Spectroscopy and Kinetics of Reactive Intermediates in the Atmosphere of Venus: the Catalytic Role of Chlorine Atoms

Citation

Chao, Wen (2024) Spectroscopy and Kinetics of Reactive Intermediates in the Atmosphere of Venus: the Catalytic Role of Chlorine Atoms. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/v9ey-m476. https://resolver.caltech.edu/CaltechTHESIS:05202024-220341626

Abstract

Chlorine chemistry plays essential roles in both industrial and academic fields due to the special reactivity required to initiate or catalyze important reactions in our daily life. For example, Cl atoms are the most common oxidation agents in combustion and are famous for the catalyzed destruction of ozone that eventually leads to the ozone hole.

As terrestrial planets, Venus and Earth’s atmospheres have similar origins but very different evolutions. Compared to Earth, Venus suffers strong water loss in hydrodynamics escape due to its distinct distance from the Sun; as a result, high abundances of chlorine and sulfur species are not trapped in the sea and survive in Venus’ atmosphere. For example, a dense cloud, made with sulfuric acid, has been observed at the middle altitude (50 - 70 km) and the concentration profiles for distinct species (SO₂, CO, CO₂, O₂ ..., etc.) have been measured during the Venus Express mission operated by the European Space Agency. Digging into the discrepancy between the observations and model simulations, a few important questions arose, and further laboratory studies are needed to solve the puzzles, including (1) the unknown UV absorber, (2) the SO₂ concentration inversion at high altitude and (3) the extremely low O₂ and high CO₂ abundances, wherein a reactive chlorine atom is proposed for explaining these phenomena.

In this thesis, we performed the pulsed-laser photolysis experiments with a homemade time-resolved broadband UV-Vis transient-absorption spectroscopy coupled with a temperature- and pressure-controlled flow reactor to study the spectroscopic and kinetic properties of key intermediates (ClSO, ClCO and ClCO₃) in the oxidation process of sulfur and carbon to ultimately form SO₂ and CO₂. The recorded spectra are analyzed, and highlevel ab initio calculations were performed to rationalize the electronic structures of target molecules to reveal the catalysis role of Cl atoms. In addition, key reaction rate coefficients (ClSO + Cl, _kClSO+Cl(292 K) = (1.48 ± 0.42)x10^-11 cm³ molecule^-1 s^-1; ClCO + O₂, _kClCO+O₂(0) = (9.0 ± 2.3) × 10^-32 cm⁶ s^-1 a cm³ molecule^-1 s^-1, and thermodynamic property (Cl + CO ⇌ ClCO, K_eq = 1.8 x10^-18 molecules cm⁻³) have been measured to further assist the model simulations.

This thesis not only offers essential data for model simulations to understand the complex chemistry in Venus' atmosphere but also provide new insights to guide future tasks to explore Venus, e.g. DAVINCI+ and VERITAS by NASA.

Thesis Files