None of that really matters unless you're talking about noise pollution. Aircraft fly in the air and air moves around so it's irrelevant where they are.
The stacks are set to reduce noise impact above all else. Aircraft produce the most pollution on takeoff whereas the engines tend to be at lower speeds on approach. There are local pollution and noise sensors at airports for this reason as operators have to reduce takeoff engine speeds wherever possible.
Look at any pollution map of the area near an airport, it's always worse directly around the takeoff runway but relatively non-existent elsewhere.
The worst I've seen for pollution in London is on the underground. I had to go to London for work last Tuesday and I only took a few trips on the underground but afterwards my nose was pouring out and full of black gunk.