WP2, 3 and 4 will perform the research needed for a pre-operational policy-relevant GHG observation system. The three WPs cover the data collection and development of advanced algorithms for the estimation of fossil fuel combustion CO2 emissions (WP2), terrestrial CO2 sources and sinks and carbon stocks (WP3) and all types of CH4 and N2O emissions (WP4). This will provide Product 1 and Product 2. WP2, 3 and 4 follow the same follow a common structure that includes: i) the collection and processing of all needed input data with yearly updates, ii) the integration into pre-operational systems of bottom-up models currently developed for research applications (ecosystem models, very detailed emission mapping models) and atmospheric inversion models and iii) targeted research actions to further improve these model tools to use recent and upcoming space-borne observations and develop cutting-edge new model data fusion techniques. For the bottom-up component, selected models will be integrated under the same protocol to deliver annual GHG flux maps at 10 km, weekly across the EU (covering all land use types and emission sectors). For the top-down component, there will be two stages: in the first stage we will use existing inversion systems to provide GHG flux estimates at 50 km, weekly resolution during the first three years of the project, then, in the second stage, a unified Community Inversion Framework (CIF, see text-box) that will be built to integrate different transport models and inversions, paving the way for a future operational system.

Observation-based methods to quantify fossil fuel combustion CO2 emissions (WP2)
Improving and independently verifying fossil15 fuel CO2 (ffCO2) budgets on the national scale, for reporting to the UNFCCC, and for regional emission hot spots (e.g. large urban areas) have a high political priority. However, current CO2 observation systems are not well suited to quantify ffCO2 on either spatial scale.
To address this challenge, WP2 will develop a dynamic emission-mapping model that will ingest near-real-time activity data (e.g., energy demand, traffic intensities, socio-economic drivers) and proxies for predictive modelling of these activities (e.g. meteorology), and will further optimise the resulting emission estimates using various independent data streams. This model will provide high spatially (< 10 km) and temporally (hourly) resolved emissions of ffCO2 and of co-emitted species. The model output will be evaluated using isotopes and tracer measurements collected over the selected hotspot region of the Rhine valley (Germany). Several measurement campaigns will be performed there to characterise ffCO2-to-coemitted species ratios (CO and NO2) taking advantage of existing air-quality networks, as well as in Eastern Europe (Russia) with activities related to WP3.
Based on the detailed emission mapping model and emission ratios, VERIFY will develop the first atmospheric inversions to monitor specifically ffCO2 emissions. These systems will use CO and NO2 from satellites covering the last 10 years (IASI, MOPITT, OMI) innovating on recent work [Konovalov et al. (2016)]. This atmospheric inversion, oriented to quantify ffCO2 emissions (called Fossil Fuel Data Assimilation System; FFDAS), will provide independent estimates of emissions at 50 km resolution with full uncertainty structure and will be later integrated in the CIF Framework. Complementary research tasks will be conducted to improve FFDAS. With the advent of new satellites, the space sector will become progressively more important for a future ffCO2 monitoring system. We will explore the potential of space-borne measurements of CO2 (GOSAT, OCO2) and reactive trace gases, and the potential of using other observations (i.e. socio-economic), to improve the FFDAS.

Natural CO2 sources and sinks and carbon stocks (WP3)
The objective of WP3 is to provide accurate estimates of ecosystem CO2 fluxes (NEE) based on land observations and ecosystem models capable of producing: i) estimates of NEE at high resolution (≈ 10 km, weekly) for all land use types and ii) attribution of NEE to land use, management intensity and climate drivers.
A first task will collate all needed input data for ecosystem models, including new high-resolution gridded climate fields, land cover changes, management intensity and carbon stocks (biomass and soils). In addition, soil carbon erosion, nitrogen deposition as well as all lateral carbon from lakes and rivers, coastal oceans, and human appropriation of ecosystem resources (harvested products) will be collected. The ecosystem models (see figure 6) will provide high-resolution (≈10 km, weekly) estimates of NEE split into its components (Gross Primary Production, Respiration, disturbances) and carbon stock (biomass and soil). Factorial simulations will allow us to disentangle the contribution of different drivers (climate, CO2, N deposition, management, and land cover change). Model evaluation and recommendation for model selection and improvement for an operational observation-based monitoring and verification system will be drawn.
The second task consists in integrating a regional (higher resolution than global models) atmospheric inversion following the Jena Carboscope approach nested into a global CO2 inversion (Kountouris et al., 2016, http://www.bgc-jena.mpg.de/CarboScope/) in order to assimilate ICOS and other atmospheric CO2 data into 50 km maps of NEE, using a priori fluxes from the first task. In the last year of the project, the NEE maps will be additionally provided by the CIF allowing the use of several transport models to estimate NEE uncertainties from transport modelling. Complementary research activities will test the use of a comprehensive Carbon Cycle Data Assimilation System (CCDAS) (Rayner et al. 2005; Peylin et al. 2016) where ecosystem model parameters are jointly optimised from land and atmospheric observations as well as synergies with the FFDAS of WP2. A research task (including campaign measurements in Russia) is also focused on Eastern Europe, potentially a large carbon sink (Reuter et al. 2016), but where fewer observations are currently available.

CH4 and N2O emissions (WP4)
The objective of WP4 is to provide accurate estimates of all types of CH4 and N2O emissions at various scales across the EU combining bottom-up statistical and ecosystem modelling and atmospheric inversions (top-down).
The first task is dedicated to collect all needed input data at high spatial resolution for all types of anthropogenic and natural (wetlands, lakes, rivers) emissions of CH4 and N2O, as well as atmospheric CH4 and N2O concentrations from surface networks (e.g. ICOS) and satellites (for CH4). Then, we will integrate a number of complementary bottom-up models using input forcing data on climate and management (see WP3). In a second task, we will run regional atmospheric inversions specifically for CH4 and N2O. This will allow WP4 to provide spatio-temporally resolved CH4 and N2O fluxes at 50 km, weekly. In the last year, the CIF will integrate these inversions in a common, unified system, allowing the use of several transport models to estimate the associated uncertainties.
Complementary research activities will develop a specific data assimilation system for wetland fluxes of CH4, develop methods to assimilate new satellite CH4 data (TROPOMI on Sentinel 5P) into regional inversions, and specific tracers (ethane and δ13C) to better separate and attribute CH4 emissions to different sectors, and develop the prototype of an advanced very high resolution inversion (≈ 10 km).