Carbon Dioxide Measurement Based on Off-axis Integrated Cavity Output Spectroscopy Technology
Carbon dioxide(CO2)accounts for about 0.04％of the atmospheric composition and is one of the major greenhouse gases.With the development of industrial society,anthropogenic CO2 emissions are increasing every year,which undoubtedly aggravates global warming.Therefore,it is of great significance to monitor the CO2 concentration in the atmosphere to manage the CO2 emission scientifically.In this paper,a simple atmospheric CO2 gas detection equipment based on Off-axis Integrated Cavity Output Spectroscopy(OA-ICOS)was constructed using a 1.573 μm distributed feedback diode laser.Firstly,the CO2 detection system was built and optimized.In this paper,the absorption spectrum of CO2 at 6 358.65 cm-1 with a line intensity of 1.732×10-23 cm-1/(molecule·cm-2)is selected,and then the CO2 direct absorption signal of 400×10-6in the sealed cavity is measured,and the experimental results show that an effective optical range of about 2.4 km is realized in the lens reflectivity of 99.98％and 60 cm cavity length.There is a large amount of residual cavity mode noise in the transmission signal acquired in a single time,and the system noise can be reduced by averaging calculation.The experimental results show that the optimal value of the averaging number is at 1 000 times,and at this averaging time,the relative error of the absorbing area is averaged at 7.67× 10-3 and the STD is 5.87× 10-3.Second,the performance of the CO2 detection system was analyzed.By measuring CO2 gas from 400 to 2 000X 10 6 with an interval of 200 X 10-6,the linearity of the OA-ICOS system was obtained to be 0.996,and the maximum value of the STD of the absorbed area at each volume fraction was 3.23× 10-3(1 200X 10-6)and the minimum was 2.34X 10-3(1 800X10-6).By measuring the CO2 direct absorption signal in the sealed chamber for a long time,the optimal integration time of the system was obtained from the Allan curve analysis to be 98.3 s,and the minimum detectable limit was 0.63X10-6.The system response time is able to analyze the response speed of the system to the change of the measured value,which is an important parameter to characterize the performance of the system.In order to obtain the system response time,the gas control valve of the mass flow meter was repeatedly switched to alternately pass 2 000 X 10 6 of CO2 and N2,and the ventilation time was 180 s.The experimental results showed that the average value of the system rise(CO2 pass)response time was 50 s,and the fall(N2 pass)response time was＞180 s.The fall time was longer than the rise time due to the fact that the CO2 density was larger than that of N2,and CO2 was deposited at the bottom of the cavity,resulting in CO2 residuals inside the cavity after 180 s of N2 pass-through(～300×10-6).Finally,the application of indoor CO2 measurement was carried out on the OA-ICOS system.The results of 96 h of continuous indoor CO2 detection experiments show that the system can better reflect the activities of laboratory personnel,and some details of indoor CO2 changes,such as the intermittent stay of experimental personnel near the experimental platform to carry out other experiments,can also be monitored,which verifies the reliability and stability of the measurement device,and at the same time,provides a practical scientific basis for the management of indoor CO2 emissions.Conclusion,the CO2 detection equipment proposed in this paper is characterized by simple structure,high sensitivity and robustness,which is suitable for the detection of CO2 in the atmospheric background,and lays the foundation for the further development of CO2 detection instruments for atmospheric CO2.
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