Not only because of the knowledge you taught me, but also because of your kindness and concern for us. You taught me to be nice and kind to the people around me, no matter how stressful it is and how much pressure I'm under. You taught me most of the things I know in the lab, gave me a career-changing opportunity when I had to decide what I wanted out of my graduate career, and gave me honest criticism when I needed it most.
You two have been very kind to me despite all my flaws, talking to me at length about life in general and guiding me through hardships throughout the years I've been here. Erin, you are such an inspiration to me and have become a great friend of mine since we worked together. Thank you for all you have taught me, my research would not have been possible without all your great work.
Introduction: Atmospheric Methane and Its Chemistry
These ice core records revealed that the current average global methane concentration is the highest in the last 420,000 years and has increased by a factor of 2.5 since industrialization. (Etheridge et al., 1998; Petit et al., 1999) Atmospheric methane monitoring since 1983 has given us more understanding of it. However, methane concentration has started to increase rapidly since 2007 at more than 10 times the rate between 2000 and 2006. (Butler and Montzka, 2017; Dlugokencky et al., 2011; Saunois et al., 2016b) Despite the environmental and economic importance of it, many of the formation and destruction mechanisms remain poorly constrained. It has been suggested that this recent rapid rise is mostly biogenic with smaller contributions from fossil fuel uses and wetland contributions. (Saunois et al., 2016b) This rapid change in the last decade calls for a response in methane source identifications and possibilities of emission control.
Tropospheric OH oxidation accounts for about 90% of the total methane sink. (Kirschke et al., 2013) Due to the high temperature of the reaction with OH and the global distribution of OH radicals, the oxidation of methane with OH mainly occurs in the lower to middle tropics. 3 shows possible methane sources and Martian methane sinks. (Atreya et al., 2007; Chassefière and Leblanc, 2011) In the Earth's atmosphere, more than 90% of methane molecules originate from biological activities. In the presence of certain catalysts, H2 can reduce carbon dioxide to methane by a Fischer-Tropsch (FTT) type synthesis. (Abrajano et al., 1990; Berndt et al., 1996; Neubeck et al., 2011).
Mid-Infrared Cavity Ring-down Spectroscopy Measurements of Atmospheric
Cavity pressure is monitored with a 1000 Torr Baratron absolute capacitance manometer (MKS 626C13TBE). The simpler Voigt profile was chosen because of the complex absorption profiles of the ethane transitions. The remaining samples were then expanded into the cavity and equilibrated to measure ethane abundance.
Ethane to methane ratio as a function of the isotopic content of the carbon dioxide in the same sample. Measurements of C2H6 came from the average of the spectra collected in the spectral window reported in this work. The laser beam was then manipulated to be smaller than the active area of the AOM crystal prior to AOM.
Beam profile of the beams through the AOM measured 5 inches away from the AOM output. HITRAN simulation of spectral lines of ethane, methane and water in the tuning range of the laser.
Soil Microbial Response to A Massive Natural Gas Leak: Case Study of the Porter
These leaks offset the advantage of gas over oil. (Brandt et al., 2014) Estimates of the extent of NG leakage are highly variable. Variants of MMO enzymes prefer longer chains. (Sayavedra-Soto et al., 2011; Van Beilen and Funhoff, 2007) The ability to survive on methane as the sole source of carbon and energy (methanotrophy) is usually an obligate lifestyle, but some MOBs are able to utilize methane. Literature describing MOB in semi-arid soils is limited, despite the documented role of semi-arid soils as methane sinks and that 20% of the land mass consists of such soils. (Aronson et al., 2013) Moisture content, pH, and temperature drive methane biooxidation in these soils hosting both cultivated and untreated MOBs. (Angel and Conrad, 2009; Horz et al., 2005; Hou et al., 2012; Judd et al., 2016; Smith et al., 2000) The extent of the natural gas leak at Porter Ranch and its localization to the chaparral biome offers a unique an opportunity to study the natural mechanisms of alkane cycling in semi-arid soils and also to better understand the pathways of methane consumption during leakage.
In the SS-25_well soils, a shift in the bacterial community towards gammaproteobacteria ("gamma shift") was detected. these soils, approximately twice the value seen in the background samples. Sequences associated with facultative MOBs in the family Beijenrickiaceae (e.g. Methlocapsa, Methylocella and the upland soil group, "USC"), were detected in very low abundance (< 0.1 % of reads) sporadically throughout the set of data. (Ricke et al., 2005) These sequences were present in 19% of background cores (highest abundance = 0.07% of reads), and in 41% of SS-25_well cores (highest abundance = 0 .09% of readings). Facultative MOB is understood to consume atmospheric levels of methane and has been shown to be enriched in high methane environments. (Ricke et al., 2005) Our results agree with the current understanding that environmental Methlocapsa, Methylocella and USC are likely optimized for atmospheric methane levels.
The second and third were incubated with 12CH4 and 13CH4 for SIP analysis. (Neufeld et al., 2007) From these three soil samples, three fractions (heavy, medium and light) were collected for iTag analysis (total 9 DNA samples). The genus Sphingobium, in Alpharoteobacteria, is a strictly aerobic bacterium known for its ability to bioremediate organic pollutants, including hexachlorocyclohexane, phenoxybenzoate, toluene, and trichlorophenol. (Basta et al., 2005) Sphingobium typically encodes biodegradation genes on extra-chromosomal megaplasmids. These were the same three samples that showed very elevated methane concentrations, and both methane and carbon dioxide values are among the highest reported in the literature. (Lovell et al., 1983; Pumpanen et al., 2003; Yonemura et al., 2013).
Natural gas has a significantly lighter carbon isotopic signature (thermogenic natural gas is typically greater than -50 ‰), while atmospheric CO2 has a δ13C value of -9 ‰. (Golding et al., 2013) Soil carbon dioxide is often lighter than atmospheric carbon dioxide due to. A two-step iTag amplification method was used to determine the microbial composition in each DNA extract (Kozich et al., 2013). GTCTCGTGGGCTCGGAGATGTGTATAAGAGACAGCCGYCAATTYMTTTRAGTTT) include a 21–22 nucleotide Illumina adapter sequence, a 10 nucleotide primer sequence, 2. Reagents, cycling conditions and unique index sequences were used as described in (Kozich et al., 2013).
Horizon Oil Spill (DWH). (Hazen et al., 2010; Redmond and Valentine, 2012) Specifically, a robust flowering of the coding sequence of non-methanotrophic lineages—divergent MMOs was followed by a smaller but persistent flowering of MOBs. Methane-oxidizing bacteria comprised less than 0.1% of the total microbial population in the 0–5 cm (open squares) and 23–28 (filled circles) samples. Methane-oxidizing bacteria comprised up to 13.8% of the total microbial population in several samples between March 2016 and January 2017, the dates following the repair of well SS-25.
Measurements of Doubly-Substituted Methane Isotopologues by Frequency
The frequency range of the spectrometer is from 2200 cm-1 to 2300 cm-1, which enables detection of the five most common isotopologues of methane. The physical length of the cavity is actively stabilized with respect to the frequency-stabilized He/Ne laser, which improves the sensitivity and accuracy of the spectrometer. A small amount of MIR light (< 1%) was captured by a MgF2 window to be sent to a wavemeter (Bristol, 621A-IR) for laser frequency.
Spectra were acquired by tuning and frequency-locking the MIR probe laser to frequency-stabilized cavity sequential longitudinal modes (FS-CRDS). Thus, the uncertainty of the FSR measurement is only a consequence of the uncertainty of the wavemeter's frequency measurement. To test the accuracy of the instrument, we monitored the number density of methane isotopologues in a natural methane sample at a pressure of 5.1 Torr.
The frequency range of the spectrometer is 2200 cm-1 to 2300 cm-1, which provides the ability to detect the five most common methane isotopologs. The physical length of the cavity is actively stabilized with respect to a frequency-stabilized He/Ne laser, which improves the sensitivity and precision of the spectrometer. The acquisition rate is ~50 Hz with the estimated minimum detectable absorption from the spectrometer as cm-1.
Spectra of the first 3 isotopologues were generated based on the line parameters reported in the HITRAN database. Spectra of the two disubstituted methane isotopologues were from normalized high-resolution FTIR measurements of pure 13CH3D and 12CH2D2. Sample spectra of a pair of 12CH2D2 and 12CH3D abundance measurements of a 5.1 Torr natural abundance methane measurement.
At natural isotope abundances, the isotope ratios of the monosubstituted methane are approximately equal to the bulk isotope ratios.
Infrared Kinetic Spectroscopy Studies on HO2 Radicals Produced from Criegee
Both the reactions brought more attention to CI in light of SOA formation and heterogeneous chemistry in the boundary layer. In this study, we use infrared kinetic spectroscopy (IRKS) to simultaneously monitor HO2 and IO to study the self-reaction kinetics of the smallest Criegee intermediate. The HO2 self-reaction kinetics were studied to test and optimize the performance of the apparatus.
Thus, the absorption of 220 nm light at t = 0 is assumed to originate from the contribution of HO2 absorption. Most of the HO2 was found to be generated from CI-related reactions, while there is a rapid formation of HO2 by a non-CI-related mechanism. We also monitored IO kinetics simultaneously with HO2 using a blue LED to provide a complimentary understanding of HO2 kinetics.
Pressure and temperature dependence of the HO2 and IO kinetics with or without HFA were then investigated to shine more light into the reaction mechanism of CH2OO related. Finally, we investigated the pressure and temperature dependence of the CH2OO-related HO2 kinetics in the same pressure and temperature ranges as above. A negative pressure dependence was observed for the HO2 formation rate, which is consistent with the predicted pressure-dependent branching ratio of the product channels in the CH2I + O2 reaction.
In order to assist in understanding the fate of Criegee intermediate in the. atmosphere, we investigated the kinetics of HO2 produced from the photolysis of CH2I2/O2/N2. 2014b) Measure Rate Constants for Reactions of the Simplest Criegee Intermediate (CH2OO) by Monitoring the OH Radicals. Most of the HO2 was found to be made from CI, while there is a rapid HO2 formation mechanism unrelated to CI, most likely from direct CH2I + O2 reaction.
From the HFA experiments, we found that some IO originates from the reaction between CH2OO and I.
