At the CHE-VERIFY Joint General Assembly, Hugo Denier van der Gon (TNO) and Paul Palmer (U Edinburgh) introduce the progress and plans for the VERIFY WP2. The overarching objective of WP2 is to develop the components of the observation based monitoring and verification system (summarized in an Algorithm Description Document, ADD and a Product Description Document, PDD) dedicated to fossil fuel CO2 emissions (ffCO2), using in situ and remotely sensed atmospheric measurements of CO2 and co-emitted tracers. They will become part of the Community Inversion Framework (CIF, see description in WP3). To address this objective, we will construct a Fossil Fuel Data Assimilation System (FFDAS) to estimate ffCO2 emissions at a sub-national resolution (25-50 km) by combining spatio-temporal information from a dynamical emission (inventory) model for ffCO2 and co-emitted species (CO, NOx) (Task2.1), natural CO2 fluxes estimates (from WP3), and independent in situ and space-borne observations of atmospheric CO2, 14CO2, and co-emitted species (CO, NO2, Black Carbon). The dynamical emission model and the inverse modelling strategy using ffCO2 proxies like CO and NOx will be evaluated using measurements from dedicated field campaigns. To prepare for the future, we will explore the theoretical potential of new and future satellite data products (e.g. Sentinel-5P).