Impacts of Heterogeneous Chemistry on Vertical Profiles of Martian Ozone. Issue 11 (10th November 2022)

• Record Type: Journal Article
• Authors: Brown, M. A. J.; Patel, M. R.; Lewis, S. R.; Holmes, J. A.; Sellers, G. J.; Streeter, P. M.; Bennaceur, A.; Liuzzi, G.; Villanueva, G. L.; Vandaele, A. C.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-11-10
• Subjects: Mars -- atmosphere -- ozone -- heterogeneous chemistry -- photochemistry -- water ice; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2022JE007346
• Languages: English
• ISSNs: 2169-9097

Abstract

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 Nadir and Occultation for Mars Discovery instrument. This is particularly apparent during L S = 0°–180°, Mars Year 35 at high southern latitudes, when the water vapor abundance is low. Ozone and water vapor are anti‐correlated on Mars; Clancy et al. (2016, https://doi.org/10.1016/j.icarus.2015.11.016 ) also discuss the anti‐correlation between ozone and water ice. 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 by creating a separate tracer for the HO x adsorbed state, causes ozone abundance to increase when water ice is present (30–50 km), better matching observed trends. When water vapor abundance is high, there is no consistent vertical correlation between observed ozone and water ice and, in simulated scenarios, the heterogeneous chemistry has a minor influence on ozone. HO x, which are by‐products of water vapor, dominate ozone abundance, masking the effects of heterogeneous chemistry on ozone, and making adsorption of HO x have a negligible impact on ozone. This is consistent with gas‐phase‐only modeled ozone, showing good agreement with observations when water vapor is abundant. Overall, the inclusion of heterogeneous chemistry improves the ozone vertical structure in regions of low water vapor abundance, which may partially explain GCM ozone deficits. Plain Language Summary: Ozone gas is found in small quantities in the martian atmosphere, highly variable both in time and space. Ozone quantity is controlled by photochemical reactions involving other species too difficult to detect with remote sensing. Two main ways ozone is destroyed in the martian atmosphere are by: (a) absorbing ultraviolet sunlight; (b) reacting with hydroxyl radicals, highly reactive chemicals formed by water vapor absorbing ultraviolet sunlight. The latter leads to a well‐known anti‐correlation between ozone and water vapor. Ozone is underpredicted in climate models, implying chemical reactions are missing or incorrect in models. We investigate reactions between hydroxyl radicals and water ice as a potential explanation for the ozone underprediction by using a model and observed vertical profiles. We find observed ozone and water ice have a positive vertical correlation and, when simulating a model with improved chemistry, ozone abundance increases at altitudes where water ice is present due to the decrease in hydroxyl radicals. However, in areas where water vapor is abundant, no such correlation is seen and the chemistry has little impact on modeled ozone. This is due to large abundances of hydroxyl radicals which inhibit the increase in ozone caused by the addition of the improved chemistry. Key Points: Observed ozone and water ice profiles show a positive vertical correlation, contrary to global ozone and water vapor column anti‐correlation Heterogeneous chemistry increases ozone abundance at altitudes where water ice is present, matching locations where ozone is underpredicted High hydroxyl radical abundance, assumed proportional to high water vapor abundance, masks the effects of heterogeneous chemistry on ozone