According to the Kyoto protocol, 174 countries shall have in place, no later than 2008, a national system for the estimation of anthropogenic emissions from all sources and removals by all sinks of the greenhouse gases not controlled by the 1992 Montreal Protocol. Countries are however committed to show substantial progress by the year 2002, thus invoking for an urgent need for a system of monitoring of sinks and sources.
In recent years it has become clear that the traditional atmospheric greenhouse gas observing system has severe gaps. Until recently, continental monitoring stations have not been considered relevant because of the large variability of the signals, caused by the proximity to the landbiosphere atmosphere exchange fluxes and the large, spatially concentrated sources.
The horizontal gradients of greenhouse gas concentrations, which carry the information on the magnitude and spatial distribution of sources and sinks, are quite small. If we are to infer fluxes at the regional level, it is thus necessary to sample close to the earth surface and on a continuous basis to capture the signal of greenhouse gas exchange fluxes. This calls for measurements in the boundary layer. Here the variability in concentrations (diurnal cycles) is huge, because the air is to a large extent influenced by local sources. In order to separate the effect of local (few tens of km) variability from the regional signal, one needs to continuously monitor concentrations above the surface layer (100 meters), complemented for CO₂ by eddy flux towers to characterize the contribution of "local" biospheric exchange.
If the gases are measured at sufficient height above ground (ideally a few hundred meters), then a fairly homogeneous signal that integrates fluxes over a footprint on the order of a circle of 500 to 1000 km is obtained. Continuous measurements also permit to optimise the signal to "noise" ratio of the measurements.
Because of the natural variability of the signals in the atmosphere, the time series needed for successful inverse calculation of fluxes by atmospheric transport models (ATM's) should extend over a long time frame of at least several years. Time series of 5-10 years or longer will be required in order to be able to distinguish trends in time, because of a response to e.g. climate change on ecosystems CO₂ fluxes or emission reduction measures. At this time no infrastructure yet exists that will provide us with that data.
The new approach followed in this project is thus to sample continuously CO₂ and other greenhouse gases (some also related to the carbon cycle) like CH₄, CO, N₂O, and SF₆ together with tracers that help validate the realism of transport simulation with atmospheric models like C₂Cl₄ and ²²²Rn on tall towers. We think that it is possible to monitor the European carbon balance and in addition to this also the emissions of other greenhouse gases on the regional scale (areas of approximately 1 million km²) in support of the Kyoto Protocol using a pan-European observing system of a carefully designed and well calibrated network of atmospheric concentration measurement sites. We propose here to start implementing such an approach using existing and new tall tower observation sites.
In Europe tall towers in the Netherlands, Hungary, Germany and Sweden already are equipped with devices that permit online CO₂ concentrations and in some cases related tracers. In the CHIOTTO-infra project these towers will extended with new towers in Great-Britain, France, Poland and Italy to increase the spatial coverage of the studied area. The footprints of these towers together cover Europe fairly well. The objective of the proposal is to help sustain existing and partly implement new measurements, and ensuring high data quality and a high standard of calibration and inter-comparison.
The tower-based observations will also be useful and important as ground-truthing data for calibration and verification of future remote-sensing (satellite) data.

Specific objectives

• Implement new tall tower stations and complement existing tall tower stations enabling the continuous monitoring of the sources and sinks of the most important greenhouse gases in Europe;
• Design and implement a standardized protocol for calibrating measurements of CO₂, CH₄, N₂O, CO and SF₆ at selected tall towers, to a level of precision enabling the use of tall tower concentration records in atmospheric transport models for estimation of annual average regional fluxes;
• Set up a near real-time transmission system of tall towers data, including archiving, quality control, and accessibility;
• Implementation of a technical support and servicing unit for tall tower measurements;
• Establish a link between tall tower concentration measurements and CO₂ surface flux data for evaluation of the representativity of tall tower data for regional scales.