Dissertation/Thesis Abstract

Aerosol chemistry on the early Earth and Titan
by DeWitt, H. Langley, Ph.D., University of Colorado at Boulder, 2010, 116; 3404045
Abstract (Summary)

Understanding the atmospheric conditions before and during the period when life first appeared on the planet, approximately 3.5-4 billion years ago (Ga) is an important part of understanding the conditions under which life developed. Titan, a moon of Saturn, is often cited as a possible analogue to early Earth's atmospheric conditions: it is made up of primarily nitrogen (N2) with a few percent methane (CH4) that is photolyzed into a thick organic haze, obscuring the surface of the moon. The evidence of liquid water at 4 billion years ago, despite the faintness of the young sun, necessitates the presence of larger amounts of greenhouse gases in the early Earth's atmosphere. CH4, carbon dioxide (CO 2), and hydrogen (H2) have been proposed to have been present in the early Earth's atmosphere in elevated amounts, varying in amounts with the rise of methanogens on the surface of the Earth. Unlike present Earth, the presence of sulfur mass independent fractionation (S-MIF) in sediments older than 2.45 Ga is thought to be evidence for an early anoxic atmosphere. To preserve this signal, the tropospheric formation of at least two different forms of sulfur-containing aerosols, usually thought to be elemental sulfur (S8) and sulfuric acid (H2SO4), would be necessary. Thus, CH4, CO2, H2, O2, and H2O could influence the chemistry of sulfur-bearing aerosols. In this work, we use aerosol mass spectrometry (AMS) to examine the aerosols formed form the photolysis (to simulate the solar spectrum, at wavelengths from 115-400 nm) and electrical discharge (to simulate lightning on the early Earth and the free electrons present in Titan's upper atmosphere) of varying gas mixtures under different laboratory conditions to probe the chemistry of early Earth and Titan aerosols. We examined the N-incorporation into organic aerosols formed from the electrical discharge of 2% CH4 in N 2 to simulate the haze formed in the upper atmosphere of Titan. We found an N/C ratio of 0.25 and determined that the majority of the N-containing organic signal was derived from nitriles. We also probed the effect of H 2 on the photolysis reactions of CH4 and CO2 in N2 and the effect H2 had on the relative amounts of organic haze formed. We found that H2 reduced organic haze formation and thus would prevent any cooling from scattering of incoming solar radiation away from the lower atmosphere and Earth's surface by an overly thick aerosols layer. Finally, when we examined the chemistry of SO2, CH 4, H2O, and CO2, we found that S8 aerosols formed only under very reduced circumstances and that the pressure of SO2, the amount of H2O vapor present, and the amount of reducing gases (such as CH4 or H2) could change the S-aerosol chemistry. We found that organo-sulfur compounds could be another type of S-bearing aerosol important for the preservation of S-MIF signal and could have been prevalent in the early Earth's atmosphere under reasonable atmospheric conditions. Understanding the chemical properties of these aerosols will broaden our understanding of the conditions under which complex organic molecules were formed and life began to develop.

Indexing (document details)
Advisor: Tolbert, Margaret A.
Commitee: Jimenez, Jose L., Koval, Carl, Toon, Owen B., Vaida, Veronica
School: University of Colorado at Boulder
Department: Chemistry
School Location: United States -- Colorado
Source: DAI-B 71/06, Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Atmospheric Chemistry, Analytical chemistry
Keywords: Aerosols, Archean, Methane, Nitrogen, Prebiotic atmosphere
Publication Number: 3404045
ISBN: 9781109782233
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