Acid Rain Formation

     Acid rain is caused by emissions of sulfur dioxide and nitrogen oxides.  Natural sources of these gases do exist; however, more than 90% of the sulfur and 95% of the nitrogen emissions are from human activities.  The use of coal for electricity, fuel for vehicles, and many other factors contribute to the pollution.  Once these toxins are in the air, they are converted to nitric and sulfuric acid by reacting with water, oxygen, and oxidants.  The basic reactions for each follow:

SO2 + 2OH --- H2SO4 (sulfuric acid)
NO2 + OH --- HNO3 (nitric acid)
Both of these acids are easily dissolved in water.  Consequently, acidic water returns to Earth in the form of rain snow or fog.  The sulfur dioxide and nitrogen oxide emissions can also chemically transform into sulfur and nitrogen salts.  In this state, they are deposited as "dry" acid rain in the form of gases or particles.  This "dry" rain causes the same type of damage as regular acid rain, but it can be transferred hundreds of miles by prevailing winds.