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Comparing the Advantages and Limitations of Different Techniques Used to Quantify Methane Emissions From Municipal Solid Waste Landfills - 3.1 Open-Path Fourier Transform Infrared Spectroscopy

3.1 Open-Path Fourier Transform Infrared Spectroscopy

Open-path Fourier transform infrared (OP-FTIR) spectroscopy is a method that takes advantage of how well methane absorbs into the infrared portion of the electromagnetic spectrum.  Open-path FTIR works by first generating a beam of infrared light using an infrared source within the instrument.  The light is sent to a beam splitter, where one portion of the beam is directed to a fixed mirror, while the other portion is directed to a moving mirror.  These mirrors direct the beams back to the beam splitter where they are recombined either constructively or destructively, resulting in a certain amount of interference.  This recombined beam is then projected over the defined open-path where some of the infrared energy is absorbed by the gases that are present.  The beam is then reflected to an infrared detector where the interference pattern is detected and translated into a standard single beam infrared frequency spectrum using a Fourier transform algorithm.  To obtain the data in terms of absorbance, the negative log base-10 is taken of each data point, and the concentration of a specific gas (i.e.  methane) is proportional to the amount of infrared energy absorbed from the beam.  The output is given in path integrated concentration (PIC), which has units of concentration per unit length (i.e.  ppmm – parts per million per meter) (USEPA 2011).

Open-Path FTIR Diagram

Open-Path FTIR Diagram (USEPA 2011)

The advantages of OP-FTIR spectroscopy are that it is capable of simultaneously measuring large numbers of compounds in real-time and the spectra can be saved to be analyzed later.  The instrument itself is durable and portable.  Daily calibration of the device is unnecessary, therefore; it can support unattended sampling for up to one week.  The technique can also be used to locate hotspots which are common in landfill gas applications because the gas tends to find weak spots in the landfill cover and exit via the path of least resistance (USEPA 2011).

The limitations of OP-FTIR spectroscopy are that it is not simple to run and therefore requires experienced operators.  The instrumentation itself takes two people about 5-8 hours to set up, and measurements must be taken along multiple paths to calculate emissions.  It also cannot measure concentrations of homonuclear diatomic gases, or any other compounds that do not absorb infrared radiation.  Additionally, water vapor, carbon monoxide, and carbon dioxide may cause undesirable interference.  The path length of OP-FTIR is also limited to 400-500 meters, and the infrared detector requires weekly refills of liquid nitrogen for cryogenic cooling (USEPA 2011).

Some examples of studies that have used OP-FTIR spectroscopy as an optical remote sensing technique to measure methane emissions at municipal solid waste landfills include the following: Galle et al. 2001; Börjesson et al.  2009; Mønster et al. 2014; and Mønster et al. 2015.

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