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: email@example.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
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.