Research On Application Of Highly Sensitive Laser Spectroscopy For Detecting Atmospheric Greenhouse Gases And SF₆ Decomposition Gases

The detection of atmospheric greenhouse gases and SF₆ decomposition gases plays a crucial role in environmental protection,public health,and ensuring the safe and stable operation of electrical power systems.Laser spectroscopy,with its outstanding robustness,rapid response,high sensitivity,and high precision,has been widely applied in the field of gas detection.Cavity ring-down spectroscopy（CRDS）and photoacoustic spectroscopy（PAS）,owing to their extremely high sensitivity and accurate measurement capabilities,can precisely identify trace components in gases and are suitable for rapid analysis of complex gas systems.This dissertation presents a study on the detection of atmospheric environmental gases and SF₆ decomposition gases using CRDS and PAS.It expands the application domains of these spectroscopic techniques and provides reliable technical support for environmental protection and the safe and stable operation of electrical power systems.The main contributions are summarized as follows:Theoretical research on gas detection using CRDS and PAS was conducted,providing theoretical support for the application of these two laser spectroscopy techniques in the detection of atmospheric greenhouse gases and SF₆ decomposition gases.The mechanisms behind the broadening of different spectral line shapes were analyzed,laying a foundation for laser spectroscopy gas detection.The relationship between cavity ring-down signals and gas concentrations was derived,and factors affecting the finesse of optical cavities were analyzed,along with the study of mode matching between the cavity and the laser.Furthermore,the relationship between photoacoustic signals and gas concentrations was derived.Simulation analysis was conducted on the impact of different background gas mixtures（N₂,He,and SF₆）on the performance of differential Helmholtz photoacoustic cells,highlighting the importance of background gas consistency in multi-gas detection with PAS.Experimental setups for gas detection using CRDS and PAS were developed based on a quantum cascade laser with a central wavelength of 7.6μm and a tunable range of1230-1380 cm^-1.Suitable lasers were selected according to the basic principles of CRDS and PAS and the characteristic spectra of atmospheric greenhouse gases and SF₆decomposition gases.The CRDS experimental setup was constructed and optimized,including the response of the acousto-optic modulator,reflectivity of cavity mirrors,mode matching between the cavity and the laser,and testing of the decay signal.The PAS experimental setup was also constructed and optimized,testing the laser power modulated by the acousto-optic modulator,the signal from the differential Helmholtz photoacoustic cell,and the resonance frequencies of the photoacoustic cell under different background gases.The performance of the acousto-optic modulator and mechanical chopper was compared.A dynamic gas dilution system composed of five mass flow meters was established to precisely control and adjust the concentration of gas samples under test,ensuring the reliability and consistency of detection results.Based on experimental setups for CRDS and PAS,detection studies were conducted for atmospheric greenhouse gases CH₄,N₂O,SF₆,and SO₂F₂,achieving for the first time high-sensitivity CRDS measurements of SF₆ and simultaneous PAS measurements of SF₆and SO₂F₂.Using the CRDS setup,the impact of residual H₂O under static and dynamic air conditions on the simultaneous measurement accuracy of CH₄ and N₂O was investigated,achieving higher precision detection of the greenhouse gases CH₄ and N₂O.Research on the detection of the greenhouse gas SF₆ using CRDS was carried out,achieving ppb-level detection of SF₆.Measurement studies of the greenhouse gases SF₆and SO₂F₂ were performed using the PAS setup,realizing the simultaneous determination of these two greenhouse gas concentrations,with detection sensitivities reaching sub-ppm and sub-ppb levels,respectively.Utilizing experimental setups for CRDS and PAS,research on the detection of decomposition gases in SF₆ gas-insulated equipment was carried out,marking the first achievement of SF₆ decomposition gas measurement using CRDS and simultaneous PAS measurement of four decomposition gases:SO₂F₂,CF₄,SO₂,and SOF₂.Measurement studies on SF₆ decomposition gases using the CRDS setup realized the simultaneous detection of SO₂F₂,CF₄,and SO₂ with 1%SF₆/N₂ background gas,achieving detection limits in the ppb to sub-ppb range.Research employing the PAS setup on SF₆decomposition gases analyzed the saturation phenomena of SF₆ photoacoustic signals,enabling the simultaneous detection of SO₂F₂,CF₄,and SO₂ in pure SF₆with minimum detection limits in the ppb range.Further considering the constraints of SOF₂ samples,the influence of different background gases（N₂,He,and SF₆）on the photoacoustic signals of SO₂F₂,CF₄,SO₂,and SOF₂ was analyzed.A 95%SF₆/5%He background gas was determined to be optimal,under which simultaneous PAS detection of these four SF₆decomposition gases was achieved.