results (near real-time data)
ratios of CO2 are measured at 10,
48, 82 and 115 m above the ground. Air is
pumped through 9.5 mm diameter tubes (Dekoron
Type 1300) to a CO2 analyzer located
in the TV transmitter building. A 47 mm diameter
(Whatman EPM) particle filter is located at the
inlet of each tube. Diaphragm pumps (KNF Neuberger
type UN73MVP) are used to draw air continuously
through each of the tubes from the four monitoring
levels at a flow rate of about 2 l/min. After
the pump, the air at 40 kPa overpressure enters
a glass trap for liquid water which is cooled
in a regular household refrigerator, to dry the
air to a dew point of 3-4oC. Liquid
water is forced out through an orifice at the
bottom of each trap.
The four inlet
tubes and the standard gases are connected to a computer controlled,
16-position valve (VICI AG, Valco Europe), that selects which monitoring
level or standard gas is sampled by the analyzer. The valve head is
protected by 7 m m in-line filters. Ambient air flows continuously
through the multiport valve so that the system is constantly flushed.
The (expensive) standard gases are shut off when not in use by means
of computer-controlled solenoid valves. The air leaving the multiport
valve through its common outlet is further dried to a dew point of about
-25oC by passage through a 182 cm long Nafion drier (Permapure,
type MD-110-72P), so that the water vapor interference and dilution
effect are less than 0.1 ppm equivalent CO2. The Nafion drier
is purged in a counter-flow (100 cm3/min) arrangement
using waste sample air that has been further dried by passage through
for CO2 is carried out using an infrared
gas analyzer (IRGA) (Li-Cor Inc. model LI-7000).
A constant sample flow rate of 100 cm3/min
is maintained by a mass flow controller (Tylan
model FC-260). The reference cell of the CO2
analyzer is continuously flushed at a flow rate
of 5-10 cm3/min with a compressed reference
gas of 330-340 ppm CO2 in synthetic
air (Messer Hungarogáz). Calibration of
the analyzer is carried out using four standards
spanning 330-420 ppm CO2, that were
prepared by NOAA/CMDL.
The basic measuring
cycle is two minutes, consisting of one minute flushing and one minute
signal integration. Each one minute average and standard deviation is
based on 6-7 measurements. The multiport valve steps through the four
monitoring levels in eight minutes. Every 32 minutes, after four 8 minutes
measuring cycles, the standard gas with the lowest CO2 mixing
ratio is selected and analyzed, and we term this measurement a "zero".
After every sixth cycle (every 202 minutes) a full four-point calibration
is carried out. The reference and sample cells of the CO2
analyzer are not pressure or temperature controlled. The "zero"
measurements are used to account for any short-term drift of the analyzer
due to changes in ambient pressure or temperature. A quadratic response
function is fit to each set of calibration gas measurements. The "zero"
offset and response function are linearly interpolated in time to obtain
values appropriate to calculate CO2 mixing ratio from the
To flush the tubing
the solenoid valves for the standard gases are open for 2 minutes prior
to measurement, so that 400 cm3 of standard gas is used per
zero check or calibration (2 min flushing, 2 min measurement).
The lifetime of the lowest standard used for zero checking is more than
half a year, while the other standards may last as long as 3 years.
The standards are calibrated before shipping to Hungary and after their
return by NOAA/CMDL. So far no significant drift has been observed.
The CO2 standards prepared by NOAA/CMDL have been found to
be very stable over time, so separate working standards are not needed.
The off-line postprocessing
of the profile data consists of the calculation of the response functions
for the CO2 analyzer and the conversion of the voltage data
into physical units. If the change in the response function causes more
than 2 ppm change between two consecutive calibrations, the data for
the period is rejected. Such periods are rare, and almost always caused
by significant change in room temperature. The usual change of the response
function is below 0.3 ppm. It should be noted that the drift equally
influences all monitoring levels, therefore the relative mixing ratio
profile is correct even if the absolute accuracy is temporarily lower
than usual. As this type of error is random, the long-term accuracy
of the values is close to that of the standards (about 0.1 ppm).
At the highest monitoring
level, 115 m above the ground, wind speed (Vaisala WAA15A), wind direction
(Vaisala WAV15A) and air temperature/humidity sensors (Vaisala HMP35D)
are mounted along with the air sampling tube at the end of a 4.4 m long
instrument arm projecting north. The analog signals of the meteorological
sensors are digitized by means of a 12 bit A/D converter and are transmitted
to the data acquisition computer via an RS232 serial link. Proper shielding
of the cables and sensors, as well as the digitalization of the signals,
are essential to avoid the pick-up of noise in the long cables, which
may be caused by the nearby high power antennas. Grounding is also important
to minimize the possibility of damage from lightning.
instrumentation at 82 m above the ground is similar, but a wind direction
sensor is not installed there. In April 1997 a sonic anemometer was
installed at this level for eddy flux measurements.
data acquisition and system control for the CO2
profiles and meteorological data we use a PC.
The analog signals of the CO2 analyzer
and mass flow controller are read by a multiplexer-A/D
converter (PCL-711B). The data acquisition and
system control software is written in Delphi and
runs under MS Windows. During the data integration
period the computer consecutively reads data from
the meteorological sensors at each monitoring
level through its serial port (RS232), then it
reads the CO2 analyzer and the mass
flow controller through the PCL-711B card. The
profile system generates 5 MByte/month of data
which is stored on a floppy diskette after compression.
The data are mailed or carried to our laboratory
in Budapest for processing.
more details please see:
L. (editor), 2011: Atmospheric Greenhouse Gases:
The Hungarian Perspective. Springer, Dordrecht
- Heidelberg - London - New York. ISBN 978-90-481-9949-5,
e-ISBN 978-90-481-9950-1, doi: 10.1007/978-90-481-9950-1
L., and Barcza, Z., 2010. Climate variability
as reflected in a regional atmospheric CO2
record. Tellus B, 62, 417-426. doi: 10.1111/j.1600-0889.2010.00505.x
L., Barcza, Z., Hidy, D., Szilágyi, I., Dlugokencky,
E., Tans, P., 2008. Trends and temporal variations
of major greenhouse gases at a rural site in Central
Europe. Atmospheric Environment 42, 8707–8716.
P. S., Davis, K. J.,Yi, C., Wofsy, S. C., Munger,
J. W., Haszpra, L., Barcza, Z., 2004. Regional
carbon dioxide fluxes from mixing ratio data.
Tellus B, 56 (4), 301-311.
Haszpra, L., Barcza, Z., Bakwin, P. S., Berger,
B. W., Davis, K. J., Weidinger, T. 2001: Measuring
system for the long-term monitoring of biosphere/atmosphere
exchange of carbon dioxide. Journal of Geophysical
Research. Vol. 106D, 3057-3070.
Haszpra, L., 1999:
On the representativeness of carbon dioxide measurements, Journal
of Geophysical Research, Vol. 104D, 26953-26960.