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Comparing the Advantages and Limitations of Different Techniques Used to Quantify Methane Emissions From Municipal Solid Waste Landfills - 3.3 Cavity Ring-Down Spectroscopy

3.3 Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy (CRDS) uses a tunable laser source, like TDLAS; however, the laser can emit light ranging from the far-infrared to the ultraviolet portions of the electromagnetic spectrum.  This laser is cast through a gas sample which is bounded on each side by concave high reflectivity mirrors.  The light beam continuously reflects off these mirrors while a photodetector is measuring the rate of light intensity decay.  This rate of decay is a function of the length of the cavity, the reflectance of the concave optical mirrors, and the absorptivity of the sample.  Because the cavity length and mirror reflectance are constant, the rate of decay is dependent only on the absorptivity of the sample.  The amount of time that is elapsed until the intensity of the light beam decays to 1/e of its original value is called the cavity ring-down time (RDT).  The difference between the RDT curve of the sample and the RDT curve of the empty cavity is proportional to the concentration of the target gas within the sample (USEPA 2011).

CRDS Diagram

CRDS Diagram (USEPA 2011)

The advantages of CRDS are that the instrumentation is compact, user-friendly, easy to install, low maintenance, and does not require a large power source.  It also delivers real-time results and frequent calibration is unnecessary.  Because the technique measures time as opposed to absorbance, it is resistant to environmental fluctuations such as temperature and humidity, and it has a wider linear dynamic range.  Due to the high reflectivity of the mirrors, CRDS can also achieve longer path lengths than some other techniques (USEPA 2011).

The limitations of CRDS are that it is extremely difficult to detect multiple species due to the small wavelength range of the high reflectivity mirrors.  Sample filtering may also be necessary to avoid interferences.  The limitations of CRDS are that it is extremely difficult to detect multiple species due to the small wavelength range of the high reflectivity mirrors.  Sample filtering may also be necessary to avoid interferences.  Lastly, the instrumentation for the CRDS is more expensive due to the high-quality requirements for the laser and the mirrors (USEPA 2011).

Some examples of studies that have used CRDS as an optical remote sensing technique to measure methane emissions at municipal solid waste landfills include the following: Green et al. 2010Mønster et al. 2014Mønster et al. 2015; and Cambaliza et al. 2017

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