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New Theoretical and Laboratory Rates of HOx Reactions and Impact on Models of Mars Atmospheric Chemistry

Presentation #213.04 in the session Martian Aurora, Atmosphere, Winds, and Dust (Poster)

Published onOct 23, 2023
New Theoretical and Laboratory Rates of HOx Reactions and Impact on Models of Mars Atmospheric Chemistry

The rates of many of the key radical reactions used in photochemical models of Mars atmosphere are are poorly characterized or unknown, especially at the low temperatures of Mars. Uncertainties in both the rates and deuterium Kinetic Isotope Effect (DKIE, most of which are not measured at all) in hydrogen radical (HOx) chemistry can have a quantitative impact on modeling the gas phase processes that influence hydrogen escape and H2O loss from Mars. We have re-examined some of the key reactions that could affect HOx chemistry and H/D ratios in the Mars atmosphere, using both computational and laboratory studies.

The primary photochemical pathway for molecular hydrogen H2 formation in the lower Mars atmosphere is the reaction H + HO2 -> H2 + O2 which is a minor channel. Laboratory studies are challenging, due to complexities of competing reactions, and there have been few reports in the literature. We have performed accurate quantum chemistry and transition state theory calculations on the absolute reaction rate and branching ratio, the temperature dependence, and the deuterium kinetic isotope effect. Our results indicate that the rates of H2 formation are significantly differ from previous results, and that the DKIE used in current models is incorrect.

There is limited information on the formation of HO2 by the termolecular recombination reaction H + O2 + M -> HO2 + M under the conditions of the Martian atmosphere. Most critically, photochemical models typically rely on rates measured using M=N2. We have performed experiments showing that the rate is significantly faster when M=CO2.

The impact of these changes on modeling of the hydrogen and oxygen chemistry in the lower Mars atmosphere will be discussed.

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