Acid rain is rainfall that has a pH lower than seven (neutral) caused by pollution.
  • acid rain is caused by the reaction between sulfur dioxide or nitrogen dioxide and water (creating sulfuric or nitric acid)
  • in Pennsylvania alone, approximately 890,000 metric tons of sulfur dioxide each year
  • the acidified water droplets then fall to the earth (precipitation) as acid rain, across large areas

pH of acid rain in the US
pH of acid rain in the US

Effects of acid rain:
  • low pH (5 or less) in streams and lakes will kill aquatic life and eggs, reducing ecologic diversity
    • according to the EPA, 50% of streams in the US are acidified, and 75% of lakes and ponds are acidified by acid rain
  • soil chemistry is altered:
    • death of microbes reduces breakdown of plant waste, increases soil toxicity
    • leaching of calcium and magnesium (removal from soil by acid rain)
  • loss of plant life:
    • forest regrowth is damaged
    • crop fields must be limed and fertilized
  • damage to structures:
    • limestone statues, steps, etc. are dissolved
    • faster rate of oxidation of metals

How acid rain can be prevented or treated:
  • reduce the amount of sulfur dioxide and nitrogen dioxide being emitted by coal power plants, burning of other fossil fuels
  • use 'wet scrubbers', or flue gas desulfurizaition, to remove sulfur dioxide from exhaust
  • enforce automobile emissions standards (use of catalytic converters)

Typically, wet scrubbers treat flue gases with aqueous sodium carbonate, which does two things:

  • First, it hydrolyzes sulfur oxides into sulfuric or sulfurous acid:

SO3 (g) + H2O (l) --> H2SO4 (aq)


SO2 (g) + H2O (l) --> H2SO3 (aq)

  • Second, the sodium carbonate neutralizes the acids:

Na2CO3 (aq) + H2SO4 (l) --> Na2SO4 (aq) + CO2 (g) + H2O (l)


Na2CO3 (aq) + H2SO3 (l) --> Na2SO3 (aq) + CO2 (g) + H2O (l)

forming much less harmful water-soluble sulfate or sulfite salts.

For more detailed and animated deposition and pH data for the US or your region, go here.