Objective As a method for directly observing turbulent motions without parametric assumptions, the eddy correlation method is undoubtedly the most advanced and preferred flux observation method at present. This method is widely used in ecological flux observations. The basic equipment of the eddy-related measurement system mainly includes a three-dimensional ultrasonic anemometer and a gas analyzer, which calculates the flux value using the covariance of the vertical wind speed and concentration fluctuation. To capture tiny concentration variables, the concentration measurement method is crucial. Moreover, the accuracy and sampling frequency of the instrument have high requirements. The commercial gas analyzers use nondispersive infrared technology, which can only achieve a time resolution of 20 Hz, resulting in a loss of high-frequency data. Based on this research background, a new type of gas analyzer for eddy-related systems must be designed, which can achieve high-frequency flux measurements.
Methods A laser gas analyzer is developed using laser absorption spectroscopy technology. First, the absorption lines of H2O and CO2 molecules near 7181 and 4990 cm -1, respectively, are simulated based on the HITRAN2012 database. Second, open-path structures are designed based on the absorption intensity of two gas molecules. Third, a miniaturized electronic measurement system is designed, which facilitates laser drive, signal acquisition and processing, and data storage. Thereafter, concentration results are calculated using derivative absorption spectroscopy. To analyze the correlation between a known concentration and the peak value of derivative spectrum signal, the integrated instrument is tested and analyzed in the laboratory using CO2 standard gas with different concentrations. Finally, the integrated gas analyzer is installed on the Yantai National Satellite Ocean Calibration platform for one week to detect real sea-air flux and compared with the LICOR7500-H2O/CO2 analyzer.
Results and Discussions We conduct performance tests on the developed prototype. First, the direct absorption and second derivative spectrum signals of the instrument are obtained using different concentrations of CO2 standard gas at normal temperature and pressure in the laboratory [Fig.4 (a) and Fig.4 (b)].The linear correlation between different concentrations and peak values is 0.998 (Fig.5). The result shows that the derivative spectrum technique is suitable for concentration inversion. A 4 h data comparison on the Yantai National Satellite Ocean Calibration platform between the TDLAS-H2O/CO2 and LICOR7500-H2O/CO2 analyzers shows good consistency. Moreover, a 1 s data comparison shows that the former exhibits a higher sampling frequency (100 Hz) and can capture more fluctuations in the concentration than the latter [Fig.7 (a) and Fig.7 (b)]. To analyze the detection limit of the TDLAS-H2O/CO2 analyzer, 20 min stable data are selected for Allan variance analysis. The result shows that the detection limits of the H2O and CO2 analyzers are 8.17×10 -6 and 0.40×10 -6, respectively, when the time resolution is 100 Hz (Fig.8). Overall, the main performances of the TDLAS-H2O/CO2 and LICOR7500-H2O/CO2 analyzers is compared (Table 1). Additionally, for a continuous week, the data results show that the CO2concentration exhibits obvious changes, while the daily change in the H2O concentration is not obviously affected by seawater and rain (Fig.9).
Conclusions In this study, the miniaturized laser gas analyzer for the eddy correlation method is integrated using laser absorption spectroscopy technology and derivative absorption spectroscopy. Based on the HITRAN2012 database, the absorption lines of H2O and CO2 molecules near 7181 and near 4990 cm -1 are simulated. Moreover, open-path structures and miniaturized electronic measurement system are designed. The main performances of the integrated analyzer are listed as follows: the detection limits of the H2O and CO2 analyzers are 8.17×10 -6 and 0.40×10 -6, respectively, and the time resolution is 100 Hz. The integrated gas analyzer is installed on the Yantai National Satellite Ocean Calibration platform for one week to detect real sea-air flux and compared with the LICOR7500-H2O/CO2 analyzer. The result shows that the laser gas analyzer is stable and reliable and can be widely used in ecological flux detection applications. Future studies will involve combining the wind speed data of the ultrasonic anemometer to analyze the flux results at the time resolution of 100 Hz and comparing these data with the data at the time resolution of 20 Hz to analyze the contribution value of flux observation.
