Not being a climate guru I can believe rain chemistry is very dependant on the local area. Chemical and power plants, big city with lots of vehicles, etc. Here in rural Northeast PA I periodically measure the pH of rainwater and it is always as close to 7.0 as I can measure with a pool test kit. The only effect I have ever seen, even after many inches of rain, is smple dilution. I catch rain from gutters when I need to top the pool. I love it.
Al