Hydrogen chloride uptake on water ice aerosols in the Martian atmosphere from the ACS/ExoMars data

In 2020, molecules of hydrogen chloride were discovered in the atmosphere of Mars using the ACS (Atmospheric Chemistry Suite) instrument from the ExoMars project [1]. Observations showed a seasonal increase in volume mixing ratio up to 5 ppbv (parts per billion by volume) during perihelion, followed by a sudden drop in values below the detection limit (<0.1 ppbv) [2, 3]. This rapid disappearance of HCl from the atmosphere contradicts the widely accepted estimate of the lifetime of HCl (> 70 Earth days) [3, 4].

According to the laboratory measurements [5], the uptake of HCl onto water ice cloud particles may play a significant role in the removal of hydrogen chloride from the Martian atmosphere [1, 3].

In this work, will demonstrate examples of anticorrelation between HCl and water ice at altitudes above 20 km. This result suggests a rapid mechanism (1-2 Earth days) for the removal of HCl from the Martian atmosphere, which can explain its seasonal fluctuations.
In this work, we have processed ACS data over a period of 2.5 Martian years with 784 solar occultation sessions. From each session, vertical profiles of temperature, concentrations of HCl (if any), water vapor, aerosol extinction, and water ice mass loading were retrieved.

The retrieval of HCl concentration profiles was performed using Levenberg-Marquardt iterative algorithm with Tikhonov regularization from the ACS MIR data covering 2922.5–2927.4 cm⁻¹ spectral range. Temperature and water vapor profiles were retrieved from the ACS NIR data. The determination of the microphysical properties of dust and water ice particles was conducted through simultaneous analysis of ACS NIR data in the spectral range of 0.76–1.6 µm and ACS MIR data in the range of 3.09–3.46 µm. To retrieve vertical profiles of aerosol extinction from the transmission profiles, the "onion peeling" method (inverse Abel transform) was used. From the obtained spectral dependencies of the extinction coefficient, effective radius, number, and mass concentrations of water ice were retrieved.

This work has been funded by grant #23-12-00207 of the Russian Science Foundation.

[1] Korablev et al., 2021. Transient HCl in the atmosphere of Mars. Sci. Adv. 7 (7), 1–9. https://doi.org/10.1126/sciadv.abe4386

[2] Olsen et al., 2021. Seasonal reappearance of HCl in the atmosphere of Mars during the Mars year 35 dusty season. Astron. Astrophys. 1–14 https://doi.org/10.1051/0004-6361/202140329

[3] Aoki et al., 2021. Annual appearance of hydrogen chloride on Mars and a striking similarity with the water vapor vertical distribution observed by TGO/NOMAD. Geophys. Res. Lett. 48 (11), 1–11. https://doi.org/10.1029/2021GL092506.

[4] Krasnopolsky, 2022. Photochemistry of HCl in the martian atmosphere. Icarus 374. https://doi.org/10.1016/j.icarus.2021.114807

[5] Kippenberger et al., 2019. Trapping of HCl and oxidised organic trace gases in growing ice at temperatures relevant to cirrus clouds. Atmos. Chem. Phys. 19 (18), 11939–11951. https://doi.org/10.5194/acp-19-11939-2019.