*3.3. Experimental Field Measurements*

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, as depicted in Figure 6. Figure 6a shows the installation of our integrated instruments and an ultrasonic anemometer on an offshore platform in the Yellow Sea of Yan-tai city in Shandong province (Site-A) with high-windspeed marine environment. Figure 6b shows the installation on the Jue-hua Island of Huludao city in Liaoning province (Site-B) 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.

**Figure 6.** Photographs of the installation for field measurements at two sites of different environmental conditions. (**a**) Site-A of marine environment. (**b**) Site-B of terrestrial environment.

The results of comparison measurements for an hour in the marine environment (Figure 6a) with wind speed of about 13 m/s are displayed in Figure 7a,b, which show CO2 and H2O concentrations determined by our laser gas analyzer and the commercial LI-7500 analyzer. Our H2O sensing unit was installed close to the LI-7500, while the CO2 sensing unit was slightly further away. The H2O concentration measurements show good agreement between the two instruments. The difference of CO2 concentration trend between our laser gas analyzer and the LI-7500 analyzer was partly due to its installation location, which is slightly further away, therefore had different wind conditions. Another contributing factor might be the mounting and housing of the multi-pass sensing unit, which we will investigate in future studies. Furthermore, we measured CO2 absorption in the wavelength region of its ~2.0 μm absorption band, while the LI-7500 analyzer operated around the ~4.2 μm stronger absorption band, resulting in a higher sensitivity with shorter optical path length. The detected concentration of CO2 for both analyzers in the marine environment ranges from 380 ppmv to 410 ppmv, and the detected concentration of H2O ranges for 22 mmol/mol to 25 mmol/mol.

**Figure 7.** Comparison measurements between the laser gas analyzer and an LI-7500-CO2/H2O for an hour. (**a**) CO2 and (**b**) H2O measurements. The data rate of the TDLAS was at 100 Hz (with only one-fifth of the data points plotted in the graphs), whereas the LI-7500 was at its max 20 Hz.

For the investigation of the frequency characteristics of the measurements and the time response of the instruments, we applied the frequency power density method to analyze the recorded time series of the concentration data [37]. Figure 8a,b show the comparison of power density spectra for CO2 and H2O concentrations between the laser gas analyzer and LI-7500 analyzer via a fast Fourier transform method. The slope of spectra (ploted in log–log scales) is approximate to −5/3 of the Kolmogorov theory in the inertial subrange [38], indicating that the laser gas analyzer is capable of measuring turbulence fluxes of CO2 and H2O via EC method.

**Figure 8.** Power density analysis of concentration measurements between laser gas analyzer and LI-7500 analyzer for (**a**) CO2 and (**b**) H2O concentration measurements.
