Permanent pollution absolutely. You don't even need lighting to see that, it's the giant mako-producing city in the middle of a wasteland that we're told used to be plains and grasslands when the towns were separate.
But pollution doesn't work like that. Even in the smoggiest cities in the world, you're not gonna reach perma-night levels of light blocking, at least not the absolute pitch black that people seem to think exists.
The argument isn't whether or not Midgar has sunny days chasin' the clouds away. It's whether or not it's permanently night-time levels of light because of it. And with respect to Mr. Ite and others, that's a hearty "no".
It all boils down to the good old days of PSOne where you basically had one light setting per locale. Mr. Ite mentioned Treno and Deling City, but those weren't caused by pollution. We were to assume that those places just always were in eternal midnight? It's not even like Burmecia having constant rain either, as that's a weather effect and, well as anybody who lives in Seattle can tell you that does happen lol.