This talk by Frederic Chevallier (CEA) at the 1st CHE General Assembly presented an overview of the activities in Work Package 4. WP4 (Coordinating Efforts on Attributing CO2 emissions from in-situ measurements) will explore the practical implications of distinguishing between anthropogenic and biogenic CO2 fluxes. It is dedicated to optimising the space-time sampling of 14C, Carbon Monoxide and Atmospheric Potential Oxygen (APO, a tracer based on the atmospheric mixing ratio of O2 with a correction for the influence of land biosphere photosynthesis and respiration, Stephens et al., 1998). The attribution problem will be addressed with an ensemble of five forward and inverse modelling tools, again providing a representative cross-section of current methodologies. They will be applied to a series of hypothetical in-situ networks with varying spatial and temporal density to evaluate their capability to extract the signal from anthropogenic emissions within the domain. As a preparatory step, the current European in-situ observation network for measurements of atmospheric mixing ratios of CO2, CO, 14C-CO2, and APO will be surveyed and documented.