IV. Field Measurements of Methane Emissions
With the ongoing push to reduce greenhouse gas emissions, researchers and the solid waste industry are spending time and money developing methods to measure landfill methane emissions—not only to be able to quantify landfills’ contributions to greenhouse gases, but to have a way to assess whether technologies and processes for emission reduction are working, and to quantify progress. We have been at the forefront of a considerable amount of the research and resulting innovations. Some methods we have researched that have been extensively used over time are:
· Flux chambers, which measure gas from point sources
· Vertical radial plume mapping (VRPM), which measures area sources,
· Tracer gas correlation, which measures whole-landfill emissions
· Ambient air landfill surface measurements using a gas detector, which estimates area sources and or whole-landfill emissions.
FSU Tracer Gas Correlation Capabilities: Accurate Determinations of Methane Emissions.
It is of course important to accurately determine methane concentrations with probes and sensors. However accurate assessment of methane flux from a surface to the atmosphere is a more difficult parameter to evaluate. Constraining the accuracy of models and default values for fugitive methane emissions and landfill cover methane oxidation is becoming increasingly important as the focus of reporting narrows down the facility level. Often regulatory and voluntary schemes, of methane abatement estimate facility emissions on the basis of modeled gas generation and assumptions about gas collection efficiency and methane oxidation. In situ field measurements of methane emission and oxidation (with the stable isotope approach) serve to determine the accuracy of these approaches. We recommend the acetylene tracer gas correlation (TGC) method of determining emissions using a cavity ring-down spectroscope (CRDS) as described by Foster-Wittig, et al. (2015) and originally by Galle et al. (2001)
We own A Picarro cavity ring down analyzer that can be used to simultaneously measure concentrations of acetylene (tracer gas) and methane at ppt and ppm levels. The unit is mounted in a full sized pickup truck fitted with an external snorkel for gas sample collection. The analyzer is integrated with a GPS (Hemisphere R100) and a compact weather station including self-aligning sonic anemometer (Climatronics AIO Compact Weather Station). Concentration, position, and meteorological data are recorded in a time-synchronized data file while the truck is in motion or at a single location downwind of the landfill. After a calibration check, mobile transect measurements are made by driving the analyzer along roads located around landfill. If roads are not available then the staionary approach can be used, placing the instrument down wind of the source. Continuous measurements of acetylene and methane concentration are recorded as the analyzer makes transects through the plumes (Figure 1),or as the wind pushes the plume back and forth across the instrument site. The emission rate of methane is determined as the product of the release rate of the tracer and ratio of integrals of the concentrations of acetylene and methane in the plume transects.
Measuring Methane in Remote or Challenging Environments
Understanding and quantifying methane emissions is challenging because this colorless, odorless gas enters the atmosphere from ubiquitous natural sources and very widespread industrial releases—so-called fugitive methane. An important source of methane are the landfills that every nearly community relies on for disposal of solid waste. However, fugitive methane releases often occur in obscure or under-sampled environments. These include pipeline leaks, energy production sites, unsecured industrial facilities, and ports. The best studied of these fugitive sources occur on land, but may be difficult to access for sampling and quantification. Other, much less understood sources may be found in marine and lake environments. Some of the largest pools of methane are found in marine gas hydrates and arctic permafrost [Kvenvolden, 1999; Kvenvolden and Rogers, 2005]. The stability of these pools under conditions of climate change is an area of active research and it has been suggested that warming of polar waters has increased the decomposition of gas hydrates and increasing the flux of methane to the atmosphere [Westbrook et al., 2009]. The test of this hypothesis, and of many related scientific or management questions, depends on the ability to quantify methane emissions in adaptive and cost-effective ways.
Recently, variability in the methane discharge from a natural seep in the Gulf of Mexico was quantified with use of a monitoring system designed by FSU scientists [Razaz et al., 2020]. Pohlman and others challenged the Westbrook hypothesis of increased based on CO2 uptake in Arctic waters [Pohlman et al., 2017]; they were unable to definitively measure methane at the air-water interface of coastal waters; they suggested that methane released from seabed sources would be oxidized in the water column and therefore would not reach the atmosphere. However, Mason and others [Mason et al., 2019] recently demonstrated a flux of methane to the atmosphere of at least 750 kg/day from a leaking oil platform located at 135 m depth in the coastal Gulf of Mexico. These researchers utilized technological innovations developed at FSU.
MacDonald lab continues to improve its mobile measurement capabilities using cavity ring-down spectrometry methods configured for remote application. Most recently, the group has begun fabrication of a remote sampling application that would utilize drones to collect discrete samples from point sources. Such samples could be analyzed real-time with use of the mobile measurement facility. Development of technology and application experience supports a wide range of potential investigation of fugitive methane emissions.
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