Hi Fishmyster here are some links to some sites that you may find interesting. If you want me to I'll have a look at what I can find out about "There was a sudden drop in rain ph to 3.5 in 1994" but at this point I don't think "Mt Pinatubo erupted in June of 1991 and by 1994 our rain ph drops down to 3.5." has a correlation. I could be wrong but I personally think you need to track back to a closer source for why the pH dropped in that area in your data. What would be interesting to know about the 1994 data is, was the pH seasonal or was it consistent through the year. Another thing that would be telling is, does an other site that is close show the same thing. What about a site that is further away? Does the data match?
Here is something that you said that I sure agree with you and can relate.....
"I read a hell of alot of scientific reports. Some require much filtering to extract the raw facts."
https://aura.gsfc.nasa.gov/
https://aura.gsfc.nasa.gov/science/feature-20110228.html
http://www.ec.gc.ca/air/default.asp?lang=En&n=1F36EFBB-1&offset=12&toc=show