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

The ICOS (Integrated Carbon Observation System) network of atmospheric measurement stations produces standardized data on greenhouse gas concentrations at 46 stations in 16 different European countries (March 2023). The placement of instruments on tall towers and mountains results in large influence regions ("concentration footprints"). The combined footprints for all the individual stations create a "lens" through which the network sees the European CO2 flux landscape. In this study, we summarize this view using quantitative metrics of the fluxes seen by individual stations and by the current and extended ICOS networks. Results are presented from both country level and pan-European perspectives, using open-source tools that we make available through the ICOS Carbon Portal. We target anthropogenic emissions from various sectors, as well as the land cover types found across Europe and their spatiotemporally varying fluxes. This recognizes different interests of different ICOS stakeholders. We specifically introduce "monitoring potential maps" to identify which regions have a relative underrepresentation of biospheric fluxes. This potential changes with the introduction of new stations, which we investigate for the planned ICOS expansion with 19 stations over the next few years. In our study focused on the summer of 2020, we find that the ICOS atmospheric station 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 considering the pan-European view. Exceptions include representation of grass and shrubland and broadleaf forest which are abundant in south-eastern European countries, particularly Croatia and Serbia. On the country scale, the representation shows larger imbalances, even within relatively densely monitored countries. The flexibility to consider individual ecosystems, countries, or their integrals across Europe demonstrates the usefulness of our analyses and can readily be reproduced for any network configuration within Europe.