Measurement and profile inversion of atmospheric N<sub>2</sub>O absorption spectrum based on laser heterodyne detection

Abstract

aser heterodyne spectroscopy detection has rapidly developed in recent years due to its high spectral resolution, small size, and light weight. It can be used to measure the atmospheric greenhouse gas vertical profile and calibrate the carbon satellite ground. This paper reports a laser heterodyne system for measuring atmospheric N<sub>2</sub>O, with a 3.939-µm interband cascade laser used as a local oscillator light source. A homemade high-precision solar tracker collects sunlight as a laser heterodyne signal source. The tracking accuracy reaches 7 arcsec, and the spectral resolution of the laser heterodyne system arrives at 0.004 cm<sup>–1</sup>. The atmospheric N<sub>2</sub>O absorption spectrum in Hefei area (31.902°N, 117.167°E) is measured, and two strong absorption peaks respectively at 288.336 and 2539.344 cm<sup>–1</sup> are obtained. In addition, the wavelength calibration of the absorption signal, and the entire atmospheric transmittance spectrum of N<sub>2</sub>O molecules are obtained, and the signal-to-noise ratio is 93. The high-resolution spectrum data are normalized and frequency is corrected, and the N<sub>2</sub>O atmospheric concentration profile is obtained by using the reference forward model and the optimal estimation algorithm. The standard deviation of volume fraction is in a range of 0.000031—0.0026 ppm, and the corresponding relative error range is 0.009%—0.83%. The research results show that the laser heterodyne system built in this work can be used to measure the absorption spectrum of N<sub>2</sub>O in the atmosphere and realize the inversion of the N<sub>2</sub>O profile, which provides a guarantee for long-term observation of atmospheric N<sub>2</sub>O concentration.