How long does it take air to circle the world?
It takes about 2 weeks for a mass of air to circle the globe along its latitude from West to east. I used to work for a French climate research lab in the 70s, and was based on Crozet island (South Indian Ocean) . At the time I was carrying atmospheric radioactivity measurements, and the French army was carrying nuclear test above ground in the pacific, I could see the radioactive cloud passing above me about 10 days later and then again about every second week after that and about completely disappeared after 6 months.
In the case you raise, you will benefit from cleaner air about one week after the decommissioning