Optogalvanic spectroscopy for atmospheric carbon-dioxide concentration and isotopic ratio measurement.

摘要

The status of CO2 as a critical greenhouse gas and a useful ingredient and byproduct of several important ecological processes is well established. Due to this importance, it has attracted enormous resources toward research and monitoring of atmospheric levels across the globe. A major restraint hindering the true globalization of CO2 monitoring and research activity is the high cost and operational limitations of appropriate measurement devices such as IRMS and IR spectrometers. The use of optogalvanic effect as an alternative tool for CO2 measurements and analysis has been demonstrated for high concentration (>1%) CO2 samples.; The goal of this thesis is to establish parameters for OGE studies at atmospheric CO2 concentration ∼400 ppm. At such low concentrations, several factors combine to significantly degrade the SNR, inhibiting precise signal measurements. A study of the relevant parameters that influence measurement results is carried out in this thesis with the goal of achieving sub-delta precision and concentration measurements. Analysis of results is done to establish the threshold noise levels necessary for achieving the targeted precision. To stabilize the laser wavelength, software and hardware methods are evaluated for best results. In the dynamic software control, feedback mechanism is used. In this mode, the tightness with which 13CO2 and 12CO2 laser drifts need to be individually controlled to realize the results is studied. Control of laser wavelength drift by use of periodic signal with different characteristics such as frequency, peak-to peak and off set voltages is analyzed for optimum settings.; In an attempt to improve SNR, a variety of signal representation schemes are studied. These include the FFT and harmonic combinations as well as the area representation. A major problem with area representation is the observed DC level drifts. Techniques for eliminating this are presented and results compared with corresponding FFT representation. The effect of laser modulation frequencies on the signal magnitudes, both in the FFT and area formats is studied. Results provide a benchmark necessary for realizing sub-delta measurements in field deployed instruments.