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Spotlight on Optics


  • March 2014

Optics InfoBase > Spotlight on Optics > Atmospheric CH4 and N2O measurements near Greater Houston area landfills using a QCL-based QEPAS sensor system during DISCOVER-AQ 2013

Atmospheric CH4 and N2O measurements near Greater Houston area landfills using a QCL-based QEPAS sensor system during DISCOVER-AQ 2013

Published in Optics Letters, Vol. 39 Issue 4, pp.957-960 (2014)
by Mohammad Jahjah, Wenzhe Jiang, Nancy P. Sanchez, Wei Ren, Pietro Patimisco, Vincenzo Spagnolo, Scott C. Herndon, Robert J. Griffin, and Frank K. Tittel

Source article Abstract | Full Text: XHTML | Full Text: PDF

Spotlight summary: Methane is the most abundant reactive trace gas in our atmosphere, and is a very efficient infrared heat trapping specie. Methane and nitrous oxide trail only carbon dioxide and water vapor as total heat trapping gases at present. In spite of this importance, getting a handle on methane emission source strengths has proven elusive. In addition, the landscape is rapidly changing for the production and distribution of natural gas (predominantly methane) from unconventional extraction methods (e.g. horizontal hydraulic fracturing or hydrofracking). Still, the largest reservoir of methane is believed to be the methane hydrates trapped in oceans and arctic tundra regions. For all these reasons, and more, it is clear that more and better methane source characterizations are urgently needed.

Jahjah and colleagues address this important scientific and societal need using an innovative optical technique that involves a quantum cascade laser (QCL). While other studies and other instruments have made use of QCLs, this instrument is one of very few using a quartz tuning fork as the detector. The absorbing gas, methane or nitrous oxide in this case, absorbs the optical energy from the modulated laser, heating the gas and producing an acoustical wave that bends the tuning fork and creates an electrical signal due to the quartz piezoelectricity. While this detection method may not have the ultimate sensitivity of an optical sensor, it has the very large advantage of not requiring cooling of the detector, often using liquid nitrogen or multistage thermoelectric cooling.

An extremely attractive feature of this sensor is its ability to perform sensitive measurements with a very small (few mm3) gas sample, a tremendous advantage over many other existing sensors. The sensor is also suitable to be fabricated into an ultra-compact system based on surface mounted digital electronics. This kind of advance in the integration of all sensor components into a single surface mounted system is what is required to make this new generation of sensor broadly applicable to environmental measurements.

As this type of sensor package is made even more compact and more sensitive it will be very useful in the identification of the source strengths of methane, nitrous oxide, and other gases important to atmospheric chemistry and climate change.

--James Schwab

Technical Division: Light–Matter Interactions
ToC Category: Spectroscopy
OCIS Codes: (280.1120) Remote sensing and sensors : Air pollution monitoring
(300.6340) Spectroscopy : Spectroscopy, infrared
(300.6380) Spectroscopy : Spectroscopy, modulation
(280.4788) Remote sensing and sensors : Optical sensing and sensors
(140.5965) Lasers and laser optics : Semiconductor lasers, quantum cascade

Posted on March 25, 2014

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