Abstract. Mixing ratio measurements of atmospheric tracers like CO2 can be used to estimate regional surface-air tracer fluxes using inverse methods, involving a numerical transport model. Currently available transport models are either global but rather coarse, or more accurate but only over a limited spatial and temporal domain. To obtain higher-resolution flux estimates within a region of interest, existing studies use zoomed or coupled models. The two-step scheme developed here uses global and regional models sequentially in separate inversion steps, coupled only via the data vector. This provides a nested atmospheric inversion scheme without the necessity of a direct coupled model implementation. For example, the scheme allows an easy nesting of Lagrangian models with their potential of very high resolution into global inversions based on Eulerian models.