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Comparing the Advantages and Limitations of Different Techniques Used to Quantify Methane Emissions From Municipal Solid Waste Landfills - 3.2 Open-Path Tunable Diode Laser Absorption Spectroscopy

3.2 Open-Path Tunable Diode Laser Absorption Spectroscopy

Open-path tunable diode laser spectroscopy (OP-TDLAS) is an infrared laser technique, like OP-FTIR, except it can only measure the concentration of one compound at a time.  This technique works by directing a laser beam through a gas sample at a specific wavelength that is “tuned” to that of the target compound by adjusting the temperature and bias current.  The instrument then measures how much of the infrared energy is absorbed by the target gas, and produces absorption spectra.  When combined with gas temperature and pressure and path length, the spectra can be translated into concentration readings to be integrated over the beam’s path.  Similar to FTIR, 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).  Tunable diode lasers operate in the near infrared electromagnetic range, and because they operate at a specific wavelength, there is very little interference.  This lack of interference, in combination with the high intensity of the laser beam, make it possible for OP-TDLAS to operate over paths up to 1-2 kilometers long (USEPA 2011).

TDL

Open-Path TDL Diagram (USEPA 2011)

The advantages of OP-TDLAS are that the instrument is compact, lightweight, robust, and low maintenance.  The technique also returns near real-time measurements and supports continuous remote sensing.  Calibration of the device is simple, and interferences due to other gases are minimized due to the narrow wavelength at which the laser emits the infrared light.  The absence of interference allows the technique to achieve longer path lengths (USEPA 2011).

The limitations of OP-TDLAS are that it is only capable of measuring the concentration of one gas at a time.  Also, each laser has a limited range of wavelengths; therefore, additional lasers may need to be purchased if multiple gases need to be detected.  Only approximately twenty compounds are detectable by TDLAS because the target gas must absorb into the near- or mid-infrared range of the electromagnetic spectrum.  Lastly, a laser beam that is blocked by either dust or objects will result in no measurements (USEPA 2011).

Some examples of studies that have used OP-TDLAS as an optical remote sensing technique to measure methane emissions at municipal solid waste landfills include the following: Babilotte et al. 2010; Green et al. 2010; Goldsmith et al. 2012; and Bateman et al. 2016.

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