Monitoring of atmospheric carbon dioxide and CO₂ budget studies

(László Haszpra)

The increase in the carbon dioxide (CO₂) atmospheric mixing ratio is evident from the measurements. The risk of the intensification of the greenhouse effect and the resulted global climate change are undoubted. Considering the potential consequences of a global climate change the monitoring of atmospheric CO₂ has become part of the measurement program at the baseline stations of the World Meteorological Organization's Global Atmosphere Watch (GAW) network. The growth rate of atmospheric CO₂ has been increased until recently and reached about 0.45%/year by now. According to the measurements approximately the half of the anthropogenic CO₂emitted remains in the atmosphere, the other half is taken up by the oceans and the biosphere through processes not fully understood. The measurements show that the growth rate of atmospheric CO₂ varies in a much wider range than the anthropogenic emission. The reasons might be revealed by the measurements carried out on continental areas covered by vegetation (both source and sink of CO₂), as well as by CO₂ isotope composition (¹⁴C, ¹³C/¹²C, ¹⁸O/¹⁶O) measurements.
Long-term, regionally representative measurements for the determination of the CO₂budget of the vegetation was started first in the United States (1992). Shortly after (1994) a similar monitoring program was started in Hungary using the TV-transmitter tower of Antenna Hungária Corp. at Hegyhátsál and funded by the US-Hungarian Joint Fund as well as by the Hungarian Scientific Research Fund. The first years of the American measurements supported the model-based expectations: the biosphere-soil system in the northern temperate region took up more carbon dioxide through photosynthesis than its own emission by respiration. The Kyoto Protocol and the decisions of the follow-up conferences (COPs) allowed to take the biospheric CO₂ uptake into account, with certain limitations, in the determination of the national total greenhouse gas emission. In addition, trading the emission quotas became also possible. These decisions gave significant impulse to the research programs on the biosphere-atmosphere CO₂ exchange, and to the start of long-term monitoring programs. The research on the biosphere-atmosphere CO₂ exchange has got high priority in the European Union's research programs since 1998. The soil-biosphere system is extremely sensitive to the environmental conditions, to their changes, therefore its net carbon budget is not stable. While carbon sequestered through photosynthesis increases (up to a certain measure) linearly with the atmospheric CO₂ concentration, the respiration is the exponential function of the temperature. Consequently, parallelly with the global warming, the biospheric uptake of CO₂ of anthropogenic origin might decrease soon forcing a drastic decrease in the anthropogenic emission.
The increasing interest in the continental carbon budget gave high value to the measurements carried out at K-puszta since 1981 because there were only very few stations in Europe having such a long time series. The CO₂ mixing ratio measured there faithfully followed the global tendencies although the station was located in a densely populated, highly industrialized region (but relatively far from any direct anthropogenic influence). The common character of the monitoring stations located at low elevations in vegetated regions is the low nighttime spatial representativeness of the measurements due to the limited nighttime atmospheric mixing and the high, spatially heterogeneous emission of the biosphere-soil system. At these sites only the early afternoon measurements can be used to draw general conclusions. Unlike the nighttime values, the early afternoon data can be considered valid for up to several hundred thousand km². Due to this fact no significant deviation was observed between the measurement carried out at K-puszta and those started at Hegyhátsál in 1994. Thus Hegyhátsál could replace K-puszta where CO₂ monitoring was ceased in 1999 due to the lack of financial and technical supports.
Monitoring of atmospheric CO₂ mixing ratio and basic meteorological elements (wind, temperature, humidity) were started at four elevation levels (10 m, 48 m, 82 m, 115 m) along the TV-transmitter tower in autumn, 1994, with the in kind support of Antenna Hungária Corp. Later solar radiation sensors (global radiation, photosynthetically active radiation, radiation balance) and soil sensors (temperature, humidity, heat flux) were also installed. Air samples have been taken for NOAA (Boulder, Colorado, USA) from which the concentration of carbon dioxide, carbon monoxide, methane, nitrous oxide and sulfur hexafluoride, as well as the stable isotope composition of carbon dioxide are determined. In 1997, in co-operation with the Department of Meteorology, Eötvös Loránd University (Budapest), the long-term monitoring of surface-atmosphere carbon dioxide exchange was started on regional scale using the tall tower at hegyhátsál. In 1999-2000 this program was completed by the measurement of the carbon dioxide exchange of a semi-natural grass field.
The monitoring program carried out at Hegyhátsál had achieved international recognition, so we could joint the European-level carbon budget research right in the beginning, in 1998. The support received form the European Union allowed to start aircraft measurements over the tower to study the greenhouse gas composition of the atmosphere up to the free troposphere. The air samples collected during the flights are analyzed by Laboratoire des Sciences du Climat et de l'Environnnement, CEA-CNRS, France. It was also the support received from the European Union which made possible to start the continuous measurements of the most important non-CO₂ greenhouse gases (CH₄, N₂O, SF₆) at Hegyhátsál early 2006.
Nowadays, atmospheric CO₂ mixing ratio is monitored at several sites in Europe and the number of the monitoring stations measuring non-CO₂ greenhouse gases is also increasing. However, Hegyhátsál still has an almost exclusive feature: here high precision greenhouse gas measurements, tall tower vertical concentration profile measurements, surface-atmosphere CO₂ exchange measurements and aircraft measurements are performed at the same site. All these are in a geographical region which is fairly plain and homogeneous to make the validation of mathematical models and remotely sensed data possible. Due to these conditions and capabilities Hegyhátsál has become one of the greenhouse gas monitoring supersites of the European Union since 2006, but it also provides monitoring data for the WMO GAW program.
The details of the Hegyhátsál monitoring and research programs can be found at http://nimbus.elte.hu/hhs.

Trend in the early afternoon atmospheric carbon dioxide mixing ratio as it has been measured at K-puszta (KPU) and Hegyhátsál (HHS)

Net carbon exchange (NEE - negative values denote biospheric uptake) in the Hegyhátsál region and meteorological data from the nearby Farkasfa Observatory (46°55'N, 16°19'E, 312 m)