Once those particles cool, they fall to the ground, just like ash from burning any other hydrocarbon. One difference is that the particles from burning gasoline are smaller, and therefore, stay aloft for a longer period; but, eventually they fall to the ground. All solid particles eventually fall to the ground.

You know what they become when they fall to the ground? Fertilizer, natural fertilizer. That's what hydrocarbons do. They naturally fertilize the land - just look at forests after a forest fire.