What might be in your rainwater depends on what is in your air and it will be very dilute and unlikely to be any real concern. 30 years ago if you lived north of the all the coal-fired power plants on the eastern side of the US there was a lot more crap in your air than there is today. They put in all those tall smokestacks long ago to keep the pollutants from falling in the local area. Worked great but all that crap just fell out further north and poisoned the land and lakes in Canada. Need a really tall wall to keep that crap out so when is Trump going to build the northern wall 20 miles high?
Rain water is basically distilled water tho it's oxygen content will be a lot higher. Distilled water should be aerated to get more O2 into it and I use a small air pump to aerate my RO water before giving it to my plants just to make sure it has as much O2 in it as possible.
Air pollution is everywhere but worse downstream of industrial areas. A lot of the pollution that lands on the west coast of North America comes from China and other countries way across the Pacific that have much more lenient pollution standards than we have here. China tho is going gung-ho for cleaning up their act and are becoming world leaders in non-fossil fuel energy production.
The oxides of sulfur and nitrogen are probably why the pH of your rain water is 6. Distilled or RO water has no real pH until something is added to it as pH is determined by the amount of + or - ions in the water and pure water has none or very little. Put one drop of pH down in a liter of distilled water and the pH will drop. Do that with most tap water and it won't do a thing as there are too many ions already in it for that little amount of acid to make a difference.