*3.4. Comparison of Flux Measurements under Low and High Wind Speeds*

Measurements of wind speed form an integral part in the determination of flux of gas emissions and movements. Three-dimensional wind speeds are measured via an ultrasonic anemometer. However, the mounting of the anemometer may not be exactly vertical or horizontal to the ground. A coordinate rotation is needed to transform the measurement values of the anemometer to the three velocity components *u*, *ν*, *w* in respect to the ground, where *w* is the vertical wind velocity and is expected to have a mean value of zero. The components *u* and *ν* are the two wind velocities in the horizontal plane. The horizontal component *ν* is also aligned during the coordinate rotation to have a mean value of zero. Therefore, the component *u* represents the speed along horizontal wind direction. The results of wind speeds after rotating and H2O concentrations are plotted in Figure 9. The maximum horizontal wind speed in the marine environment of Site-A is about 13 m/s, whereas the maximum vertical wind speed is about 4 m/s in the terrestrial environment of Site-B.

**Figure 9.** Measurement data of H2O concentrations and three-dimensional wind velocities (*u*, *v* for the two horizontal components, *w* for vertical). (**a**) Data of Site-A. (**b**) Data of Site-B.

Fast measurements can capture more details of rapid small-scale turbulence in air movements, especially in a high wind speed environment. Subsequently, this is expected to be an advantage for flux determination. We numerically analyzed the impact of data sampling rate by block averaging every 5 data samples of the original 100 Hz data set to obtain a 20 Hz data set. This 20 Hz data set loses frequency contributions above 20 Hz. Both data sets are computed for flux using a 5 min time base (see Equation (5)). The results of H2O fluxes are shown in Figure 10, for both high and low wind speeds.

**Figure 10.** Comparison of H2O fluxes between 20 Hz and 100 Hz. (**a**) The comparison of H2O fluxes between 20 Hz and 100 Hz in 10 m/s wind speed environment. (**b**) The correlation analysis of H2O fluxes between 20 Hz and 100 Hz in 10 m/s wind speed environment. (**c**) The comparison of H2O fluxes between 20 Hz and 100 Hz in 4 m/s wind speed environment. (**d**) The correlation analysis of H2O fluxes between 20 Hz and 100 Hz in 4 m/s wind speed environment.

As described in Figure 10a,b, the H2O fluxes computed for data rates of 20 Hz and 100 Hz can differ by up to 16% (adjustable R<sup>2</sup> = 0.84) in the 10 m/s wind speed environment. As a comparison, the difference of H2O fluxes is about 5% (adjustable R<sup>2</sup> = 0.95) in the

4 m/s wind speed environment, shown in Figure 10c,d. 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. The data points spread out further away from the straight-line in Figure 10b than in Figure 10d.
