Fog (or smog) in Mississppi today?

Drove in to BR today from Jacksonville. Kind of a strange trip. First of all, it was overcast the whole entire way but not one drop of rain. Finally hit some wet roads in Mississippi but had missed the rain.

The weird thing though... Even though it was overcast it was clear all the way through Florida and Alabama. Just a few miles into Mississippi we ran into some really thick fog, at 4:15 in the afternoon. About halfway through, there was definitely some smoke involved, but that smell only lasted for a couple of miles, then foggy again most of the rest of the way. Just a few miles from LA I could see in the sky ahead it was going to clear some. Sure enough, by the time we went over the bridge into LA the fog/smog or whatever was gone and the rest of the drive was clear.

So it appeared to be fog, though I did see and smell smoke for a couple of miles, was present the entire span of I-10 in Mississippi between 4 and 5 pm, yet was contained to only Mississippi, and the rest of the trip was clear. So what the heck was it? Anybody know?