Environmental Sciences Department Seminar

Methane (CH4) and nitrous oxide (N2O) are long-lived greenhouse gases (GHGs) in the atmosphere with large global warming effects. These gases also are known to be produced in a number of anthropogenic settings. One example is manure management systems, which release substantial GHGs and are mandated by the EPA to be continuously monitored. Such monitoring can be easily done remotely using open-path methods. Most open-path methods for quantitative detection of trace gases utilize either Fourier Transform Infrared Spectroscopy (FTIR) or near-infrared Differential Optical Absorption Spectroscopy (DOAS). Although FTIR is suitable for ambient air monitoring measurement of more abundant gases such as CO2 and H2O, the lack of spectral resolution makes retrieval of more weakly absorbing species such as N2O difficult. As alternative, a quantum cascade laser (QCL) operating in the mid-infrared, has been successfully employed for sensing atmospheric abundance of such species. In this presentation I will discuss the use of an open-path QCL-based sensor for simultaneous detection of multiple GHGs, specifically CH4 and N2O. Spectra for these gases were recorded by tuning a QCL over the wavenumber range 1299-1300 cm-1 using a thermal down-chirp technique to achieve rapid sweeping. Careful optimization of the spectral window for absorption features of CH4 and N2O and interfering gases has enabled this technique to be incorporated into a cost-effective, robust, and rapid-response open-path laser-based monitor that can detect ambient concentrations of CH4 with ~1% accuracy and N2O with <1% accuracy over a 500 m path length. Long-time performance and other applications of the system will also be discussed.