Long Path Quantum Cascade Laser Based Sensor for Urban Monitoring of CH4 and N2O

Long Path Quantum Cascade Laser Based Sensor for 

Urban Monitoring of CH₄ and N₂O

Paulo Cesar Castillo, Ihor Sydoryk, Barry Gross, Fred Moshary

Optical Remote Sensing Laboratory. The City College of New York, 160 Convent Ave, NY, NY 10031

e-mail address: pcastcamp@gmail.com

Methane (CH₄) and Nitrous Oxide (N₂O) are long-lived greenhouse gases in the atmosphere with significant global warming effects. These gases also are known to be produced in a number of anthropogenic settings such as manure management systems, which releases substantial GHGs and is mandated by the EPA to provide continuous monitoring. One application of particular interest are natural gas leaks in urban areas which result in strong spatially inhomogeneous methane emissions as illustrated in figure 1

           

Figure 1. Point source mapping of methane leaks in NYC

http://documents.dps.ny.gov/public/Common/ViewDoc.aspx?DocRefId=%7BBD0C41AF-925B-425E-A17A-1EF5ABAFAC19%7D

Most open path methods for quantitative detection of trace gases utilize either Fourier Transform Spectrometer (FTIR) or near-IR differential optical absorption spectroscopy (DOAS). Although, FTIR is suitable for ambient air monitoring measurement of more abundant gases such as CO₂ and H₂0 etc., the lack of spectral resolution makes the retrieval of weaker absorbing features such as N₂0 more difficult. On the other hand, conventional DOAS systems can be large and impractical.

As an alternative, we illustrate a robust portable quantum cascade laser (QCL) approach for simultaneous detection of CH₄ and N₂O.  In particular, gas spectra were recorded by ultrafast pulse intensity (thermal) chirp tuning over the 1299 - 1300cm^-1 spectral window. Etalon measurements insure stable tuning was obtained.

To deal with multiple species, a LSQ spectral fitting approach was used which accounted for both the overlapping trace gases, background water vapor as well as detector drift and calibration.  In summary, ambient concentrations of CH₄ with and N₂O   with accuracy < 1%  was obtained on the order of 5ms using optical paths of 500 m path length. Furthermore, accuracy was obtained even under summer heat wave conditions where water vapor interference was significant.  In addition, unattended long term operation was demonstrated and validations using other sensors when possible were shown to be consistent. The system accuracy is limited by systemic errors which are still being explored. Finally, we will illustrate the use of the system for long path methane detection in urban street canopies.