Research On Ocean Dissolved CO₂ Gas Measurement System Based On The Compact Multi-pass Cell Absorption Spectrum

The measurement of dissolved CO2 concentration in the ocean and its carbon isotope(δ¹³CO2)is important for the development of marine ecological environment monitoring and seabed resource exploration,especially for the combustible ice resources exploration.Combustible ice is a new type of green energy,which is widely distributed and has huge reserves.And its rational exploitation after fine investigation can solve the energy crisis.Combustible ice is mainly composed of the hydrate of methane,and is named because of looking like ice and being ignited when exposed to fire.The existence of combustible ice in stable form requires strict temperature and pressure conditions,and it will crack for seawater warming and geological movements,which leads to an increase in the concentration of dissolved CH4 gas in the seawater surrounding the combustible ice.The diffusion oxidation of CH4 will generates a large amount of CO2 and increases the concentration of dissolved CO2 in the surrounding seawater.Since dissolved CO2 concentration is also influenced by geological movements and biological activities,it is impossible to distinguish whether the dissolved CO2 concentration anomaly is caused by the cracking of combustible ice,so the detection ofδ¹³CO2 isotope with tracer effect can effectively distinguish the source of concentration anomaly and realize the fine exploration of combustible ice mining area.Therefore,the development of the ocean dissolved CO2 concentration andδ¹³CO2isotope measurement system is crucial.Currently,the in-situ measurement of the dissolved gas has become a key research direction by using geochemical testing tools.Compared with the existing in-situ measurement technology of marine dissolved gases.Tunable Diode Laser Absorption Spectroscopy（TDLAS）technology has the advantages of simple system structure,good selectivity,high stability and fast response,which can meet the demand of in-situ detection of marine dissolved gases in combination with water-gas separation and deep-sea pressure-resistant devices.This paper aims to develop a TDLAS-based system for detecting CO2concentration andδ¹³CO2 isotope,and to address the problem of long response time due to insufficient efficiency of deep-sea water-gas separation devices.Also,in order to improve the response time of real-time measurement and meet high precision measurement performance simultaneously,this work starts from the design of compact multi-pass cell with small volume and long optical path length（OPL）and optimizes system performance to meet the needs of sea trial applications by combining with high intensity absorption spectra in the mid-infrared.The specific research work is as follows:Firstly,the light ray transmission model within the multi-pass cell is investigated.To address the problems of near-axis error in the near-axis matrix model and the low utilization rate of single circular spot pattern mirror,a light ray transmission model based on spatial equations and the law of reflection was established to realize the accurate calculation of each light ray angle,spot position and OPL.The limitation that the near-axis matrix model is only applicable to near-axis angle and single circular spot pattern is removed.For the problem of spherical aberration and divergence angle changing spot shape,leading to interfering noise,a filling model with a large number of parallel light rays to fill the incident light ray was established,so that the spot shape change could be calculated and the output hole size could be designed accurately,as well as improving the spatial equation model.Based on the above modeling,a dense spot pattern is realized,and the mirror utilization is improved.Compared with the near-axis matrix model,the effective OPL of the spatial equation model is improved by 4.74times and the signal-to-noise ratio is improved by 6.6 times by designing the same volume multi-pass cell（MPC）with the same mirror and incident position.It provides a modeling basis for designing compact MPC.Secondly,parameter design method of the compact MPC is investigated.To address the problem of the simulation attempts struggling to find the optimal parameter of MPC and consuming a lot of time,a compact MPC with a volume of 15.44 m L and an effective OPL of 6.3 m was developed by using parallel multiple population genetic algorithm（PMPGA）with intelligent search.The mechanical design and integration of the developed MPC was verified by gas tightness test within one hour with the result of no air leakage,and the optical path stability was tested after meeting the usage requirements.Combining the system design of the gas detection scheme with the absorption spectral lines,the response time of the system based on the compact MPC is 40 s and its detection limit is in the order of ppbv.Thirdly,the overall system performance optimization is investigated.Using the mid-infrared CO2 absorption spectral lines at 4319 nm,the simultaneous measurement of CO2 concentration andδ¹³CO2 isotope was realized by a single laser.After analyzing the effect of temperature and pressure on the absorption spectral lines,a measurement scheme was developed for 25℃and 30 Torr.Based on the joint multiplexing of multiple spectral lines with N2 dilution,a wide range（0～3000 ppmv）of concentration measurement was achieved.The electrical modules of the measurement system were developed to ensure the measurement performance of the system,while the linear compact optical path structure considering the laser,MPC and detector was designed to reduce the number of optical components required for the system.Using multi-frequency modulation to enhance the second harmonic signal amplitude,the lower detection limit of the system was reduced from 23.5 ppbv to 7.1 ppbv at an average time of 2 s.The Pau Ta criterion and the second harmonic acquisition correction method were used to remove bad points from the test data and to improve the long-term operating stability of the system.The standard deviation of CO2 after applying Kalman filter was 0.26 ppmv and the standard deviation ofδ¹³CO2 isotope was 0.73‰.The theoretical limiting resolution ofδ¹³CO2 isotope for the measurement system was as low as 0.29‰at 5 min averaging time based on Allan variance.Comparing with the measurement accuracy of the commercial Picarro G2201-i,the developed system has an absolute error of 9 ppmv in the concentration measurement,and has an absolute error of 0.78‰in the isotope measurement.Finally,the developed measurement system was applied to the sea trial near the Seahorse Cold Spring in the southeast sea area of China by carrying a research vessel with the assistance of a deep-sea bracket and a pressure-resistant bin.The sea trial is mainly for the exploration of combustible ice mineral resources at a depth of 400～1400m.The system dived longitudinally and measured the dissolved CO2 concentration andδ¹³CO2 isotope in real time of 40 s.During the sea trial application,the overall working condition of the measurement system was well,the measured dissolved CO2concentration was around 800 ppmv without abnormality,and the dissolvedδ¹³CO2isotope was around-10‰without abnormality.The sea trial has realized the in-situ real-time measurement of dissolved CO2 concentration andδ¹³CO2 isotope in the ocean,which provides an effective support for the exploration of combustible ice and ocean chemical analysis.The topic of this dissertation is based on the National Key Research and Development Program of China,"Research on In-situ Multi-parameter Geochemical Measurement Technology and Device for Near-Seafloor".