If your talking about straight from the sky, without using anything other than a sterile bucket, in the middle of a large field, without any trees near by, to collect the rain water in. Then as you mentioned, anything that is in the surrounding air is likely to be in it. From any dust particles (and there elements), to acid rain (from smog). Daily conditions will change. As the raindrops fall, they collect everything that's in it's path (sticking to the raindrop). That's why it always smells nice after a good rain, the rain filters the air. Taking all the contaminates with it, leaving fresh air. Anyone who has lived in a large city knows the smell of the air after a good rain.

Wow, EC of .7 millisiemens? Mine measures at about 0.05. I have no clue why yours is so high.

One thing that I found funny was that rainwater measures higher than AC condensate for me -- the condensate coming in at about 0.03. But the rainwater tastes a *lot* better. I think the condensate is devoid of everything but iron, and that it's loaded with the latter.