The Open University's repository of research publications and other research outputs Martian atmospheric temperature and density profiles during the 1st year of NOMAD/TGO solar occultation measurements

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We present CO density profiles up to about 100 km in the Martian atmosphere obtained for the first time from retrievals of solar occultation measurements by the Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). CO is an important trace gas on Mars, as it is controlled by CO2 photolysis, chemical reaction with the OH radicals, and the global dynamics. However, the measurements of CO vertical profiles have been elusive until the arrival of TGO. We show how the NOMAD CO variations describe very well the Mars general circulation. We observe a depletion of CO in the upper troposphere and mesosphere during the peak period, LS = 190°–200°, more pronounced over the northern latitudes, confirming a similar result recently reported by Atmospheric Chemistry Suite onboard TGO. However, in the lower troposphere around 20 km, and at least at high latitudes of the S. hemisphere, NOMAD CO mixing ratios increase over 1,500 ppmv during the GDS (Global Dust Storm) onset. This might be related to the downwelling branch of the Hadley circulation. A subsequent increase in tropospheric CO is observed during the decay phase of the GDS around LS = 210°–250° when the dust loading is still high. This could be associated with a reduction in the amount of OH radicals in the lower atmosphere due to lack of solar insolation. Once the GDS is over, CO steadily decreases globally during the southern summer season. A couple of distinct CO patterns associated with the Summer solstice and equinox circulation are reported and discussed.

CO is produced by the photodissociation of CO 2 and recycled to CO 2 by the catalytic cycle involving HOx in the Martian atmosphere (e.g., McElroy & Donahue, 1972). The photochemical lifetime of CO is ∼6 years in the lower atmosphere (Krasnopolsky, 2007). The previous nadir observations revealed latitudinal and seasonal distributions of CO in the lower atmosphere, which indicate CO 2 condensation/sublimation in the polar caps and dynamics (Encrenaz et al., 2006;Smith et al., 2009Smith et al., , 2021. In the middle and upper atmosphere (>∼50 km), the photochemical lifetime of CO becomes much longer due to the decrease in HOx species density. Thus, the characteristic times of production and eddy diffusion of CO are shorter than the photochemical lifetime of CO in

Ozone (O 3) is important in the stabilisation of CO 2 in the Martian atmosphere and thus it is important to study the spatio-temporal variability of O 3. We retrieve two years of total columnar O 3 from raw spectral data provided by the SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument aboard the Mars Express. The seasonal variability is studied in tropical, mid-and high latitudes and is compared with simulations by a photochemical coupled general circulation model (GCM). The high latitudes exhibit the largest seasonal variations in O 3 , with a winter high and a summer low and a comparison with GCM results is good in general. We have studied the correlation of O 3 with dust, retrieved simultaneously from SPICAM observations. In southern tropical latitudes, the columnar O 3 is seen to increase during a global dust storm year (Martian year (MY) 28) compared to the O 3 column values during a year without global dust storm (MY27), although the water vapour column between these years remains unchanged. This indicates the radiative impact of dust on O 3 and its retrieval. We also study the ozone-carbon monoxide correlation as a tracer of dynamics. The dynamical contribution to the O 3 column is found to be the highest during winter over the southern polar region.