profiles obtained with a one‐dimensional photochemical model assigning several shapes and intensity of eddy diffusion coefficient. The comparison shows that the eddy diffusion coefficient is not constant but variable depending on altitude, season, and latitude, which suggests that the efficiency of the vertical diffusion varies with season and latitude. This fact is useful to other 1D photochemical models to reproduce the seasonal and latitudinal variation of atmospheric composition. The CO/CO2profiles from 75 to 105 km measured by NOMAD aboard TGO are used to investigate variations in the eddy diffusion coefficientThe estimated CO/CO2profiles agree well with the observed profiles if altitude‐dependent eddy diffusion coefficients are consideredOur results demonstrate a substantial seasonal variation in the eddy diffusion coefficient in the southern hemisphere The CO/CO2profiles from 75 to 105 km measured by NOMAD aboard TGO are used to investigate variations in the eddy diffusion coefficient The estimated CO/CO2profiles agree well with the observed profiles if altitude‐dependent eddy diffusion coefficients are considered Our results demonstrate a substantial seasonal variation in the eddy diffusion coefficient in the southern hemisphere

We present observations of terminator CO2ice clouds events in three groups: Equatorial dawn, Equatorial dusk (both between 20°S and 20°N) and Southern midlatitudes at dawn (45°S and 55°S east of Hellas Basin) with ESA ExoMars Trace Gas Orbiter's Nadir and Occultation for MArs Discovery instrument. CO2ice abundance is retrieved simultaneously with water ice, dust, and particle sizes, and rotational temperature and CO2column profiles in 16 of 26 cases. Small particles (<0.5 μm) prevail at dusk, while water ice likely provides most source nuclei at dawn. Clouds east of Hellas are found to be dominantly nucleated on surface‐lifted dust. CO2ice is sometimes detected in unsaturated air together with dust nuclei at dawn, suggesting ongoing sublimation. Depending on latitude and local time, the interplay between particle precipitation and the lifetime of temperature minima (i.e., cold pockets) determines CO2ice properties. The upper atmosphere of Mars is characterized by the seasonal presence of CO2ice clouds. Their properties have been long studied, as well as their formation mechanisms in relation to the thermal structure of the atmosphere and its variability. In this study, we present the first observations of these clouds at the terminator (dawn and dusk) by the NOMAD spectrometer onboard the Exomars Trace Gas Orbiter. CO2ice is detected simultaneously with dust, water ice and the temperature profile. Our results agree with previous findings in terms of how clouds are spatially distributed and their temporal occurrence. However, we also explore the sources of condensation nuclei for CO2ice particles, showing that water ice is a possible source at dawn near the Equator. We also identify surface‐lifted dust below the CO2ice clouds observed east of Hellas Basin, suggesting that, at that location, dust could provide nuclei for CO2ice. CO2ice is also sometimes detected at temperatures higher than the CO2freezing point, suggesting ongoing sublimation. In this work we explore for the first time the composition of CO2ice clouds, which is critical to advance our understanding of how CO2ice clouds form in the mesosphere at Mars. Twenty Six mesospheric CO2ice clouds were detected with NOMAD SO in Mars Year 35, simultaneously with water ice, dust, and CO2saturation ratiosEquatorial CO2clouds are observed at 50–80 km altitude at dusk, and 40–60 km at dawn, when water ice likely provides condensation nucleiSix CO2ice clouds are found east of Hellas basin; their formation is likely sourced by surface‐lifted dust at 40–65 km Twenty Six mesospheric CO2ice clouds were detected with NOMAD SO in Mars Year 35, simultaneously with water ice, dust, and CO2saturation ratios Equatorial CO2clouds are observed at 50–80 km altitude at dusk, and 40–60 km at dawn, when water ice likely provides condensation nuclei Six CO2ice clouds are found east of Hellas basin; their formation is likely sourced by surface‐lifted dust at 40–65 km