The Role and Lifetime of Dissociative Heterogeneous Processes in Improving Simulated Ozone on Mars

Ozone simulated in Mars Global Climate Models (MGCMs) is used to assess the underlying chemistry occurring in the atmosphere. Currently, ozone total column abundance (TCA) is under-predicted in MGCMs by up to 120%, implying missing or inaccurate chemistry in models. Heterogeneous reactions of hydroxyl radicals (HO_X) have been offered as an explanation for some of this bias, because they cause ozone to increase at locations where it's currently under-predicted. We use four simulations to compare modeled ozone TCA with observations from the UVIS spectrometer aboard the ExoMars Trace Gas Orbiter to improve the representation of heterogeneous processes and their impact on ozone. We use a gas-phase only run, a dissociative scheme, an adsorbed HO_X retention scheme, and a hybrid scheme that combines the dissociative mechanism with the retention of HO_X on water ice. We find retention of HO_X is dependent on water ice sublimation, and ozone abundance increases when water ice persists for longer periods (1–20 sols). Over time, the loss of HO_X causes a depletion in H₂O₂ concentration (HO_X reservoir), and thus allows ozone concentration to increase. When adsorbed HO_X are desorbed and dissociate into other by-products, HO_X are not immediately available to destroy ozone. This results in larger ozone concentrations than if desorbed HO_X are released directly back into their gaseous states. When using the hybrid scheme, ozone TCA is increased up to 50% where the ozone deficit is greatest, demonstrating the best agreement with observations, and implying that HO_X radicals are both retained when adsorbed and dissociate.