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Analysis and demonstration of atmospheric methane monitoring by mid-infrared open-path chirped laser dispersion spectroscopy

Nart S. Daghestani, Richard Brownsword, and Damien Weidmann

Open Access Open Access

Abstract

Atmospheric methane concentration levels were detected using a custom built laser dispersion spectrometer in a long open-path beam configuration. The instrument is driven by a chirped distributed feedback mid-infrared quantum cascade laser centered at ~1283.46 cm−1 and covers intense rotational-vibrational transitions from the fundamental ν4 band of methane. A full forward model simulating molecular absorption and dispersion profiles, as well as instrumental noise, is demonstrated. The instrument’s analytical model is validated and used for quantitative instrumental optimization. The temporal evolution of atmospheric methane mixing ratios is retrieved using a fitting algorithm based on the model. Full error propagation analysis on precision gives a normalized sensitivity of ~3 ppm.m.Hz-0.5 for atmospheric methane.

© 2014 Optical Society of America

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Figures (8)
Fig. 1
Fig. 1 Schematic of the CLaDS optical layout for (a) low pressure methane mixture experiments and (b) open-path atmospheric sensing.

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Fig. 2
Fig. 2 Schematic of the radio-frequency processing line of the CLaDS instrument.

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Fig. 3
Fig. 3 (a). Dependence of the noise in the FM demodulated CLaDS signal as function of the post detection LPF bandwidth (single cycle data). (b) Dependence of the noise with the number of averaged samples. The solid line indicates the model. (c) Dependence of the noise of the FM demodulated CLADS signal as function of the CNR.

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Fig. 4
Fig. 4 (a). Low pressure CLaDS spectrum for the transition of methane centered at 1283.458791cm−1. The sample was 1.3% methane in a 5 cm long cell, the CNR was 83 × 103, the total acquisition time was 10 ms. The model was fitted to the experiment. Residual expressed in percent of the maximum CLaDS signal is given in the lower panel. (b) Allan deviation for a 1200 s record of methane concentrations at a rate of one measurement per second, for a single trace acquisition (150 µs scan time) and 10 averaged traces, both normalized to one second.

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Fig. 5
Fig. 5 (a). One-sigma precision on CH4 mixing ratios measurements as function of the CNR. These were obtained from a ~1.3% methane mixture in N2, 5 cm cell length, 24 Torr total pressure, 0.325 MHz/ns, and a post detection LPF of 3.5 MHz. Several averaging conditions were recorded. The corresponding solid lines shows the expected detector noise-limited precision calculated from the CLaDS signal and noise models, using error propagation analysis. (b) Evolution of the SNR as function of the CNR exhibiting a threshold effect (single cycle data and 200 averages).

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Fig. 6
Fig. 6 (a). Experimental spectrum extracted from the open-path time series measurements. Both the experimental data and the model fit are show. (b) Corresponding full modelling of the spectrum, including simulated noise. (c) Calculated individual contributions from the three atmospheric molecules integrated into the model. CLaDS dispersion signal (top panel) and transmission (bottom) are shown.

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Fig. 7
Fig. 7 Temporal evolution of methane mixing ratio as measured by the open-path CLaDS spectrometer.

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Fig. 8
Fig. 8 (a). Calculations of SNR and methane mixing ratio error as function of the difference frequency Ω. The top x axis shows the ratio of Ω to the full width at half maximum of the most intense CH4 transition in the scan range. (b) Single scan normalized experimental sensitivity as function of the laser chirp rate for open-path and low pressure methane mixing ratio measurements. Extrapolation of the experimental data to real time data processing has been included.

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Equations (9)

Equations on this page are rendered with MathJax. Learn more.

CNR= S R N R = S R N 0 ⋅ B T = A c 2 2 N 0 ⋅ B T ⋅ R L
σ yFM = W FM 3 3⋅CNR⋅ B T ⋅k
f(ω)= 1 2π [ − S⋅ΔL c − S⋅ L C c ω⋅( dn dω | ω−Ω − dn dω | ω ) ]
B T =2 (1+β)⋅ W FM ≈2⋅ W FM
SNR= 3 2π⋅c ⋅ ω⋅ Δ MAX ( n,Ω )⋅ c k c W 3 ⋅ S R N 0
Δ MAX (n,Ω)=MAX[ dn dω | ω−Ω − dn dω | ω ]
y=f( x,b )+ε
x ^ =R( y, b ^ , x a ,c )
x ^ −x= ∂R ∂y ⋅ε
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