Who is Robert Angus Smith?
Acid rain was discovered in the 19th century (1872) by Robert Angus Smith, a pharmacist from Manchester (England), who measured high levels of acidity in rain falling over industrial regions of England and contrasted them to the much lower levels he observed in less polluted areas near the coast. Little attention was paid to his work until the 1950s, when biologists noticed an alarming decline of fish populations in the lakes of southern Norway and traced the problem to acid rain. Similar findings were made in the 1960s in North America (the Adirondacks, Ontario, Quebec). These findings spurred intense research to understand the origin of the acid rain phenomenon. (Reference)
Acid rain: rainfall with a pH less than 5.6
Additional notes on Acid Rain and Air Pollution: Britannica Encyclopaedia (click link)