*2.2. Method of Flux Measurements*

The eddy covariance (EC) method has been widely used for direct measurements of surface atmosphere exchange. It uses the covariance between vertical velocity *wi* in wind speed and fast variations of *Ci* in trace concentration. The EC flux *F* can be calculated from a recorded time series of *N* measurements as:

$$F = \overline{\mathbb{C}' \cdot w'} = \frac{1}{N} \sum\_{i=1}^{N} \left[ (\mathbb{C}\_i - \overline{\mathbb{C}}) \cdot (w\_i - \overline{w}) \right] = \frac{1}{N} \sum\_{i=1}^{N} \mathbb{C}'\_i \cdot w'\_{i\prime} \tag{5}$$

where *C* and *w* are the instantaneous deviations from their corresponding mean values *C* and *w*, respectively. For this work, an ultrasonic anemometer (GILL-HS100) was used to determine vertical wind speeds. The instantaneous gas concentrations of the species of interest (i.e., H2O vapor and CO2 gas) were measured with our newly developed laser gas analyzer at a fast data rate of 100 Hz. As eddies occur on a wide range of timescales, it is necessary to use sufficiently long averaging time for calculating mean values. For this study, a time interval of about 5 min was chosen for calculating the average value when operating at a data rate of 100 Hz (resulting in a total of 30,000 data points). Alternatively, the effective data sampling can be slowed to a lower rate (e.g., 20 Hz), or the averaging time base can be extended.
