Research Of Atmospheric CO₂ And Water Vapor Detection Based On Hollow Waveguide Sensor

Carbon dioxide and water vapor are important components of the atmosphere,both of which regulate the growth of biosphere vegetation,simultaneous measurement of changes concentration of carbon dioxide and water vapor in time and space play a crucial role in understanding the operation mechanism of carbon cycle,water cycle and energy cycle in the biosphere and the establishment of biosphere theory model.In this paper,the research of atmospheric carbon dioxide and water vapor detection based on hollow waveguide sensor is mainly demonstrated.Firstly,a set of tunable diode laser absorption spectroscopy?TDLAS?gas measuring device based on single pass cell is built,and the system performance is compared with the subsequent hollow waveguide sensor.The carbon dioxide and water vapor absorption in experimental band range are simulated using the Hitran2012 database,and selects the carbon dioxide absorption peak at 3663.85 cm^-11 which is not interfered by the water vapor absorption as the experimental absorption peak.The 10?90%response time of the system t_r has been measured as 38 s,the 0?10%delay time of the system t_d is 10 s,the measurement precision of the system is 44.5ppmv together with the corresponding precision of 3.9%,the stability time of the system can be obtained by Allan variance for 72 s and the Allan variance detection limit is 7.8 ppmv,which corresponds to an absorbance of 5.3×10^-5.Hollow waveguide has the characteristics of low loss,bendability,small size,etc.Compared with the multi-pass cell of the same optical path,the hollow waveguide as a gas cell can effectively reduce the volume of the measurement system and improve the system response time,so,the hollow waveguide has been regarded as an ideal candidate for gas absorption cell in the spectroscopic gas sensing system.In order to improve the detection limits of the system,a hollow waveguide sensor which can work under low pressure is designed.The influence of scanning frequency on signal fluctuation is studied,and the signal fluctuation is the minimum when the scanning frequency is 30 Hz;the relationship between pressure and the second harmonic?2f?is studied,the maximum 2f signal is obtained when the pressure was 165 torr,compared with the atmosphere pressure,the detection limit of the system is increased by 14.5 times under the condition of 165 torr;the 10?90%response time of the sensor t_r is 9 s and the 0?10%delay time of the system t_d is 2 s,the response time of the system is 4 times faster than the single pass cell system,the measurement precision of the system is 28.5 ppmv together with the corresponding precision of1.4%,and the precision of the system is 2 times higher than that of the single pass cell system.The stability time of the system can reach 431 s by Allan variance which is 6times than the single pass cell system,and the Allan variance detection limit is 1.17ppmv,which corresponds to an absorbance of 9.7×10^-6,the system resolution is 5.5times higher than the single pass cell system;Finally,ambient CO₂ and water vapor measurement is performed on two day in Winter and Spring separately,using Kalman filter to process the experimental data,The measurement precision can be improved to13.28 ppmv of CO₂ and 0.0229%of water vapor with average time of 1 s.