Investigation of Atmospheric Mercury Concentrations and Dry Deposition Rates Using Established and Novel Methods

This work presents new methods for and measurements of concentrations and dry deposition of atmospheric mercury. Chapter 2 reports on measurements of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and mercury bound to particles (PBM), mercury soil flux, and mercury in precipitation at two locations in northern Nevada, U.S.A. Concentrations of GEM were influenced by both local substrate emission and transport from regional source areas. Concentrations of GOM and PBM were within ranges reported for other rural sites, and mercury wet deposition rates were similar to other sites in the arid West. In Chapter 3, multiple methods were used simultaneously at the same sites to estimate dry deposition of atmospheric mercury. The ratio of dry to wet deposition was between 10 and 90%, and varied with season and with the methods used for dry deposition approximations. Chapter 4 reports on two years of measurements of atmospheric mercury fractions in Reno, Nevada. Concentrations of GEM and PBM were influenced by emission from local sources and meteorological conditions. Concentrations of GOM were higher during periods with higher temperature and lower dew point, confirming the findings of others that warm, dry air from the free troposphere is a source of GOM to the surface. Chapter 5 details work focused on development of a surrogate surface for estimating GOM dry deposition. Deposition of mercury to surfaces was well correlated with GOM concentrations (r2 = 0.84, p < 0.01, n = 326) and was not significantly influenced by temperature, humidity, or ozone concentrations. The surrogate surface is not able to mimic natural surface variability, but it is useful to measure the maximum potential for and spatial and temporal trends of GOM dry deposition. Chapter 6 reports on the development of a passive sampler for characterizing GOM concentrations. Uptake of Hg by the passive sampler was correlated with measured air GOM concentration (r2 = 0.89, p < 0.01, n = 22), and did not appear to be significantly affected by changes in temperature, humidity, or ozone concentration, but sampler performance did appear to be slightly dependent on wind speed. The detection limit for a 14 day sample was ~5 pg m-3

Observations of speciated atmospheric mercury at three sites in Nevada: Evidence for a free tropospheric source of reactive gaseous mercury

Air mercury (Hg) speciation was measured for 11 weeks (June–August 2007) at three sites simultaneously in Nevada, USA. Mean reactive gaseous Hg (RGM) concentrations were elevated at all sites relative to those reported for locations not directly influenced by known point sources. RGM concentrations at all sites displayed a regular diel pattern and were positively correlated with ozone (O3) and negatively correlated with elemental Hg (Hg0) and dew point temperature (Tdp). Superimposed on the diel changes were 2- to 7-day periods when RGM concentrations increased across all three sites, producing significant intersite correlations of RGM daily means (r = 0.53–0.76, p \u3c 0.0001). During these periods, enhanced O3 concentrations and lower Tdp were also observed. Back trajectories were applied to develop gridded frequency distribution (GFD) plots and determine trajectory residence times (TRT) in specific source boxes. The GFD for the upper-quartile RGM daily means at one site showed a contributing airflow regime from the high-altitude subtropics with little precipitation, while that developed for the lower-quartile RGM concentrations indicated predominantly lower-altitude westerly flow and precipitation. Daily mean TRT in a subtropical high-altitude source box (\u3e2 km and \u3c35°N) explained a component of the daily mean RGM at two sites (r2 = 0.37 and 0.27, p\u3c0.05). These observations indicate that long-range transport of RGM from the free troposphere is a potentially important component of Hg input to rural areas of the western United States

HPLC Method development and instrument QC for Aldehyde and Ketone compounds

The Uintah Basin periodically experiences high ozone levels during the winter season (Lyman et al). Carbonyl compounds are produced from various emission sources and are precursors to ground level ozone production. Of these compounds, several aldehydes and ketones are volatile and listed as hazardous under the Clean Air Act (1990). We regularly measure carbonyl concentrations in air via high-performance liquid chromatography (HPLC) as part of our efforts to better understand and improve air quality in the Uintah Basin. For this project, we investigated possible improvements to our HPLC system to increase resolution, identification, and quantification of carbonyls in collected air samples. The goals of this work were to (1) achieve better separation of individual carbonyls to do so we investigated the impact of changes to flow rate/pressure and eluent composition to the method. (2) improve the signal/noise ratio of our measurements. To address the second objective we determined detection limits for the modified instrument configuration by modifying sample injection volume

Aerial and Ground-Based Optical Gas Imaging Survey of Uinta Basin Oil and Gas Wells

We deployed a helicopter with an infrared optical gas imaging camera to detect hydrocarbon emissions from 3,428 oil and gas facilities (including 3,225 producing oil and gas well pads) in Utah’s Uinta Basin during winter and spring 2018. We also surveyed 419 of the same well pads from the ground. Winter conditions led to poor contrast between emission plumes and the ground, leading to a detection limit for the aerial survey that was between two and six times worse than a previous summertime survey. Because the ground survey was able to use the camera’s high-sensitivity mode, the rate of detected emission plumes was much higher in the ground survey (31% of all surveyed well pads) relative to the aerial survey (0.5%), but colder air temperatures appeared to impair plume detection in the ground survey as well. The aerial survey cost less per facility visited, but the ground survey cost less per emission plume detected. Well pads with detected emissions during the ground and aerial surveys had higher oil and gas production, were younger, were more likely to be oil well pads, and had more liquid storage tanks per pad relative to the entire surveyed population. The majority of observed emission plumes were from liquid storage tanks (75.9% of all observed plumes), including emissions from pressure relief valves and thief hatches on the tank or from piping that connects to the tank. Well pads with control devices to reduce emissions from tanks (combustors or vapor recovery units) were more likely to have detected emissions. This finding does not imply that the control devices themselves were not functioning properly. Instead, gas was escaping into the atmosphere before it reached control devices. Pads with control devices tended to be newer and have higher oil and gas production, which probably explains their higher rate of detected emissions

Whole Genome Phylogenetic Tree Reconstruction Using Colored de Bruijn Graphs

We present kleuren, a novel assembly-free method to reconstruct phylogenetic trees using the Colored de Bruijn Graph. kleuren works by constructing the Colored de Bruijn Graph and then traversing it, finding bubble structures in the graph that provide phylogenetic signal. The bubbles are then aligned and concatenated to form a supermatrix, from which a phylogenetic tree is inferred. We introduce the algorithms that kleuren uses to accomplish this task, and show its performance on reconstructing the phylogenetic tree of 12 Drosophila species. kleuren reconstructed the established phylogenetic tree accurately, and is a viable tool for phylogenetic tree reconstruction using whole genome sequences. Software package available at: https://github.com/Colelyman/kleurenComment: 6 pages, 3 figures, accepted at BIBE 2017. Minor modifications to the text due to reviewer feedback and fixed typo

An Updated Review of Atmospheric Mercury

The atmosphere is a key component of the biogeochemical cycle of mercury, acting as a reservoir, transport mechanism, and facilitator of chemical reactions. The chemical and physical behavior of atmospheric mercury determines how, when, and where emitted mercury pollution impacts ecosystems. In this review, we provide current information about what is known and what remains uncertain regarding mercury in the atmosphere. We discuss new ambient, laboratory, and theoretical information about the chemistry of mercury in various atmospheric media. We review what is known about mercury in and on solid- and liquid-phase aerosols. We present recent findings related to wet and dry deposition and spatial and temporal trends in atmospheric mercury concentrations. We also review atmospheric measurement methods that are in wide use and those that are currently under development

Detection and Quantification of Gas-Phase Oxidized Mercury Compounds by GC/MS

Most mercury pollution is emitted to the atmosphere, and the location and bioavailability of deposited mercury largely depends on poorly understood atmospheric chemical reactions that convert elemental mercury into oxidized mercury compounds. Current measurement methods do not speciate oxidized mercury, leading to uncertainty about which mercury compounds exist in the atmosphere and how oxidized mercury is formed. We have developed a gas chromatography/mass spectrometry (GC-MS)-based system for identification and quantification of atmospheric oxidized mercury compounds. The system consists of an ambient air collection device, a thermal desorption module, a cryofocusing system, a gas chromatograph, and an ultra-sensitive mass spectrometer. It was able to separate and identify mercury halides with detection limits low enough for ambient air collection (90 pg), but an improved ambient air collection device is needed. The GC/MS system was unable to quantify HgO or Hg(NO3)2, and data collected cast doubt upon the existence of HgO in the gas phase