Quantitative analysis of accuracy of concentration inversion under different temperature and pressure

Abstract

The CO₂, CH₄ and other greenhouse gases are measured by using a home-made Fourier transform infrared spectrometer at the Longfengshan atmospheric background station. Compared with the measurement results of the instrument from the background station which meets the standards of the World Meteorological Organization, the correlation coefficient and the root mean square error of the CO₂ concentration value are 0.9576 and 18.6015, so the measurement results from the home-made instrument are reliable. In the home-made instrument the calibration spectrum of standard temperature and the calibration spectrum of stand pressure are used to invert the concentration. With the temperature changing, the temperature of the measured gas will vary, thus resulting in error. The research of environmental variable factors can improve the accuracy of concentration inversion. For example, compared with CO₂ absorption spectrum under 296 K, the CO₂ absorption spectrum under 297 K will have 1.8% spectrum deviation and its inversion concentration error is 0.41%. This is the main cause of inversion concentration error. Based on the above analysis, the absorption cross section is calculated by using the high-resolution transmission molecular absorption database parameters. Combining with the instrument line shape, the calibration spectra at different temperatures and pressures can be obtained. The calibration spectra at different temperatures and pressures are used to calibrate the concentration inversion. After calibration, compared with the measurement results of the background station instrument, the correlation coefficient and the root mean square error of the CO₂ concentration value are 0.9637 and 6.7803. The correlation coefficient of CO₂ concentration value measured by self-developed instrument is improved and root mean square error is reduced. The result shows that the calibration algorithm enhances the accuracy of the measurement results to a certain extent. The above results illustrate the reliability of the home-made FTIR instrument and this experiment provides important data, which lay the foundation forstudying the home-made Fourier transform infrared spectrometer. Of course, improvement can be made in the following areas. Other minor factors may affect the effect of the inversion algorithm. The concentration inversion will have subtle differences at different bands of calibration spectra. So in order to improve the measurement accuracy, we need to choice more reasonable band inversion and more precise parameters from the high-resolution transmission molecular absorption database.