Graduate Thesis Or Dissertation

 

The Effects of Trace H2S in Laboratory Experiments of Planetary Organic Haze Chemistry Public Deposited

https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/9880vs35v
Abstract
  • Planetary organic hazes and atmospheric sulfur gases are each common in planetary atmospheres of the solar system and likely exoplanetary atmospheres. Previous thought was atmosphere sulfur would eventually lead to inorganic S8 or H2SO4 aerosol, while, separately, methane (CH4) and other carbon gases would lead to organic aerosol (i.e. organic haze). However, the likely co-occurrence of organic haze and sulfur gases in planetary atmospheres invokes the possibility of a coupled chemistry between the two. Thus, the respective chemistries of organic haze and atmospheric sulfur have been historically separated. Therefore, the main objective of this thesis is to explore the coupling between sulfur and organic haze chemistry. 

    In this thesis, I present the results of laboratory experiments exploring how trace amounts of H2S influence the compositional, physical, and optical properties of planetary organic haze analogs. I investigate the chemistry of organic haze formed in reducing (CH4/H2S gas mixtures in N2) and weakly reducing (CO2/CH4/H2S gas mixtures in N2) atmospheric conditions. In the experiments, I used quadrupole aerosol mass spectrometry (Q-AMS) to determine the bulk haze composition and mass loading. To determine the haze particle size distributions and number concentrations, I used a scanning mobility particle sizer (SMPS). Finally, I used photoacoustic spectroscopy coupled with cavity ringdown spectroscopy (PASCaRD) with two wavelengths of light (405 nm and 532 nm) to determine the optical properties of the haze particles.

    For reducing atmospheric conditions, I explored the haze aerosol compositional, physical, and optical properties as of function of trace H2S (0-20 ppmv) while CH4 remained constant at 0.1%. I found that the inclusion of trace H2S in the haze chemistry resulted in the formation oforganic reduced sulfur (ORS) compounds. Further, the amount of total organic aerosol formed and the aerosol particle effective density increased as a function of increasing H2S. In studying the haze aerosol optical properties, I found that the real refractive index (n, scattering) at 402 and 532 nm light increased with increasing H2S mixing ratios. I found that the imaginary refractive index (k, absorption) also generally increased with increasing H2S mixing ratios, but only at 402 nm. The k at 532 nm did not exhibit a discernable trend. 

    For weakly reducing atmospheric conditions, I explored the haze aerosol composition as a function of the CO2 mixing ratio (0-2%) while keeping the CH4 and H2S mixing ratios constant (0.1% and 5 ppmv, respectively). I found the aerosol mass loading increased with increasing CO2. Moreover, in the experiments with H2S, the organic aerosol production did not decrease with higher values of CO2:CH4 (>~1:1) as in experiments without H2S. Finally, I found evidence that both inorganic sulfate aerosol and organic oxidized sulfur (OOS) aerosol form in the experiments with H2S. At CO2 mixing ratios <0.5%, the total sulfate signal was entirely attributed to OOS. The contribution of inorganic sulfate to the total sulfate signal increased with the CO2 mixing ratio.

    The results of my thesis show that trace H2S greatly influences organic haze composition, physical properties, and optical properties in both CH4/N2 and CO2/CH4/N2 hazes. The observation of organic sulfur aerosol, a previously unconsidered sulfur reservoir, has potential implications for the understanding of the history and evolution of Earth’s atmosphere. Further, these results present an abiotic synthesis of biologically relevant compounds such as thiols, sulfides, and OOS. The H2S-induced changes in optical properties of haze aerosol can impact a haze’s anti-greenhouse capacity, in turn affecting a planet’s climate and habitability. Thus, my results show that trace H2S has the potential to greatly influence the chemistry and climate impact of an organic haze. Future work regarding organic haze, therefore, needs to consider the potential impacts of trace H2S. 

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  • 2023-04-14
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  • 2024-01-09
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