If collecting rainfall is viable for you, just go ahead and collect some, then have it tested. I think you'll be surprised at what you DON'T find. Do this first, before you go crazy with the purification systems.

From "Harvesting, Storing, and Treating Rainwater for Domestic Indoor Use" from the Texas Commission on Environmental Quality
http://rainwaterharvesting.tamu.edu/drinking/gi-366_2021994.pdf

"The water in a raindrop is one of the cleanest sources of water available. Rainwater can absorb gases such as carbon dioxide, oxygen, nitrogen dioxide, and sulfur dioxide from the atmosphere. It can also capture soot and other microscopic particulates as it falls through the sky. Nevertheless, rainwater is almost 100% pure water before it reaches the ground."

I see you are in NYC. I read a clever way to keep birds from perching on the peak of your roof: screw a sturdy eye-bolt into each end of your roof peak, then stretch some stainless steel wire or strong fishing line between them. That's it.

Sue