Investigating the relationship between ozone and water-ice clouds using retrieved data from the ExoMars Trace Gas Orbiter

Brown, M. A. J.; Patel, M. R.; Lewis, S. R. and Bennaceur, A. (2020). Investigating the relationship between ozone and water-ice clouds using retrieved data from the ExoMars Trace Gas Orbiter. In: British Planetary Science Conference 2020, 13-15 Jan, Oxford.

Abstract

This project will use retrievals of ozone and ice-water clouds in the martian atmosphere from the ultraviolet (UV) spectrometer, UVIS, aboard the Exo-Mars Trace Gas Orbiter (TGO). Ozone abundance will be mapped and compared to the water-ice opacity and their interaction will be used to assess the atmospheric chemistry between ozone, water-ice and hydroxyl radicals. Hydroxyl photochemistry may be indicated by a non-negative or fluctuating correlation between ozone and water-ice, which will contribute to understanding the stability of carbon dioxide and chemistry of the martian atmosphere.

Introduction

Ozone (O3) is a trace gas found in the martian atmosphere e.g. [1], which can be used for tracking general circulation of the atmosphere [2] and trace chemicals, as well as acting as a proxy for water vapour [3]. The photochemical break down of water vapour produces hydroxyl radicals, which are trace gases known to participate in the destruction of ozone [4, 5]. As a result, the relationship between water vapour and ozone follows an anti-correlation. Water-ice may also follow this theory [6].
The photochemical reactions between ozone, water-ice clouds and hydroxyl radicals are poorly understood in the martian atmosphere due to the short life-time and rapid reaction rates of hydroxyl radicals [7, 8]. These reactions are essential for the destruction of ozone, as well as indirectly contributing to the water cycle and stability of carbon dioxide (measured by the CO2 –CO ratio) in the martian atmosphere [5, 9, 10].
The detection of ozone in the presence of water-ice clouds suggests that the preexisting relationship between water-ice and ozone is not always anti-correlated[7]. Global climate models (GCMs) are in disagreement of the chemical processes occurring within water-ice clouds [4, 6, 11]. For example, the heterogeneous photochemistry described in the LMD (Laboratoire de Météorologie Dynamique) GCM did not significantly improve the model [6].
This leads to the following questions: what is the relationship between water-ice clouds and ozone, and can the chemical reactions of hydroxyl radicals occurring within water-ice clouds be determined through this relationship?

Methodology

This project aims to address these topics using nadir and occultation retrievals from the ExoMars Trace Gas Orbiter of ozone and water-ice clouds. Nadir and Occultation for MArs Discovery (NOMAD) is an instrument aboard the TGO, which contains a UV and visible spectrometer, UVIS [12, 13]. The spectrometer takes observations in two modes; nadir (total column) and occultation (vertical profile). The observations are used in an inverse process called a retrieval. DISORT is the radiative transfer model used in the retrieval, which calculates spectra with a priori of albedo, water-ice, dust and ozone. Within the iterative process, the radiative transfer model simulates spectra which are statistically compared against the observed spectra using a chi-squared test. If the test fails, the a priori are adjusted and the iteration repeats until the fit between the simulated and observed spectra is determined to be reasonable.
The data provided by UVIS will be more complete spatially and temporally than other datasets by previous instruments, reducing uncertainty and potentially increasing the reliability of the results. Analysis will include temporal and spatial binning of data to help visualize any patterns in these dimensions. Correlation tests will be conducted to determine the significance of the relationship on short term and seasonal dimensions along a range of zonally averaged latitudes.

Hypothesis

Water-ice clouds may act as a sink for hydroxyl radicals, implying that there should not be an anti-correlation between ozone and water-ice.
Ozone abundance is greatest in the winter at the polar regions, which also coincides with the appearance of the polar hood clouds [14, 15]. The use of nadir observations will enable the comparison between total column of ozone abundance at high latitudes (>60°S) in a range of varying water-ice cloud opacities, as well as the equatorial region (30°S – 30°N) during aphelion. Water-ice clouds may remove hydroxyl radicals [7, 11] and thus the suggested anticorrelation between ozone and water-ice will not hold.

Summary

Interactions between hydroxyl radicals and the surface of water-ice clouds are poorly understood. This project will address these interactions by assessing the relationship between ozone and water-ice clouds on different temporal and spatial resolutions using retrieved data from the UVIS instrument aboard the TGO. Nadir and occultations will be used in short-term and seasonal timeframes in the polar regions and during aphelion in the equatorial region.

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