The exceptional characteristics of the substance known as water have been recognized and appreciated for millennia. In particular, its ability as a solvent provides for many of the vital processes enabling life, such as acting as the medium in which red blood cells transport oxygen in our bodies. Yet water’s propensity to dissolve ions, other liquids and even gases may not always produce physical or biochemical advantages.
The water supply on Earth is continuously transported and concomitantly purified by a mechanism known as the hydrological cycle. As solar radiation heats the Earth’s surface, water molecules evaporate and then condense into cloud formations as they reach higher elevations and cooler atmospheric levels. Large-scale weather patterns transport the water in these cloud formations around the globe, and return the water to the surface as precipitation. Despite the purification of the substance by this process, rainwater is found to have a pH that is not neutral as one might expect, but mildly acidic. During its passage through the atmosphere, water’s extraordinary capacity as a solvent absorbs carbon dioxide in the air, and small quantities of carbonic acid is generated as shown:
H2O + CO2 → H2CO3→ H+ + HCO−3
In our modern industrialized world, there are other gases present in the atmosphere that, like CO2, can dissolve in atmospheric moisture. In particular, the presence of NOx and SOx, byproducts of fossil fuel combustion, is a specific concern.
NOx, formed by the reaction of nitrogenous contaminants in fuels with oxygen, can react with water in the atmosphere to generate nitric acid, HNO3. In its concentrated form, nitric acid is a corrosive material that can dissolve some metals. Likewise, sulfur oxide contaminants react with moisture yielding sulfuric acid, the viscous acid found in lead-acid car batteries. As these acids are produced and dispersed in the atmosphere, they constitute an environmental issue that transcends borders and physical boundaries.
Acid rain is then precipitation that possesses acidity greater than that of normally slightly acidic rainwater. Its effects can be noted on both biological systems and physical structures. Trees in many areas of the world bear the evidence of acid rain damage in the form of brittle, browned leaves, but the principal destruction to plants is to the root system. Increased acidity limits access of beneficial ions such as Ca2+ and Mg2+ but encourages the solubility of damaging ions such as Al3+ in the soil. Aquatic organisms such as fish experience skeletal growth problems due to limited access to calcium ions. Physical structures also bear witness to the destructive nature of acid rain; marble statues erode due to long–term exposure to acidic rainfall.