Detecting trace gases such as hydrogen chloride (HCl) in Mars' atmosphere is among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Terrestrially, HCl is closely associated with active volcanic activity, so its detection on Mars was expected to point to some form of active magmatism/outgassing. However, after its discovery using the mid-infrared channel of the TGO Atmospheric Chemistry Suite (ACS MIR), a clear seasonality was observed, beginning with a sudden increase in HCl

Super-rotation affects - and is affected by - the distribution of dust in the martian atmosphere. We modelled this interaction during the 2018 global dust storm (GDS) of Mars Year 34 using data assimilation. Super-rotation increased by a factor of two at the peak of the GDS, as compared to the same period in the previous year which did not feature a GDS. A strong westerly jet formed in the tropical lower atmosphere, with strong easterlies above 60 km, as a result of momentum transport by thermal tides. Enhanced super-rotation is shown to have commenced 40 sols before the onset of the GDS, due to equatorward advection of dust from southern mid-latitudes. The uniform distribution of dust in the tropics resulted in a symmetric Hadley cell with a tropical upwelling branch that could efficiently transport dust vertically; this may have significantly contributed to the rapid expansion of the storm.

The vertical opacity structure of the martian atmosphere is important for understanding the distribution of ice (water and carbon dioxide) and dust. We present a new dataset of extinction opacity profiles from the NOMAD/UVIS spectrometer aboard the ExoMars Trace Gas Orbiter, covering one and a half Mars Years (MY) including the MY 34 Global Dust Storm and several regional dust storms. We discuss specific mesospheric cloud features and compare with existing literature and a Mars Global Climate Model (MGCM) run with data assimilation. Mesospheric opacity features, interpreted to be water ice, were present during the global and regional dust events and correlate with an elevated hygropause in the MGCM, providing further evidence for the role of regional dust storms in driving atmospheric escape as reported elsewhere. The season of the dust storms also had an apparent impact on the resulting lifetime of the cloud features, with events earlier in the dusty season correlating with longer-lasting mesospheric cloud layers. Mesospheric opacity features were also present during the dusty season even in the absence of regional dust storms, and interpreted to be water ice based on previous literature. The assimilated MGCM temperature structure agreed well with the UVIS opacities, but the MGCM opacity field struggled to reproduce mesospheric ice features, suggesting a need for further development of water ice parameterizations. The UVIS opacity dataset offers opportunities for further research into the vertical aerosol structure of the martian atmosphere, and for validation of how this is represented in numerical models.

On Mars, saturation is the major factor constraining the vertical distribution of water vapor. Recent measurements of water and temperature profiles showed that water can be strongly supersaturated at and above the level where clouds form during aphelion and perihelion seasons. Since 2018, the near-infrared spectrometer (NIR) of the Atmospheric Chemistry Suite onboard the Trace Gas Orbiter has measured H2O and temperature profiles using solar occultation in the infrared from below 10 km to 100 km of altitude. Here we provide the first long-term monitoring of the water saturation state. The survey spans 2 Martian years from Ls=163° of MY34 to the Ls=180° of MY36. We found that water is often supersaturated above aerosol layers. In the aphelion season, water mixing ratio above 40 km in the mid-to-high latitudes was below 3 ppmv and yet is found to be supersaturated. Around perihelion, water is also supersaturated above 60 km with a mixing ratio of 30-50 ppmv. Stronger saturation is observed during the dusty season in MY35 compared to what was observed in MY34 during the Global Dust Storm and around perihelion. Saturation varied between evening and morning terminators in response to temperature modulation imparted by thermal tides. Although water vapor is more abundant in the evening, colder morning temperatures induce a daily peak of saturation. This dataset establishes a new paradigm for water vapor on Mars, revealing that supersaturation is nearly ubiquitous, particularly during the dust season, thereby promoting water escape on an annual average.

Lower atmosphere variations in the martian water vapour and hydrogen abundance during the Mars Year (MY) 34 C storm from LS=326.1◦-333.5◦ and their associated effect on hydrogen escape are investigated using a multi-spacecraft assimilation of atmospheric retrievals into a Martian global circulation model. Elevation of the hygropause and associated increase in middle atmosphere hydrogen at the peak of the MY 34 C storm led to a hydrogen escape rate of around 1.4×109 cm−2s−1 , meaning the MY 34 C storm enhanced water loss rates on Mars to levels observed during global-scale dust storms. The water loss rate during the MY 34 C storm (a loss of 15% of the total annual water loss during only 5% of the year) was three times stronger than the weak MY 30 C storm assimilation, demonstrating that interannual variations in C storm strength must be considered when calculating the integrated loss of water on Mars.

We show a positive vertical correlation between ozone and water ice using a vertical cross-correlation analysis with observations from the ExoMars Trace Gas Orbiter’s NOMAD instrument. We find this is particularly apparent during the first half of Mars Year 35 (LS=0-180) at high southern latitudes, when the water vapour abundance is low. This contradicts the current understanding that ozone and water are, in general, anti-correlated. However, our simulations with gas-phase-only chemistry using a 1-D model show that ozone concentration is not influenced by water ice. Heterogeneous chemistry has been proposed as a mechanism to explain the underprediction of ozone in global climate models (GCMs) through the removal of HOX. We find improving the heterogeneous chemical scheme causes ozone abundance to increase when water ice is present, better matching observed trends. When water vapour abundance is high, there is no consistent vertical correlation between observed ozone and water ice and, in simulated scenarios, the heterogeneous chemistry does not have a large influence on ozone. HOX, which are by-products of water vapour, dominate ozone abundance and mask the effects of heterogeneous chemistry on ozone. This is consistent with gas-phase-only modelled ozone, showing good agreement with observations when water vapour is abundant. High water vapour abundance masks the effect of heterogeneous reactions on ozone abundance and makes adsorption of HOX have a negligible impact on ozone. Overall, the inclusion of heterogeneous chemistry improves the ozone vertical structure in regions of low water vapour abundance, which may partially explain GCM ozone deficits.

We present ~1.5 Mars Years (MY) of ozone vertical profiles, covering Ls = 163deg; in MY34 to Ls = 320deg; in MY35, a period which includes the 2018 global dust storm. Since April 2018, the Ultraviolet and Visible Spectrometer (UVIS) channel of the Nadir and Occultation for Mars Discovery (NOMAD) spectrometer aboard the ExoMars Trace Gas Orbiter has observed the vertical, latitudinal and seasonal distributions of ozone. Around perihelion, the relative abundance of ozone (and water from coincident NOMAD measurements) increases strongly together below ~40 km. Around aphelion, decreases in ozone abundance exist between 25-35 km coincident with the location of modelled peak water abundances. We report high latitude (above 55deg;), high altitude (40-55 km) equinoctial ozone enhancements in both hemispheres. The northern equinoctial high altitude enhancement is previously unobserved and forms prior to vernal equinox lasting for almost 100 sols (Ls ~350‑40deg), whereas the southern enhancement persists for over twice as long (Ls = ~5-140deg;). Both layers reform at autumnal equinox, with the northern layer at a lower abundance. These layers likely form through a combination of anti-correlation with water and the equinoctial meridional transport of O and H atoms to high-latitude regions. The descending branch of the main Hadley cell shapes the ozone distribution at Ls = 40-60deg;, with the possible signature of a northern hemisphere thermally indirect cell identifiable from Ls = 40-80deg;. The ozone retrievals presented here provide the most complete global description of Mars ozone vertical distributions as a function of season and latitude.