Well, in addition to topography that can best be described as "bowl-shaped", which traps any and all particulates in a cloud hovering over the city, this part of the southeast is blessed (cursed?) with a plethora of pines and oak trees which are busily churning out various pollen granules to bedevil those of us with less than perfectly behaved sinuses.
I give you this as an example: That mess is pollen from a southern red oak tree. Which I have. Right next to my house. There's clouds of the stuff all over the yard, my roof, clogging my gutters, covering my sidewalks.