Modeling atmospheric perchlorate and chlorate

Abstract

Naturally occurring perchlorate (ClO4–) and chlorate (ClO3–) have been observed in different terrestrial and extraterrestrial environments. On Mars, per/chlorate may play important roles in driving oxidation of near-surface minerals, degrading chemical fossils of extinct life, and creating potentially habitable liquid brines for extant life. To date, it is still very unclear how most of the natural per/chlorate form on Earth and Mars. In this work, I study how the atmospheric oxidation of chlorine-containing species contributes to natural per/chlorate formation via a modeling approach. I use the GEOS-Chem global 3-D chemical transport model to simulate the production, loss, transport, and deposition of atmospheric per/chlorate. The model predictions are compared to observations of aerosol per/chlorate concentration, per/chlorate deposition flux, and isotopic composition of per/chlorate in remote desert soils. In Chapter 1, I give an overview of the atmospheric chemistry of chlorine species and the significance of per/chlorate in Astrobiology. In Chapter 2, I present the first global 3-D simulation for atmospheric perchlorate and find that gas-phase production in the stratosphere alone cannot explain measurements of 17O excess in perchlorate in remote deserts. In Chapter 3, I develop the first-ever model of atmospheric chlorate and show that chlorate likely decomposes in acidic aerosols. In Chapter 4, I demonstrate that multiphase oxidation of chlorine species on aerosols and polar stratospheric clouds can be an important source of per/chlorate in the atmosphere based on an extensive review of the literature and model experiments. In Chapter 5, I advocate for new research in laboratory and field measurements of per/chlorate via novel mass spectrometry techniques, model re-interpretation of ice-core records of perchlorate, and the development of atmospheric models for simulating halogen chemistry on Mars.