| The carbon and nitrogen cycle in the atmosphere can subtly affect climate change and the biological process of the earth,and is one of the important factors affecting carbon neutrality.Although carbon monoxide（CO）is not a direct greenhouse gas,its content affects the concentration of OH-radicals,which in turn influence the concentrations of nitrous oxide（N2O）and methane（CH4）in the long run.While N2O and CH4 are two typical greenhouse gases,which concentrations have been rising since the start of the first industrial revolution.The sources and sinks investigation of these species are of great important for predicting and controlling their changes in the atmosphere.Although some anthropogenic sources（including motor vehicles,industry,etc.）and natural sources（including agricultural soil,sewage,and animal husbandry,etc.）are considered to be the main source,however,they have not been studied in sufficient detail.For two main reasons:（1）The sensitivity,stability and time resolution of traditional sensors are not high enough,thus,it is difficult to achieve real-time measurement and analysis,which limits the dynamic observation of these sources and sinks;（2）These sources and sinks are usually unstable,especially the natural sources and sinks,which change greatly with the seasons,temperature and humidity.Therefore,high-sensitivity and dynamic monitoring of these components is essential to identify their sources and sinks,as well as their circulation in the atmosphere.Here,we develop a multispecies trace gas detection system by combining a 4.6m room-temperature continue-wave distribute-feedback cascade laser（RT-CW-DFB-QCL）with a multi-pass absorption cell（76 m）,which can achieve sub-ppb or ppb detection sensitivity for CO,N2O,and CH4.The main work includes:1.The development of a CO,N2O,and CH4 multi-gas sensing system based on MIR-MIR coupling method.We develop a multi-species detection system by combining a mid-infrared DFB-QCL with a near-infrared DFB laser.Compared to the traditional multi-laser time-division multiplexing system,the proposed method can effectively improve the time resolution of the system.The wavelength modulation spectroscopy（WMS）method is used to improve the detection sensitivity of the system.A Lab VIEW-based quadrature lock-in amplifier is developed for demodulating the harmonic signals,which can make the system more portable.The experimental results show that the best detection sensitivities for CO and N2O are～0.44ppb and～0.41 ppb,respectively,and～2 ppb for CH4 at an integration time of 900 s.The experiments carried out long-term real-time measurements of CO,N2O and CH4 in ambient air,providing strong evidence for the dynamic changes of these gases in the atmosphere.2.The development of a QCL-based N2O isotope spectral analysis system.The developed system can achieve the effective measurement of its 15N14N16O and 14N15N16O isotopes under ppm(10-6)level of N2O,when combining the QCL spectroscopic system with a 76 m multi-pass cell,which provides an excellent method for real-time N2O isotope analysis.The experimental results show that the detection sensitivity of the optimal normalized abundance can achieve～5.7‰and～0.59‰under 10 ppm and 100 ppm N2O conditions,respectively.In order to evaluate the N2O isotopic in the air,a proconcentration unit was developed,to improve the N2O concentration of ambient air through the low-temperature adsorption and high-temperature release processes.The relationships between the mass of the porous polymer and the adsorption temperature with the released concentration of N2O are investigated in detail,and the high-sensitive detection of N2O isotopic abundance in the air is achieved.3.The development of a calibration-free wavelength modulation spectroscopy（CF-WMS）-based CO and N2O detection system.The proposed method effectively improves the detection sensitivity while avoiding the complicated concentration calibration process and enhances the anti-light intensity fluctuation ability of the system.A Lab VIEW-based calibration-free iterative fitting algorithm is developed to achieve the online spectral analysis.The experimental results show that the CF-WMS can effectively improve the stability and sensitivity.In addition,the experiment also conducted research on the process of ground-air exchange in soil respiration,and measured the effect of different soil types（such as:grassland soil,reed soil,pond soil,and organic soil）,and different soil moisture（for example,grassland soil）on CO and N2O sources and sinks.For the different soil types experiments,the reed soil and organic soil with rich nitrogen sources showed the release of N2O.While grassland soil,reed soil,and organic soil showed the absorption of CO.For the different humidity experiments,a proper soil humidity can effectively increase the sink of CO and the source of N2O.In summary,we established a QCL-based spectral gas detection system,and several applications are employed,for example atmospheric multi-component measurement,air isotope measurement,and gas exchange during soil respiration.This can help to research the atmospheric carbon and nitrogen cycle process,understand the sources and sinks of these gases,and provide an effective research basis for atmospheric environment governance and carbon neutralization strategies. |
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