Atmospheric concentrations of SOx and NOx have been increasing since the industrial revolution in the early 1800s, SOx from the refining of metals and burning of fossil fuels with sulfates in them, NOx from combustion reactions.
S(s) + O2(g) --> SO2(g)
N2(g) + O2(g) --> 2NO(g) etc
SOx and NOx dissolve in water vapour in the air to form sulfuric, sulfurous and nitric acids.
SO2(g) + H2O(l) --> H2SO3(l)
2H2SO3(l) + O2(g) --> H2SO4(l)
NO2(g) + H2O(l) --> HNO3(l)
This lowers the pH of the rain and water bodies where it falls. The impact of this is the death of plant life, like in the Black forest in Germany, as many plants require a certain soil pH to survive. Acid rain also causes aluminium leaching from the soil, which is toxic to plants and to marine life, and hampers the absorption of nutrients from the soil.
Decreased pH also affects marine life adversely, by making the water too acidic, as fresh water should be close to neutral.
That's all I can think of off the top of my head, i'll come back to it later.