exchange constrained by atmospheric observations Francesco Apadula 1 ; Daniela Heltai 1 , W. Peters 2,8 , M.C. Krol 2 , G.R. van der Werf 3 S. Houweling 4 , A.J. Dolman 3 , C.D. Jones 5 , K. Schaefer 6 , K. Masarie 7 , A. Jacobson 8 J.B. Miller 8 , C.H. Cho 9 , P.P. Tans 7 , P. Ciais 10 , M. Ramonet 10 , M. Schmidt 10 , L. Ciattaglia 11 I. Levin 12 , F. Meinhardt 12 , A. di Sarra 13 , T. Aalto 14 , J. Strom 15 , L. Haszpra 16 , H.A.J. Meijer 17 M. Heimann 18 , X. Rodo 19 , A.T. Vermeulen 20 , K. Rozanski 21 , A. Manning 22 , M. Leuenberger 23 Rivista Global Change Biology USA 1 CESI RICERCA 2 : Wageningen University and Research Center, Wageningen, The Netherlands 3 : Vrije University Amsterdam, Amsterdam, The Netherlands 4 : Space Research Organization the Netherlands, Utrecht, The Netherlands 5 : Met Office Hadley Centre for Climate Change, Exeter, UK 6 : National Snow and Ice Data Center, Boulder, USA 7 : NOAA Earth System Research Laboratory, United States 8 : Cooperative Institute for Research in Environmental Sciences, Boulder, USA 9 : Meteorological Research Institute, KMA, ChungNam, Republic of Korea 10 : Laboratoire des Sciences du Climat et de l’Environnement, France 11 : Instituto di Scienze dell’Atmosfera e del Clima, Italy 12 : Institute for Environmental Physics, Univ of Heidelberg, Germany 13 : Ente per le Nuovo Tecnologie, l’Energia e l’Ambiente, Italy 14 : Finish Meteorological Institute, Finland 15 : Stockholm University, Dept of meteorology, Sweden 16 : Eotvos Lorand University, Hungary 17 : University of Groningen, Netherlands 18 : Max-Planck-Institute for Biogeochemistry, Germany 19 : University of Barcelona, Spain 20 : Energie Onderzoekscentrum Nederland, Netherlands 21 : AGH University of Science and Technology, Dept of Env. Physics, Poland 22 : University of East Anglia, Norwich, United Kingdom 23 : University of Bern, Bern, Switzerland We present an estimate of net carbon dioxide exchange over Europe during the period 2001-2007. It is derived with a data assimilation that uses a large set of atmospheric CO2 mole fraction observations (_70,000) to guide relatively simple descriptions of terrestrial and oceanic net exchange, while fossil fuel and fire emissions are prescribed. Weekly terrestrial sources and sinks are optimized for a set of 18 large ecosystems across Europe in which prescribed climate, weather, and surface characteristics introduce finer scale spatial and temporal gradients. We find that the terrestrial biosphere in Europe absorbed a net average of -164 TgC/yr over the period considered, resulting from +40 TgC/yr of fire carbon release and -204 TgC/yr annual biospheric uptake. This 42 uptake is predominantly in non-EU countries, and is found in the northern coniferous (-94 TgC/yr) and boreal mixed forests (-30 PgC/yr) as well as the forest and field complexes of eastern Europe (-85 TgC/yr). Croplands appear as a net source of carbon (+60 TgC/yr) in our system, but the spatial overlap with strong fossil fuel sources makes this estimate less robust than other ecosystems. From a suite of three different biosphere model priors we estimate the multiyear average European net terrestrial flux to fall in the range of -159 to -302 TgC/yr. This average includes a large reduction in uptake due to the 2003 drought (+147 TgC/yr below average) followed by three years of more than average uptake (-79, -200, -79 TgC/yr respectively) that more than compensated the drought year. We assess the robustness of our results using seven alternative simulations, including one with a different boundary layer mixing scheme. We also discuss the good correspondence with recent independent estimates of the European carbon balance. All results from this study (carbon fluxes, simulated CO2 time series, and full atmospheric mixing ratios in time) are freely available to the research community through http://www.carbontracker.eu.