Research Of Plant CO₂ Gas Absorption Rate Detection Based On QEPAS

Plant carbon sink refers to using plant photosynthesis to absorb carbon dioxide in the atmosphere and fix carbon to the plants and soil,in order to effectively reduce concentration of greenhouse gases,especially the concentration of CO2 in the atmosphere.Carbon sinks need to be measured and monitored while continuous,real-time,and sensitive trace gas concentration testing is required.Quartz tuning fork enhanced photoacoustic spectroscopy(QEPAS)technology is a new type of gas detection technology based on infrared absorption spectroscopy,with the advantages of great gas selectivity,fast response.However,near-infrared QEPAS also has a high detection limit and a low sensitivity in the trace detection range,and further optimization is needed to meet the trace detection requirements for carbon sink measurement.This article first describes the molecular spectrum,the Beer-lambert's law,the principle of wavelength modulation,the principle of harmonic detection,and the innovation of the QEPAS technology in QEPAS technology:Quartz tuning forks are used as photoacoustic detectors.A trace CO2 gas concentration detection system based on near-infrared QEPAS was built.According to the absorption line data of CO2 gas in the Hitran molecular absorption spectrum database,the absorption line in the near-infrared region of 1580.0395 nm was selected as the measurement line;the central wavelength was used.A 1.58?m distributed feedback semiconductor laser is used as the excitation light source;A transimpedance preamplifier and a lock-in amplifier are used to demodulate and filter the signal;And the Lab VIEW platform is used for signal acquisition and processing.After that,the gas concentration detection system was optimized.In order to study the influencing factors and corresponding relationship of signal amplitude,the direct absorption signal was first simulated and the accuracy was verified.Then,based on the basic principle of wavelength modulation technology,the harmonic component expression was deduced.Through programming,the influence of the wavelength modulation coefficient on the amplitude of the second harmonic signal was simulated,and was experimentally measured.The simulation result is highly consistent with the experimental data,verifying the effectiveness of the wavelength modulation coefficient simulation and obtaining the optimal modulation coefficient which is 2.2 and the corresponding optimal modulation current value which is 10mA.Afterwards the design of the photoacoustic detection module is optimized,and the low noise connection of the quartz tuning fork and the signal processing circuit is completed,which effectively reduced the amplitude of the noise signal;Coaxial resonant tube with optimization parameters is selected,the peak value of the second harmonic signal is increased to 1.68 times than before.The detection limit and linearity experiments were performed on the optimized near-infrared QEPAS trace gas concentration detection system.The lower detection limit of the system was reduced from 496 ppm to 71 ppm;the signal-to-noise ratio was increased to 14,which was 7 times than before.The experimental results show that the optimization method can effectively reduce the detection limit of the system,improve the signal-to-noise ratio,and enhance the detection system's adaptability to complex environments.Finally,based on the optimized detection system,a CO2 absorption rate detection system based on QEPAS was established.Four different types of plants were placed in a sealed box,and the CO2 gas concentration curve in the box environment was recorded within one hour,and the CO2 absorption rate of the four-pot plants was obtained.In order to further verify the effect of the improved detection system,two pots of the same kind of plants under weak light intensity were tested separately.Afterwards,based on the deduced formula,the daily carbon sequestration of the unit leaf area of the plants was calculated,and comparing with the accurate data obtained from the query literature,the accuracy of using this system for the measurement of plant carbon sinks was verified.