Resonant optical cavities for the measurement of atmospheric trace gases

The highly sensitive cavity enhanced and cavity ringdown laser absorption spectroscopy techniques were investigated for the measurement of trace gases in the near-infrared and visible regions of the spectrum. An instrument was developed using the cavity enhanced absorption technique for measurement of CO2, CO, and NH3 in the near-IR. The highly-reflective mirrors in the near-IR produced effective pathlengths in the range of 2.5 to 3 km. The minimum detectable fractional absorbance was determined to be 3.7x10-8 cm-1 resulting in 3sigma detection limits for CO2 and NH3 of 10 ppm and 10 ppb, respectively. The cavity enhanced absorption technique was extended to the visible region for detection of the nitrate radical, NO3. Due to the higher reflectivity of the mirrors, the sensitivity was improved offering effective pathlengths of nearly 20 km, resulting in a minimum detectable fractional absorbance of 3x10-10 cm-1 using O3 and NO 2 as test gases. Due to the strong dependence of pathlength on the alignment, frequent calibration was required. In addition, significant losses were observed when sampling NO3 through the glass inlet and cavity due to the relatively long residence times within the cavity.Nighttime ambient measurements of NO3 and N2O 5 were performed for four weeks in the summer of 2007 at York University using the cavity ringdown instrumentation modified for portability. With the exception of one evening immediately after sunset, NO3 was below the detection limit of 2.0 ppt (alpha = 9.8x10-10 cm -1) during the entire study.The cavity ringdown technique offered the best sensitivity of the techniques studied, and could be improved for ambient measurements by improving stability in the field and reducing sampling losses.Cavity ringdown spectroscopy was applied to the investigation of NO 3 with adaptations to the software and sampling instrumentation to reduce the residence time and wall losses. Since cavity ringdown is an absolute absorption measurement, pathlength calibration is no longer required. In cavity ringdown spectroscopy the rate of decay of light exiting an optical cavity is measured, making this technique immune to intensity variations of the laser source. The detection limit for NO3 was 0.3 ppt neglecting sampling losses yielding a minimum detectable fractional absorbance of l.5x10 -10 cm-1. The first-order wall loss constant of NO 3, kwall, was determined to be 0.48 s-1.