Cavity Ringdown Sensors for Atmospheric and Combustion Diagnostics Open Access
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The main objectives of the work presented in this dissertation are 1) using cavity enhanced techniques, including cavity ringdown spectroscopy (CRDS) and integrated cavity output spectroscopy (ICOS) to analyze gases extracted from various combustion systems with the goal of gaining insight into reaction mechanisms and 2) developing commercialized CRDS sensors for use as fire detectors and toxic gas sensors. CRDS and ICOS are highly sensitive types of optical absorption spectroscopy that have been applied to numerous environments for gas detection: extractive sampling from flames, advanced fire detection, and in-situ flame diagnostics. CRDS was used to obtain concentration measurements of NOx intermediates (NH3 and HCN) from gas samples extracted from various locations in a neat and a pyridine-doped methane/air flame with the goal of understanding the fate of nitrogen-containing fuels, such as pyridine, as they degrade to form environmental pollutants, including NOx. Recently, in-situ CRDS and ICOS was demonstrated by locating an axisymmetric methane/air flame in a free-space optical cavity at atmospheric pressure. Two near-infrared spectral regions were scanned with the laser at various positions in the flame to obtain line-of-sight absorption spectra containing CO2 and C2H2 vibrational-rotational lines. An Abel inversion was used to deconvolve the lateral line-of-sight absorption spectra to their respective radial distributions. The sensor projects discussed in this dissertation were completed through collaborative work with the National Institute of Standards and Technology (NIST) and with Analytical Technology, Incorporated (ATI). A portable CRDS sensor was developed in our laboratory and applied to controlled fires at NIST to test its feasibility as an advanced fire detector. Gas samples were extracted from effluents during smoldering paper fires and the sensor monitored concentrations of combustion products CO, CO2, HCN, and C2H2 near-simultaneously. The CRDS sensor detected increases in these gases prior to commercial smoke detectors alarming. ATI's interest is to commercialize CRDS sensors to target specific toxic gases. Through this collaboration, ringdown statistics were studied extensively to determine factors that influence detection limits with the goal of developing a robust, reliable, and compact gas sensor.