Near-infrared tunable diode laser absorption spectrometer for trace gas detection
A ground based near-infrared tunable diode laser absorption spectrometer (TDLAS), capable of measuring methane (CH₄) and nitrous oxide (N₂0) in their natural abundance's within the atmosphere, has been designed and constructed. The spectrometer has high resolution (0.0003 cm⁻¹), high precision (equivalent to a minimum detectable absorption of ~2 x 10⁻⁷) and uses the latest developments in near-infrared t echnology to provide similar sensitivities to mid-infrared laser spectrometers. A theoretical analysis of the noise performance of different detection schemes has been performed. It was concluded that near optimum performance could be achieved using second harmonic wavelength modulation spectroscopy in the 50-100k Hz region. Instrument sensitivity was enhanced through the use of a Herriott multipass cell. A computer model of the behaviour of a paraxial ray passing through such a cell has been developed, allowing suitable choice of cell design and spot pattern to be made. The model was modified to allow the behaviour of an astigmatic Herriott cell to be investigated. The model has shown that the astigmatic cell has all the inherent stability properties of the conventional cell. An astigmatic cell capable of supporting a 100 m pathlength in a2 litre mode volume was designed, built and tested. Performance assessment of the spectrometer was carried out. Detection limits equivalent to 560 pptV of methane and 22 ppbV of nitrous oxide were obtained. A field test and intercomparison with methane monitors (Flame Ionisation Detectors), showed that the retrieved concentrations from the spectrometer agreed to within the measurement uncertainty of the other instruments. Laboratory support for high resolution spectroscopic measurements of gases of atmospheric interest, including, methane, nitrous oxide and ammonia has been carried out. A rotational analysis of nitrous oxide was performed. A coolable Herriott cell capable of operating between 77 and 298 K has been designed, built and tested.