A strategy for a European integrated observation system to monitor fossil carbon dioxide (CO2), also referred to as a Fossil Fuel Data Assimilation System (FFDAS), has been proposed.  This FFDAS includes space-borne high resolution and spatially explicit observations of the CO2 total column abundance complemented by in situ air sampling networks, the development of state-of-the-art bottom-up emission inventories and the setup of several data assimilation systems.  Yet the atmospheric signals of fossil fuel emissions can be often be dwarfed by natural signals.  In consequence, it has been proposed to use additional trace species to separate the fossil CO2 component from the natural CO2 fluxes at regional scale.  WP4 aims to describe an optimal configuration for a measurement network with respect to trace species and station location and density to better complement the proposed FFDAS.  The tracers on which the work is focusing are radiocarbon CO2 (δ14CO2), carbon monoxide (CO), and atmospheric potential oxygen (APO). The target quantities will be the relative uncertainty reductions of fossil CO2 emissions in a set of predetermined regions and time periods.  Besides network design, WP4 already tests two additional objectives of the FFDAS: the use of state-of-the-art bottom-up emission inventories and the setup of several data assimilation systems. 

The different modelling groups participating in WP4 will address its objectives with different focuses ranging spatially from urban areas, to regional, to country to continental scales, temporally from hourly, to weekly, to monthly, to yearly scales and thematically from differentiating only between fossil fuel and non-fossil fuel emissions to differentiating emissions processes explicitly.  Consistent with the work in WP2 and the fluxes prepared therein, the year 2015 is chosen as the study period.  Partners EMPA and MPG will perform European-wide modelling using COSMO and WRF-STILT respectively at ~5 km x 5 km spatial resolution and hourly time steps.  Regional modelling will be carried out by NILU for the Oslo region at 2 km x 2 km and by CEA for northern France/Benelux/Western Germany (except Northern Netherlands) at 2 km x 2 km resolution. 

This deliverable aims to ensure that all the modelling groups are working with a harmonized set of initial conditions and uncertainty assumptions in order to obtain comparable estimates of uncertainty reduction.  This includes a common set of a priori flux estimates for each of the tracers involved, boundary conditions, comparable measurement and model error estimates, common a priori uncertainty estimates for prearranged regions, a common set of station arrangements and a similar simulation protocol.