Ozone Variations due to Heterogeneous Chemistry in the Martian Atmosphere

Brown, Megan (2023). Ozone Variations due to Heterogeneous Chemistry in the Martian Atmosphere. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.000166be

Abstract

Martian atmospheric ozone is a chemically sensitive trace gas, which can be used as a proxy for short-lived atmospheric species and underlying chemical processes. There is an ozone deficit in global climate simulations with photochemical schemes, implying there is currently a lack of understanding of martian chemistry. As a consequence of this, climate models have missing or inaccurate chemical reactions. In this work, heterogeneous chemistry is investigated as an explanation for some of the ozone deficit in models when compared to observations. Recent observations from the Nadir and Occultation for MArs Discovery instrument, aboard the Trace Gas Orbiter provide vertical profile and total column abundance (TCA) measurements of ozone, allowing the vertical distribution, as well as the global spatial and temporal variation of ozone to be analysed. In this thesis, a heterogeneous scheme has first been developed in a 1-D model, implemented into a global climate model, and then further developed by combining different approaches and assumptions to model realistic and representative heterogeneous reactions. Ozone is highly sensitive to small fluctuations in hydroxyl radicals (OH and HO₂ collectively known as HO_x ), due to rapid chain reactions HO_x undergo, destroying ozone. The heterogeneous chemical scheme in this work shows that water ice acts as a sink for HO_x , and ozone abundance increases at altitudes water ice forms. This increase in ozone abundance is dependent on the water vapour TCA, with a greater water vapour TCA leading to a reduction in the increase in ozone, due to the larger abundance of HO_x. Heterogeneous reactions are most effective at increasing ozone abundance at polar regions, particularly during the winter season when water vapour TCA is low and water ice polar hood clouds are present. It is during this season and at polar regions that the greatest deficit in ozone abundance occurs. Different desorption mechanisms of adsorbed HO_x result in a similar increase in ozone TCA, but due to different underlying chemistry. When these different mechanisms are combined together to produce a better representation of heterogeneous chemistry, ozone TCA is, under certain circumstances, greater than either of the heterogeneous schemes, further reducing the deficit between model and observation. Adsorbed HO_x interact with themselves at the surface of water ice and dissociate into other by-products when desorbed. The length of time HO_x remain adsorbed onto water ice is dependent on the lifetime of water ice clouds. The more interactions adsorbed HO_x undertake and the greater the dissociation, the larger the ozone abundance. The heterogeneous chemical schemes developed in this work cannot explain all of the deficit between models and observations. However, the inclusion of these reactions increases ozone abundance by up to 56% (observations are currently 120% greater than models) by reducing HO_x concentration within water ice clouds and causing a decrease in ozone destruction.

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