Data preparation

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Data preparation

For the calculation of the turbulent fluxes in question continuous 60 min time series are needed. Time series of wind speed are constructed directly from the sonic anemometer data (the computer stores information from the fast response sensors in 10 min long files). Time series of temperature fluctuations are constructed directly from the data measued by the aspirated thermocouple. As it was described in section , the data acquisition computer of the vertical flux monitoring system stores raw voltage data for CO $_{2}\protect$ and H $_{2}\protect$ O together with physical quantities of pressure, temperature and humidity measured by the slow sensors immediately behind the sampling cell of the IRGA. Based on the calibration procedure described in section , the calibration constants for CO $_{2}\protect$ and H $_{2}\protect$ O are utilized during this step.

After construction of the 60 min raw voltage time series, the voltage values are converted into physical quantities. Based on the equations and notations presented in the description of the calibration, water vapor mole fraction is calculated as follows:

$\begin{displaymath} q=\left( m_{q}V_{q}\frac{p_{0}}{p}+b_{q}\right) \frac{T}{T_{0}}\: . \end{displaymath}$

(2.44)

Dry air mixing ratio of water vapor, $r_{q}$ is calcualted as

$\begin{displaymath} r_{q}=\frac{q}{1-\frac{q}{1000}}\frac{M_{H_{2}O}}{M_{d}}+c_{rq}\: , \end{displaymath}$

(2.45)

where $M_{H_{2}O}$ and $M_{d}$ are the molecular weights of water vapor and dry air, respectively, and $r_{q}$ is given in g H $_{2}\protect$ O/kg dry air.

The mole fraction of carbon dioxide can be calculated similarly:

$\begin{displaymath} c=\left( m_{c}V_{c}\frac{p_{0}}{p}+b_{c}\right) \frac{T}{T_{0}}+c_{rc}\: , \end{displaymath}$

(2.46)

where $c_{rc}$ is the mole fraction of CO $_{2}\protect$ in the reference gas, and the result is given in $\mu$ mol mol $^{-1}$ . For the calculation of the dry air mixing ratio of carbon dioxide $r_{c}$ , the mole fraction of water vapor has to be taken into account:

$\begin{displaymath} r_{c}=\frac{c}{1-\frac{q}{10^{3}}-\frac{c}{10^{6}}}\frac{M_{CO_{2}}}{M_{d}}\: , \end{displaymath}$

(2.47)

where $r_{c}$ is given in mg CO $_{2}\protect$ /kg dry air. Division with $10^{3}$ and $10^{6}$ in the denominator is neccessary to calculate the mixing ratios in mol/mol, since $q$ is given in mmol/mol and $c$ is given in $\mu$ mol/mol.

Next: Wind vector rotation / Up: Data processing Previous: Averaging time Contents

root 2001-06-16