The NOMAD instrument suite on the ESA-Roskosmos ExoMars Trace Gas Orbiter (TGO) observes the physical and chemical composition of the Martian atmosphere with highly resolved vertical profiles and nadir sounding in the IR and UV-vis domains. Vertically resolved profiles of many species (water vapor, HDO, ozone, CO, CO2, oxygen airglow, … ) and of dust and clouds were obtained for more than one Martian year [1-6]. In particular, the simultaneous detection of H2O and HDO in highly resolved profiles provide a unique dataset allowing to investigate present-day fractionation of water vapor on Mars . We will provide simulations with the GEM-Mars General Circulation Model (GCM) [7-9] of HDO and the fractionation of water vapor upon cloud formation. The simulations will be compared in detail with the vertical profiles of the D/H ratio obtained from NOMAD observations. During its first year of operations, NOMAD witnessed the 2018 Global Dust Storm (GDS) during its onset, peak and decline. The redistribution of water vapor to high altitudes and latitudes observed during the GDS was explained using the GEM-Mars GCM . The impact of the GDS on D/H can be estimated from these simulations, and is confirmed by the data. GEM-Mars also includes atmospheric chemistry calculations , and we compare these to several of the new observational datasets obtained by NOMAD. As the photolysis products of water vapor are a major driver for the atmospheric chemistry on Mars, the redistribution of water vapor over the atmosphere during the GDS is expected to have considerable impact on many other species. We present some results of the simulated impact of the GDS on atmospheric chemistry and on several of the observed species.
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