单光束多组分温室气体的腔衰荡光谱同步检测

doi:10.3964/j.issn.1000-0593(2019)07-2046-07

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Abstract The cavity ring down spectroscopy (CRDS) has been a proper detecting method for the trace gas with its ultrahigh sensitivity and super spectral resolution. However, the common CRDS is designed for a single gas or the measurement of multiple species with several laser sources. In this paper, a CRDS instrument has been developed for multicomponent greenhouse gas synchronous detection with a single laser. Considering the balance of the absorption losses, it utilizes the strong absorption peak of methane (CH₄) and weak ones of vapor (H₂O) and carbon dioxide (CO₂) in the range of 1 653～1 654 nm simultaneously. The wavelength scan of that range is completed by a tunable distribution feedback laser diode. The corrected concentration of greenhouse gas has been determined by the CRDS instrument with a high finesse (F≈1×10⁵) cavity and calculated with the spectral superposition inversion matrix. It demonstrates that the remove of data points, called the filter region, at the preliminary stage of the decay has an influence on the noise equivalent absorption coefficient, which has a interrelation with the measuring sensitivity. In most cases, the Levenberg-Marquardt (L-M) algorithm, which shows a good precision, is better than the discrete Fourier transform (DFT) algorithm on the measuring sensitivity as a fitting algorithm. But this conclusion will be dubious when the ring-down curve is deviated from the single exponential form. For studying this phenomenon, a CRDS instrument with a low finesse cavity (F≈6×10³) is set up. Compared to the high finesse cavity, the low finesse cavity has a faster decay rate, and a bigger deviation from the single exponential form, which can be easily seen from the residual analyses. When the filter region is not long enough, the noise equivalent absorption coefficients calculated by L-M algorithm is larger than the ones calculated by DFT algorithm. Meanwhile, according to the definition of the fluctuation of the noise equivalent absorption coefficient, the influence of the DFT algorithm is less than that of the L-M algorithm affected by the length of the filter region in both high finesse cavity and low finesse cavity. The best length of the filter region in our CRDS instrument is 20 data points, which are basically the same at different averaging time. And according to Allan variance, the measuring sensitivity of the CRDS instrument can reach 2.4×10^-10 cm^-1 for an 8 s integration time. At 25 ℃ and 1 atm, the measuring sensitivities of CH₄, H₂O and CO₂ are approximately 0.64 ppbv, 3.5 ppmv and 4.0 ppmv separately. Calculated with the spectral superposition inversion matrix, the atmospheric concentrations of CH₄, H₂O and CO₂ in the lab are measured to 2.018 ppmv, 3 654 ppmv, and 526 ppmv separately with multiple wavelengths, in contrast to the results of 2.037 ppmv, 3 898 ppmv and 630 ppmv in the classical CRDS method. Using the temperature control of the DFB laser, an absorption spectrum of the greenhouse gas has been acquired with the wavelength scan. Compared to this measured results, the residuals of the complex fitting curve using the data from the multiple wavelength measurements are less than the ones of the simple fitting curve with data from the classical method.

Key words：Cavity ring down spectroscopy; Greenhouse gas; Trace gas detection; Fitting algorithm

Received: 2018-05-22 Accepted: 2018-09-16

ZTFLH:

O433

Corresponding Authors: YU Jin E-mail: jinyu@aoe.ac.cn

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2019)07-2046-07 OR https://www.gpxygpfx.com/EN/Y2019/V39/I07/2046