There's nothing wrong with rain water;.
That statement is not supportable.
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.
Most rain is caused when moisture condenses around small particulates in the air 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.
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 clean.
It does not take much looking to find out about this. Here is a current
Scientists fear impact of Asian pollutants on U.S.
Sun Aug 31 McClatchy Newspapers
... estimates more than 10 billion pounds of airborne pollutants from Asia — ranging from soot to mercury to carbon dioxide to ozone — reach the U.S. annually.
Particulates such as dust and soot, along with heavy metals, pesticides, PCBs, mercury, ozone, carbon dioxide, nitrogen dioxide and sulfur dioxide have all been found.
Viruses, bacteria and fungi also can be transported on dust particles, though, so far, they've been found only on the dust and sand blowing off African deserts, not Asian ones.
One study estimated a fifth of the mercury entering Oregon's Willamette River comes from overseas, with China as the mostly likely source.
Days after a major dust storm in the Gobi Desert in Asia , visibility in the Grand Canyon was obscured. Dust from deserts in North Africa has reached Florida . U.S. air pollution can reach across the Atlantic to Europe , even as pollution from Europe can circle the globe and reach the U.S.
"Air can circulate around the world in three weeks or less. "
That report is only about pollutants from Asia that reach the US. We generate our own which add to the atmosphereic mess. As I mention at the top of my post the acid rain caused by domestic pollution has severe effects upon our eastern regions. Atmosphereic pollution respects no borders, and rainfall carries those pollutants down with it.