  It is of crucial importance to be able to identify the location of
atmospheric pollution sources in our planet. Global models of atmospheric
transport in combination with diverse Earth observing systems are a natural
choice to achieve this goal. It is shown that the ability to successfully
reconstruct the location and magnitude of an instantaneous source in global
chemical transport models (CTMs) decreases rapidly as a function of the time
interval between the pollution release and the observation time. A simple way
to quantitatively characterize this phenomenon is proposed based on the
effective -undesired- numerical diffusion present in current Eulerian CTMs and
verified using idealized numerical experiments. The approach presented consists
of using the adjoint-based optimization method in a state-of-the-art CTM,
GEOS-Chem, to reconstruct the location and magnitude of a realistic pollution
plume for multiple time scales. The findings obtained from these numerical
experiments suggest a time scale of 2 days after which the accuracy of the
adjoint-based optimization methodology is compromised considerably in current
global CTMs. In conjunction with the mean atmospheric velocity, the
aforementioned time scale leads to an estimate of a length scale of about
1700km, downwind from the source, beyond which measurements, in conjunction
with current global CTMs, may not be successfully utilized to reconstruct
continuous-in-time sources. The approach presented here can be utilized to
characterize the capabilities and limitations of adjoint-based optimization
inversions in other regional and global Eulerian CTMs.
