Reciprocity of atmospheric transport of trace species

We present a new approach for the inversion of atmospheric transport of trace species based on the time symmetry of the transport equations. Whereas the classical use of Lagrangian back-tracking only accounts for large-scale transport, our approach can also be applied to other transport processes such as turbulent mixing and linear sources and sinks. It hence provides a simple and systematic way of quantifying the source to detector connection with a wide field of potential applications. We present both theoretical background and numerical illustrations in the context of the European Transport Experiment (ETEX).