Abstract
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
(LS=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 HOX. 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. HOX, 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 HOX 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.