RAIN WATER would seem to be naturally clean and pure most places. Water gets dirty when it hits the ground etc. Air generally contains very few bacteria etc. Bird poop etc is the most likely source of contamination. I mean really rain is close to being distilled water!!!!!!
There is a lot of pollution in the air. The water that precipitates out of the sky passes through those pollutants. Ever heard of "acid rain"? If you go the the Smokies national forest you can see the effects of acid rain. Visit the Shenandoah and see the effects of industrial pollution. The Park Service can show you pictures taken from view sites many years ago to compare with today. In the mid atlantic and many other regions there is also major pollution from internal combustion engines.
All this in the atmosphere where the water vapor collects, the water molecules often clustering around particles until they make a mass heavy enough to fall, whereupon they carry the particles down to ground level.
I look at how dirty my car is after after a light to moderate rain. It is much dirtier than before. A heavy, prolongued rain may leave it less dirty, but still not the condition as after washing with clean water from a hose fed by the local water system.
Sorry. We can debate or discuss how much pollution is in the atmosphere, and how much comes down with the rain. But there is no plausible foundation for claiming rain water is anything like distilled water. Maybe 300 yrs ago, but probably not then, either.