It's residual ash from the fires. The ash and snoke mix with the moisture droplets in the clouds. When the rain evaporates off it leaves behind the residue. It's not as bad as volcanic ash but isn't great for your paint.I live in Hampton Roads Va.
In the last few weeks since the NC/SC Wildfires the rain has been leaving behind greyish/white dirt like residue behind on cars and I'm wondering if anyone could identify what it is and if it will damage the paint. I washed a couple of weeks ago to get rid of any salt on the under carriage and now I washed a second time to rid the car of the white residue.