New insights into the Martian ozone deficit problem from an uncertainty propagation study

Abstract

Photochemical models of the Martian atmosphere have long suffered from a persistent “ozone deficit”. This discrepancy is typically attributed to missing physical or chemical processes, particularly involving odd-hydrogen chemistry. Here, we instead quantify the impact of uncertainties in chemical reaction kinetics on modelled ozone, which has received comparatively little attention. We perform a global sensitivity analysis using the 1-D configuration of the Mars Planetary Climate Model (MPCM) by perturbing all photochemical rates within their uncertainty ranges. Rank correlation coefficients (RCCs) are used to identify reactions that exert the strongest control on ozone variability. We find that reactions governing hydroxyl radical (OH) production and direct ozone loss dominate the anticorrelation with ozone, while oxygen-producing pathways are positively correlated. Several reactions involving the hydroperoxyl radical (HO2) and hydrogen peroxide (H2O2) that do not produce OH also contribute positively. Guided by these results, we implement targeted rate adjustments in the 3-D MPCM. These modifications produce systematic increases in ozone column densities and vertical mixing ratios across all seasons and latitudes, without perturbing the simulated water cycle. The resulting ozone fields show improved agreement with ACS MIR vertical profiles. Our results demonstrate that uncertainties in reaction kinetics can account for a substantial fraction of the ozone deficit, reducing the need to invoke additional processes, such as heterogeneous uptake on dust. We provide a list of key uncertainty reactions that should be prioritized in new lab campaigns under Mars conditions to improve model performance and better quantify biases before implementing new processes.