We present a performance assessment of the European Integrated Carbon Observing System (ICOS) atmospheric network for constraining European biogenic CO<sub>2</sub> fluxes (hereafter Net Ecosystem Exchange, NEE). The performance of the network is assessed in terms of uncertainty in the fluxes using a state-of-the-art mesoscale atmospheric inversion system assimilating hourly averages of atmospheric data to solve for NEE at 6 h and 0.5° resolution. The performance of the ICOS atmospheric network is also assessed in terms of uncertainty reduction compared to typical uncertainties in the flux estimates from ecosystem models that are used as prior information by the inversion. The uncertainty in inverted fluxes is computed for two typical periods representative of summer and winter conditions in July and in December 2007, respectively. These computations are based on a robust Observing System Simulation Experiments framework. We analyze the uncertainty in two-week mean NEE as a function of the spatial scale, with a focus on the model native grid scale (0.5°), the country scale and the European scale (including western Russia and Turkey). Several network configurations, going from 23 to 66 sites, and different configurations of the prior uncertainties and atmospheric model transport errors are tested in order to assess and compare the improvements that can be expected in the future from (1) the extension of the network, (2) improved prior information or (3) improved transport models. Assimilating data from 23 sites (a network comparable to present day capability) with the estimate of errors from the present prior information and transport models, the uncertainty reduction on two-week mean NEE should range between 20 and 50% for 0.5° resolution grid cells in the best sampled area encompassing eastern France and western Germany. At the European scale, the prior uncertainty in two-week mean NEE is reduced by 50% (66%), down to ∼ 43 Tg C month<sup>-1</sup> (resp. 26 Tg C month<sup>-1</sup>) in July (resp. December). Using a larger network of 66 stations, the prior uncertainty of NEE is reduced by the inversion by 64% (down to ∼ 33 Tg C month<sup>-1</sup>) in July and by 79% (down to ∼ 15 Tg C month<sup>-1</sup>) in December. When the results are integrated over the well-observed western European domain, the uncertainty reduction shows no seasonal contrast. The effect of decreasing the correlation length of the prior uncertainty, or of reducing the transport model errors compared to their present configuration (when conducting real-data inversion cases) can be larger than that of the extension of the measurement network in areas where the 23 stations observation network is the densest. We show that with a configuration of the ICOS atmospheric network containing 66 sites that can be expected on the long-term, the uncertainties in two-week mean NEE will be reduced by up to 50–80% for countries like Finland, Germany, France and Spain, which could bring a significant improvement of (and at least a high complementarity to) our knowledge about NEE derived from biomass and soil carbon inventories at multi annual scales.
Kadygrov, N., Broquet, G., Chevallier, F., Rivier, L., Gerbig, C., & Ciais, P. (2015). On the potential of the ICOS atmospheric CO2 measurement network for estimating the biogenic CO2 budget of Europe. Atmospheric Chemistry and Physics, 15(22), 12765–12787. https://doi.org/10.5194/acp-15-12765-2015