A Local Model and Calibration Set Ensemble Strategy for Open-Path FTIR Gas Measurement with Varying Temperature

Abstract

Open-Path Fourier Transform Infrared OP-FTIR spectrometers are commonly used for the measurement of atmospheric pollutants and of gases in industrial processes. Spectral interpretation for the determination of gas concentrations is based on the HITRAN database line-by-line modeling method. This article describes algorithms used to model gas spectra and to determine gas concentration under variable temperatures. Integration of individual rotational lines has been used to reduce the impact of spectrometer functions on the comparison of both measured and synthetic modeled spectra. Carbon monoxide was used as an example. A new algorithm for gas concentration retrieval consisting of two ensemble methods is proposed. The first method uses an ensemble of local models based on linear and non-linear PLS (partial least square) regression algorithms, while the second is an ensemble of a calibration set built for different temperatures. It is possible to combine these methods to decrease the number of regression models in the first ensemble. These individual models are appropriate for specific measurement conditions specified by the ensemble of the calibration set. Model selection is based on comparison of gas spectra with values determined from each local model.

The influence of temperature non-uniformity on retrieval of gas concentration in OP-FTIR spectroscopy

“…Recently, the influence of temperature on the accuracy of the gas determination was performed. In many papers [17][18][19] it was shown that even a few degrees of temperature difference can result in noticeable and significant errors. Therefore, various methods of temperature correction are created.…”

Optical switching method based on two diffraction gratings bistable system

“…In many problems the temperature has significant influence on the analyzed transmission spectra. In some works a synthetic calibration due to the temperature impact is performed, especially in spectroscopy [22,23]. For the purposes of this work no temperature and strain changes have been assumed.…”

Passive Open-Path FTIR Measurements and Spectral Interpretations for in situ Gas Monitoring and Process Diagnostics

“…For example, the 20202100 cm −1 spectral range is used to determine of CO and CO 2 [45]. Simulations of issues at hand can be performed to correctly analyse the measurement data [46] and the impact of various factors such as the temperature on the retrieval process [47]. For this purpose, the HITRAN database is utilised [48].…”