acid+rain

[|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



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:

SO 3 (g) + H 2 O (l) --> H 2 SO 4 (aq)

or

SO 2 (g) + H 2 O (l) --> H 2 SO 3 (aq)


 * Second, the sodium carbonate neutralizes the acids:

Na 2 CO 3 (aq) + H 2 SO 4 (l) --> Na 2 SO 4 (aq) + CO 2 (g) + H 2 O (l)

or

Na 2 CO 3 (aq) + H 2 SO 3 (l) --> Na 2 SO 3 (aq) + CO 2 (g) + H 2 O (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.