There may have been some other airborne pollution that was transported from a factory or plant. However, the fact that the "dirty rain" was noticed in more widespread areas across the Miami Valley would suggest it would have had to have occurred by a larger source.
Most likely, the dust/dirt was just lofted into the air by gusty winds blowing across vegetation-free farmland to our south. It is possible even some ash, which can be very light compared to dirt, was also picked up by gusty winds from recent fires earlier in the fall and early winter across the Appalachian Mountains.
Whatever the source, it is not uncommon for rain to be somewhat dirty, especially if it is very light and brief. The rain does do a good job of cleaning the air of dirt, dust and pollen if it is heavy enough, but when the rain is light and brief, the dust that mixes with the initial raindrops can deposit on your car and other surfaces as the water evaporates.
It is important to note, that raindrops actually require condensation nuclei, such as dust particles, to even form. So the dust is not necessarily a bad thing. But that would all depend on the source of the dust, of course.
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