FLEXPART simulations of North American CO tracer for the lowest model layer (from surface to 100 m agl) showing the progress of the North American pollution plume over Europe (a) on 19 November 2001 at 12 UTC, (b) on 20 November at 00 UTC, (c) at 12 UTC, (d) on 21 November at 00 UTC, (e) 12 UTC, and (f) on 22 November at 00 UTC. Credit: Journal of Geophysical Research.