7.3 Acid Deposition

Notes

Acid rain (acid deposition) is precipitation more acidic than normal.

• Normal rain is slightly acidic with a pH of about 5.6 because dissolved $CO_2$ forms carbonic acid. Acid rain has a pH below 5.6.
• It forms when $SO_2$ (from burning coal) and $NO_x$ (from vehicles and power plants) react with water in the air to form sulfuric and nitric acids.
• The pH scale is logarithmic: each one-unit drop makes a solution 10 times more acidic.
• Effects: acidifies lakes (killing fish and other organisms), damages forests, harms soil nutrients, and corrodes limestone and marble buildings and statues.
• Solutions: scrubbers, using low-sulfur coal or switching to renewables, and the Clean Air Act cap-and-trade program for $SO_2$.
• Regions with limestone soils neutralize acid better (they are buffered); regions with thin granite soils are more vulnerable.
• Example: rain with pH 4.6 is 10 times more acidic than pH 5.6.