A view of the European carbon flux landscape through the lens of the ICOS atmospheric observation network

Abstract. The ICOS (Integrated Carbon Observation System) network of atmospheric measurement stations produces standardized data on greenhouse gas concentrations at 36 stations in 14 different European countries (November 2022). The network targets a strongly heterogeneous landscape and the placement of instruments on tall towers and mountains make for large influence regions (footprints). The combined footprints for all the individual stations create the “lens” through which the observing network sees the European flux landscape. In this study, we summarize this view using quantitative metrics of the fluxes seen by individual stations, and by the current and future ICOS network. Results are presented both from a country-level and pan-European perspective, using open-source tools that we make available through the ICOS Carbon Portal. We target anthropogenic emissions from various sectors (e.g., energy production, industrial emissions), as well as the land-cover types found over Europe (e.g., broadleaf forests, croplands) and their spatiotemporally varying fluxes. This recognizes different interests of different ICOS stakeholders. We specifically introduce “monitoring potential maps”, which quantify the sensitivity of the network with regards to specific properties of each pixel compared to the averages across all pixels, to see which regions have a relative underrepresentation of land-cover, or biospheric fluxes. This potential changes with the introduction of new stations, which we investigate for the planned ICOS expansion with 20 stations over the next few years. The monitoring potential concept is novel and a useful addition to traditional quantitative network design methods.

We find that the ICOS network has limited sensitivity to anthropogenic fluxes, as was intended in the current design. Its representation of biospheric fluxes follows the fractional representation of land-cover and is generally well balanced, with exceptions for a country like Norway where the southerly station Birkenes predominantly senses coniferous forest fluxes instead of the more abundant northerly grass & shrublands. Grass & shrubland fluxes are relatively underrepresented in ICOS, with the largest monitoring potential in Scandinavia, Croatia, and Serbia. These easterly countries similarly show a relative underrepresentation of broadleaf forest fluxes, partly due to a lack of monitoring stations, and partly due to the abundant sensitivity to broadleaf forests in the most densely monitored countries such as France and Germany. We stress that this does not imply these latter countries to be fully monitored and of lesser interest for network expansion: for example, inclusion of Schauinsland in the future network expands the network lens to mostly unmonitored mixed- and broadleaf forests which are relatively underrepresented at the national level. Such considerations demonstrate the usefulness of our analyses and can readily be re-produced for any network configuration within Europe with tools offered through the Carbon Portal.