To begin, it does accumulate in the region where it is created, but of course it diffuses over time, so 10 pollution could be 7 locally and 3 globally. The heavy elements (soot) never get very far and naturally fall out of the air. Some elements are broken down by sunlight. Some bind to rain and are washed into the ground. If pollution is constant, you reach an equilibrium in the air quality. Decreasing output slowly lowers that equilibrium and improves the air quality in the end.