Measurement results (near real-time data)

Mixing 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 3-4^oC. 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 -25^oC 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 anhydrous CaSO₄.

Analysis 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 instrument response.

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.

The meteorological 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.

For 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.

For more details please see:

Haszpra, 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

Haszpra, 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

Haszpra, 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. doi:10.1016/j.atmosenv.2008.09.012

Bakwin, 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.