Theoretical study of 14CO2 spectrum measurement technology based on sub-Doppler wm-NICE-OHMS

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The massive emission of greenhouse gases, particularly CO2, has led to severe damage to the Earth’s ecological environment and poses a threat to human health. Many countries have therefore proposed policies to curb the greenhouse effect. Carbon monitoring is a critical prerequisite for realizing these goals, and tracking carbon emission sources can support the precise implementation and advancement of related policies more effectively. The contribution of fossil fuel combustion to greenhouse gas emissions can be inferred by detecting the abundance of 14C in carbon dioxide in a specific region. Conventional 14CO2 detection methods have significant drawbacks, including complicated operation, high cost and large equipment size. Laser absorption spectroscopy (LAS) offers advantages such as real-time, online in-situ measurement and simple operation, making it suitable for the online detection of isotopes. Among the various LAS techniques, noise immunity cavity enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is the most sensitive. This method integrates frequency modulation spectroscopy (FMS) into cavity enhanced spectroscopy (CES) to suppress the low-frequency noise while increasing the absorption paths, providing a minimum detectable absorption coefficient as low as 10–13. Additionally, the accumulation of high intracavity laser power in NICE-OHMS can stimulate saturation absorption, which has a narrow spectral width that can mitigate spectral overlap. In this work, we model the spectral signals of 14CO2 at different locations and select the transition line of 14CO2 at 2209.108 cm–1 as an optimal measurement target based on the principles of high-intensity and well-resolution. The theoretical analysis of the NICE-OHMS technique is then carried out, and theoretical simulations of a mixed sample of 14CO2 and its nearby interfering gases (13CO2, 12CO2, and N2O), are performed under the simulated experimental conditions. The results of the simulation show that the Doppler broadened spectral signal of 14CO2 is covered by the other gases’ signals with a very low amplitude, which is adverse to the detection of 14CO2. To eliminate the linear slope of the Doppler broadened signal and to further improve the signal-to-noise ratio, we perform 14CO2 spectral measurements by using wavelength-modulated NICE-OHMS (wm-NICE-OHMS). The results of the simulation show that the spectral lines are effectively separated, and the detection accuracy of the 14CO2 ratio is greatly improved. Finally, the effects of pressure and modulation index on the 14CO2 wm-NICE-OHMS signal are analyzed. The results show that when the pressure is 42 mTorr and the modulation index is 1.07, the signal amplitude of wm-NICE-OHMS reaches its maximum. This work lays a theoretical foundation for the high precision detection of 14CO2 in real-time environmental monitoring. The potential for large-scale application of wm-NICE-OHMS in carbon emission tracking is highlighted, providing a more cost-effective alternative to traditional detection methods. Furthermore, the technology is able to suppress spectral interference from other gases and achieve high resolution in 14CO2 measurements, which will greatly help monitor and reduce greenhouse gas emissions.

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