Broadband Absolute Absorption Measurements of Atmospheric Continua with Millimeter Wave Cavity Ringdown Spectroscopy PDF Download

Author: Andrey I. Meshkov
Publisher:
ISBN:
Category : Absorption spectra
Languages : en
Pages : 146

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Book Description
Abstract: It has been known for more than sixty years that all of the atmospheric absorption can not be accounted for by the standard lineshape theory. Remarkably, to this day there is no consensus on the physical origin of the "excess" absorption, called an atmospheric continuum. This is certainly due in no small part to the lack of accurate laboratory measurements, performed for a wide range of pressures, temperatures and frequencies. Unlike in low pressure gas phase spectroscopy where rapid change in the narrow lines absorption is used to remove the slowly varying baseline, quantification of slowly varying with frequency atmospheric absorption is much more difficult. In this thesis we will describe the instrument for the measurement of continua and other slowly varying absorption phenomena in the millimeter and submillimeter (a.k.a. terahertz) spectral region. By replacing the traditional frequency domain phase lock measurement of cavity Q with the time domain cavity ringdown this instrument can measure continua and broad line absorptions at ~6000 distinct cavity frequencies from 170 to 260 GHz in ~ 3 seconds. These near simultaneous measurements can include both regions dominated by contributions from near line center effects and other regions where "continua" effects are important. The result is a data set that contains, without the use of parameters from other experiments, all of the required experimental information. This approach significantly reduces systematic effects and makes possible more realistic assessment of errors. The instrument was used to study the absorption of nitrogen, oxygen, dry and moist air at a number of temperatures in -40 to +70 C range and pressures from 0 to 3 atmospheres. Results of the measurements were compared with current absorption models and previous laboratory measurements. In addition, we have used the developed technique to study the pressure broadening and pressure shift temperature behavior of the 183.310 GHz water line, important in the atmospheric remote sensing.