Development Of An Open-Path Laser Gas Analyzer For Eddy Covariance Flux Measurement

The exchange of energy and mass between the ocean and atmosphere has significant impacts on the global environment,climate,and ecological balance.With decades of technological development,the eddy covariance method has become a preferred method for direct flux estimations in turbulent motions without parametric assumptions,and is widely used in ecological flux observations.Generally,the physical principle for the eddy covariance method is to measure the quantity of molecules moving upward or downward over time,and the speed in which they travel.Mathematically it can be represented as a covariance between measurements of vertical velocity of the upward or downward movements,and the CO2 and H2O concentration of the entity of interest.The basic equipment for a flux measurement system mainly includes a three dimensional ultrasonic anemometer and a gas analyzer with a max data rate of 20 Hz.Turbulent changes happen very quickly,and the respective changes are very small in concentration,density,or temperature.It is therefore necessary to use an instrument with high precision and fast data rate of measurements,especially in high wind environments.Nevertheless,data rates of flux measurements reported in literature are typically around 20 Hz or slower.The 20 Hz frequency detection may cause data loss and inaccuracy for analyzing the gas exchange and flux.For trace gases measurements,tunable laser absorption spectroscopy was developed decades ago as an ideal analysis and measurement technology,which has the advantages of high resolution,high selectivity,and high sensitivity.In this work,we have developed a simple and compact laser gas analyzer with a data rate of 100 Hz,based on laser absorption spectroscopy and derivative absorption spectroscopy.The analyzer is designed by using two diode DFB lasers operating at wavelengths of?2004 nm for CO2 and?1392 nm for H2O measurements.Meanwhile,we have designed a multi-pass cell with a 20 m optical path length for CO2 absorption measurements and a single-path cell of 15 cm optical path length for H2O absorption measurements,as well as a miniaturized TDLAS electronics system with 0.2 mA current control accuracy,0.001 cm-1 temperature control accuracy,60 dB dynamic range gain adjustment and 0.04 dB fine adjustment for spectral signal intensity.By developing a fast data processing of derivative absorption spectroscopy,we were able to achieve gas concentration measurements at a 100 Hz data rate.To evaluate the performance of numerical analysis of derivative absorption spectroscopy,we prepared a series of reference gas mixtures of CO2 and H2O with different concentrations and made measurements at atmospheric pressure.The results show a good linear dependence(adj.R2=0.995 for CO2 and adj.R2=0.999 for H2O),and demonstrate that the algorithm is valid for trace gas measurements.Meanwhile,the detection limit of the developed laser gas analyzer is evaluated by using Allan variance plots.The measurement noise at 100 Hz data rate(i.e.,0.01 s averaging time)is about 0.40 ppmv for CO2,and 8.17 ppmv for H2O,respectively.In order to test the performance of the developed gas analyzer for H2O and CO2 fluxes measurements under low and high wind speed environments,we conducted the field measurements at two distinctive sites with an ultrasonic anemometer on an offshore platform in the Yellow Sea of Yan-tai city in Shandong province with high-wind-speed marine environment and on the Jue-hua Island of Huludao city in Liaoning province with low-wind-speed terrestrial environment.The commercial LI-7500-CO2/H2O instrument is also installed nearby to calibrate and compare the accuracy of measurements.By studied the data preprocessing algorithm,the two times coordinate rotation algorithm,Ogive algorithm for optimal time length and the auto flux calculation algorithm,the final flux measurements are calculated with different wind speeds and different data rates.The H2O fluxes computed for data rates of 20 Hz and 100 Hz can differ by up to 16%(adjustable R2=0.84)in the 10 m/s wind speed environment.As a comparison,the difference of H2O fluxes is about 5%(adjustable R2=0.95)in the 4 m/s wind speed environment.The difference suggests that the contribution of 100 Hz flux measurements increases by about 11%,as wind speed changes from 4 m/s to 10 m/s.In this paper,a laser gas analyzer based on a second derivative laser absorption spectroscopy method has been developed to achieve a 100 Hz fast data rate,which is faster than that of a well-established 20 Hz commercial instrument.Meanwhile,we made field measurements by installing the integrated instruments in two different environments to verify the influence of different wind speeds on flux measurements against a commercial instrument LI-7500.In general,the 100-Hz gas analyzer we developed has a wide prospect for flux measurement applications,especially when rapid turbulence is involved.