Photochemistry of HCl in the Martian atmosphere

The recent discovery of HCl on Mars (Korablev et al. 2021, Olsen et al. 2021) indicates its strong seasonal variations and correlation with dust. To study this idea, we develop photochemical models for summer midlatitudes at aphelion and perihelion. We assume that HCl is formed in heterogeneous reactions of FeCl₃ and/or NaCl in the dust with H, and the reaction probability is γ≈0.03 expected for FeCl₃. NaCl is more abundant than FeCl₃, while the energy yield is 2.01 eV for FeCl₃ and 0.18 eV for NaCl, so that the reaction probability may be much greater for FeCl₃. HCl is lost from the atmosphere in our model in a reaction with ice, and γ=3×10^-5. Temperature profiles, column H₂O and dust are based on the MGS/TES observations and our calculations (Smith 2004, Fig. 1, 2).

Fig. 1. Temperature profiles using the TES nadir(red) and limb (blue) observations

Fig. 2. Density, H₂O, dust and H₂O ice profiles in aphelion (blue) and perihelion (red)

Our models involve 35 reactions of the standard CO₂-H₂O-NO chemistry, 38 reactions for 9 chlorine gases, heterogeneous loss of OH, HO₂, and H₂O₂ on ice, and five heterogeneous reactions of chlorine species. Calculated chemical compositions at aphelion and perihelion are shown in Figures 3 and 4. The calculated abundances of ozone, peroxide, and the O₂ dayglow at 1.27 μm agree with observations and other models.

Fig. 3. Chemical composition of mars' atmosphere near aphelion

Fig. 4. Chemical composition of Mars' atmosphere near perihelion

The probabilities of the two major processes of formation and removal of the gaseous chlorine species are chosen to fit the observed HCl mixing ratio of 0.5-5 ppb at 5-35 km in perihelion (Figure 5). The predicted increase in HCl to ≈10 ppb above 55 km in perihelion can be reduced and eliminated, if the dust is significantly reduced above 40 km. However, the model cannot simulate the observed cutoff in HCl above 40 km.

Fig. 5. Vertical profiles of HCl in aphelion and perihelion on Mars in our models and the ACS observations in perihelion. The dashed plot is for no dust above 40 km.

Lifetime of HCl is 70 days in our model for perihelion and explains the lag between the onset of the dust storm at L_S = 190º and the first HCl detections at L_S = 230º. (This interval is 75 days.) The annual cycle of gaseous chlorine can be approximated by its production at L_S =200 to 300º with a mean abundance of 2 ppb and loss beyond this period with the calculated lifetime. Then the expected abundance at aphelion is 0.06 ppb, in accord with the upper limit of 0.1-0.2 ppb, but greater than ≈1.5 ppt calculated at the equilibrium (Figure 5). Our model at two seasons is not aimed to reproduce the complicated dynamics in the HCl observations, including two HCl detections at L_S ≈ 110º. Number densities of the major chlorine species near perihelion and at equilibrium in aphelion are shown in Figure 6. HCl dominates among those.

Fig. 6. Chlorine species on Mars in perihelion (solid lines) and at equilibrium in aphelion (dashed lines)

Chlorine species participate in photochemical cycles that stimulate production of O₂ and net reactions of CO with O and O₂. However, their column rates even at perihelion are smaller than those of the odd hydrogen cycles by four orders of magnitude. Therefore we conclude that the chlorine chemistry on Mars does not significantly affect abundances of the major photochemical products.

References

Korablev, O.I., et al., 2021. Transient HCl in the atmosphere of Mars. Sci. Adv. 7, eabe4386.

Olsen, K.S., et al., 2021. Seasonal reappearance of HCl in the atmosphere of Mars during the Mars year 35 dusty season. Astron. Astrophys. 647, A161.

Smith, M.D., 2004. Interannual variability in TES atmospheric observations of Mars during 1999-2003. Icarus 167, 148-165.