Measurements of the vertical flux of CO
, HO, momentum
and sensible heat by eddy covariance at 82 m above the ground was facilitated
by mounting an ultrasonic anemometer (Gill Solent Enhanced), an aspirated thermocouple
(50 cm from the anemometer) for fast-response temperature measurements, and
an additional air sampling tube for CO and HO measurements
to the 4.4 m long instrument arm. The measuring system was developed and operated
by László Haszpra (Hungarian Meteorological Service).
Measurements of CO and HO are made at 
4 Hz
using a fast response IRGA (LI-COR model LI-6262). Air is pumped through the
115 m long sampling tube and the analyzer at about 15 l min
, producing
a pressure drop of approximately 45 kPa. Pressure fluctuations generated by
the pump are damped by means of a 6 l buffer volume. Immediately behind the
sample cell of the analyzer we measure pressure (MKS Instruments model 122A
barotron), temperature and relative humidity (Vaisala HMD20YB). The pressure
and temperature data are used to correct the instrument response for variations
in these parameters (see below). The humidity data are used to determine the
calibration function for water vapor measurements by the LI-COR 6262 IRGA.
The CO analyzer runs in relative mode. Dry, synthetic air with a
CO mixing ratio of 330-340 ppm is used as a reference gas (Messer
Hungarogáz) during the measurements. The flow rate is 5-10 cm
min
through the reference cell of the IRGA. The analog output of the analyzer for
CO and HO, as well as the signals of the pressure and
temperature/humidity sensors are digitalized by the common A/D-RS232 converter.
A separate data acquisition computer (486 PC with a 40 MHz CPU, 4 MByte RAM,
1 GByte HD) is used to read data from the fast-response instruments (sonic anemometer,
thermocouple, and IRGA). The computer communicates with the instruments through
two standard serial ports: COM1 receives the data from the sonic anemometer
while the two A/D-RS232 devices are controlled via COM2. The data acquisition
cycle is triggered by the signal from the sonic anemometer. After reception
of a data package (horizontal and vertical wind speed, wind direction and error
code) the computer requests data from the aspirated thermocouple, the CO/HO
analyzer and its accessory sensors (pressure, temperature/humidity), and the
CO profile analyzer. The position of the multiport valve of the profile
system is also determined, which allows synchronization between the two independent
data acquisition computers. The data acquisition software is written in Turbo
Pascal language and runs under DOS 5.0. The eddy covariance system produces
data at a rate of about 600 MByte/month, and the data are stored on a CD-R without
compression.
Data coverage from the beginning of the EC measurement (end of April, 1997) until the failure of the IRGA (which caused data loss from the beginnig of 2000) was around 85%. After data screening it reduced to 78%. The average data coverage for the FLUXnet community is 69% (Falge et al., 2001).