Our atmosphere functions as living ecosystem of chemical reactions. Through the help of the water cycle, chemicals pass through the atmosphere and are eventually taken up by the soil, surface water, or organic materials. Human beings have added enormously to the atmospheric burden of many toxic substances. The most prominent evidence of this is the presence of acid rain: precipitation and particles that have been made acidic by air pollution.Acid rain is a direct consequence of the atmosphere's self-cleansing process. The tiny droplets of water that make up clouds continuously capture suspended particles and soluble trace gases. Not all trace gases can be removed by precipitation, sulfur dioxide (SO2) and nitrogen oxides in the atmosphere are chemically converted into forms that are readily added into cloud droplets: sulfuric and nitric acids, the main acids involved in acid rain.The reaction cycle takes place in the troposphere. It begins when sunlight hits an ozone molecule (O3) the result is a molecule of oxygen (O2) and a single oxygen atom, which combines with a water molecule (H2O) to form two hydroxyl radicals (HO). This sparse but active molecule forms nitrogen dioxide (NO2) into nitric acid (HNO3) which initiates the reactions that transforms sulfur dioxide into sulfuric acid (H2SO4). What this boils down to is, as a result only the amount of pollutant in the air determines how much acid is ultimately produced.The acid rain may fall hundreds of miles from the pollution source. Once it hits the ground chemical alterations still go on which can reduce the acidity of the water that will reach lakes and streams. Alkaline soils can neutralize acid directly. In acidic soils two other processes can stop the effects of the acidic pH. The acid can be immobilized as the soil vegetation retains sulfate and nitrate ions (from the nitric and sulfuric acids). It can also be buffered through a process that is known as cation exchange. In c...
