Natural Acid Rain
The phenomenon of acid rain was discovered in Great Britain in the late 1800's, but was then essentially forgotten until the 1960's. It refers to precipitation that is significantly more acidic than natural, unpolluted rain which itself is mildly acidic due to the presence of carbon dioxide dissolved in atmospheric moisture forming carbonic acid:
CO2(gas) + H2O(liquid) H2CO3(aq)
The H2CO3(aq) is a weak acid, meaning it only partially ionizes to release H+(aq), with a resultant reduction in the pH of the system. pH is the - log [H+(aq)] therefore as the concentration of the H+(aq) increases the ph decreases. This partial ionization occurs in equilibrium as carbonic acid only ionizes 1.7%:
Due to this source of acidity, the pH of unpolluted, natural rain is about 5.6. Only rain that is appreciably more acidic than this, with a pH of less than 5.0 is considered to be truly acid rain.
The two predominant acids in acid rain are sulfuric acid (H2SO4) and nitric acid (HNO3). These acids are created during the transprt of the air mass that contains the primary pollutants. Generally speaking, acid rain is precipitated far downwind from the source of the primary pollutants, namely gaseous sulfur dioxide (SO2) and nitrogen oxides such as NO2. Thus acid rain is a pollution problem that does respect provincial or national borders.
On a global scale, most SO2 is produced by volcanoes and by the oxidation of sulfar gases produced by the decomposititon of plants. Because this natural sulfur dioxide is emitted high into the atmosphere or far from populated centers, the concentration of the gas in unpolluted air is quite small. However, considerable amounts of SO2 is presently being emitted into ground - level air, particulary over land masses in the Northern Hemisphere. The main human source of SO2 is the combustion, especially in electric power plants, of mined coal that tends to contain 1 to 5% sulfur. About half of the sulfur is trapped as inclusions in the mineral content of coal. If the coal is pulverized before combustion, this type of sulfur can be removed mechanically. The other half of the sulfur however, is bonded in the complex carbon structure and cannot be removed without expensive processing.
Both nitrogen and sulfur oxides are known to react with water in the atmosphere to form acids. These reactions are the source of polluted acid rain. The formation of acid solutions by SO2 is explained as a two - step process. assume that sulfur dioxide molecules first react with water molecules, forming molecules of sulfurous acid:
Sulfurous acid molecules then react with water producing an equilibrium with H+(aq) and hydrogen sulfite. Sulfurous acid is considered a weak acid as it only partially ionizes into H+(aq):
Sulfur dioxide can also react with oygen or ozone to form sulfur trioxide:
2 SO2(gas) + O2(gas)2SO3(gas)
This sulfuric acid is a strong acid that ionizes 100% in atmospheric precipitaion to produce H+(aq) ions:
Car engines burn gasoline using air air as a source of oxygen. The primary component of air is nitrogen and its exposure to the high temperatures inside car engines forms the pollutant nitrogen monoxide (NO). Sunlight provides the energy that promotes a series of secondary reactions producing nitrogen dioxide and ground level ozone that both participate in acid rain reactions.
Direct scavening of NO2 bt atmospheric water contributes little nitric acid (HNO3) on account of the low solubility of NO2 in water:
Acid rain containing considerable amounts of nitric acid likely originated without water:
NO2(gas) + O3(gas) NO3(gas) + O(2(gas)
The gaseous NO3 then reacts with any reactive hydrogen donor ( X ) in the atmosphere:
NO3(gas) + XH(gas) HNO3(aq)
Nitric acid like sulfuric acid is strong an completely ionizes into aqueous hydrogen and nitrate ions:
Technologies are now developing for the recovery of oxides from exuast gases. For example, some industries have added sulfur oxide recovery units to smoke stacks at large smelters. In addition to reducing sulfur dioxide emissions by over 50%, the sulfuric acid produced is sold as a valuable bi-product.