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Comparing the Advantages and Limitations of Different Techniques Used to Quantify Methane Emissions From Municipal Solid Waste Landfills - 2.3 Emission Isolation Flux Chambers

2.3 Emission Isolation Flux Chambers

Emission isolation flux chambers have been around for about 25 years; therefore, the scientific principles are well known and documented.  Flux chambers are in situ point sampling instruments.  The standard chamber has a volume of approximately 30 liters, encapsulates an area of 0.13 square meters (Klenbusch 1986), and has a standard geometry of a dome superimposed on a cylinder; however, the volume, encapsulated area, and geometry can depart from the standard.  A variety of flux chambers of different shapes and sizes have been successfully deployed in the field.  The best performing flux chambers are those which optimize the mixing of the chamber atmosphere (Eklund 1992).  Some chambers enhance this mixing by using small fans installed on the inside to help with circulation.  Each chamber comes with a separable collar which should be inserted one inch into the soil with the chamber installed and sealed on top.  The goal is to isolate a known volume of air above the ground surface and measure the accumulation of methane.  This is achieved by taking frequent concentration measurements, typically once every 1 to 5 minutes for approximately 25 minutes.  Some chambers are equipped with internal sensors that can directly measure the concentration, while other methods require air samples to be taken at the prescribed interval.  If samples are taken from the chamber, they can be analyzed for methane concentration using a gas chromatograph equipped with a flame ionization detector (Green et al. 2010).  Methane emission flux can be determined by plotting the accumulation of the methane inside the chamber (i.e. concentration versus time).  

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Various Flux Chambers (Photo by Julie Bateman) (Babilotte 2011) (Almund-Hunter et al. 2015)

Some advantages to flux chambers are that they are cheap and relatively simple to use.  A critical limitation; however, is that they only provide point data; therefore, several measurements are required to characterize the methane emissions of an entire landfill.  Additionally, methane travels the path of least resistance and tends to gather and leak out of weaknesses in the landfill cover.  This creates methane hotspots as opposed to uniform methane emissions across the entire landfill surface.  It has been shown that as little one third of the area of a landfill can be responsible for 99% of the methane emissions due to these hotspots (Spokas et al. 2003), which is problematic for a point-source sampling method.  To overcome this challenge, most flux chamber methods systematically divide the land area into a dense grid to decide where to place the devices for methane measurement.  Due to the abundance of points contained in this grid, and the fact that each measurement takes at least 30 minutes, the method quickly becomes extremely time and labor intensive.  

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