Retrieving surface–atmosphere fluxes from the combination of atmospheric observations with atmospheric transport models can benefit from combining multiple species in a single inversion. The underlying effect is that species such as CO₂ and CO have partially overlapping emission patterns for given sectors and fuel types and so share part of the uncertainties, both related to the a priori knowledge of emissions, and to model–data mismatch error. We show this for airborne profile data from IAGOS.