The LUMIA (TM5-FLEXPART) algorithm is a regional atmospheric CO2 inversion system, build primarily to compute estimates of European surface CO2 fluxes for the EUROCOM project. It relies on an off-line coupling between the global transport model TM5-4DVAR, and the FLEXPART model ran on a regional domain. The coupling of the transport models follows the two-step method proposed by Rödenbeck et al., 2009: First, a global, coarse-resolution inversion is performed with TM5, to compute the influence of non-european CO2 fluxes. Then a regional, high-resolution regional inversion is performed using FLEXPART transport, to optimize CO2 emissions within the European domain. That second step relies on pre-computed FLEXPART response functions, and on a 4DVAR inversion algorithm, derived from the one used in TM5.