NH3 aliasing absorption spectra at 1103.4 cm–1 based on continuous quantum cascade laser

Abstract

Due to the important role of NH₃ in atmospheric aerosol chemistry, rapid and accurate inversion of ammonia concentration is very important for environmental issues. In this paper, a 9.05 μm continuous quantum cascade laser (QCL) is used as the light source at room temperature, and the scanned-wavelength direct-absorption tunable diode laser absorption spectroscopy (TDLAS) is used to study the spectral characteristics of the QCL at 1103.4 cm^–1. A low-pressure experimental platform based on two-level temperature control was designed to measure the six aliasing absorption lines of ammonia at 1103.4 cm^–1. The broadening of spectral line becomes smaller under the condition of reducing the pressure, and the aliasing spectra are separated. The line strength of each absorption line is calculated, and the measurement uncertainty is further analyzed. A method for accurate inversion of single-spectrum gas concentration by low-pressure separation was proposed for severely aliased spectra, and experimental verification was performed. By comparing the results with the HITRAN database, it is concluded that the experimental measured line strength of ammonia gas at 1103.4 cm^–1 has a deviation from the database of . The uncertainty of the line intensity measurement is mainly related to the separation and extraction of aliasing absorbance, which is about 2.42%–8.92%. The deviation between the inversion concentration and the actual value under the condition of extreme low pressure is between 1% and 3%, while the calculated deviation of the line intensity value in the 2.71%–4.71% HITRAN database is about 3% to 5%. The results above indicate that the experimental data are reliable. The non-separative aliasing spectral line method is used to invert the concentration at normal pressure, and the low-pressure separated single spectral line method is used to invert the concentration at low pressure. The results of the two are compared. The analysis results show that the low-pressure separation single-spectrum spectral line inversion concentration value has smaller deviation and higher accuracy from the original concentration. The study of this method provides reference for future inversion of gas concentrations inversion in the atmospheric environment and other fields.