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 (L_S=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 HO_X. 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. HO_X, 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 HO_X 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.