Description Rain is normally slightly acidic, having a pH of 5.6. (Pure water, in contrast, has a pH of 7.0. The lower the number, the more acidic the liquid. For example, most fruit juices and soft drinks have a pH in the range of 2 – 3. A pH lower than 1 is not only unpleasant to the taste but is actually dangerous because it burns the skin).
The natural acidity in rain is due to dissolved carbon dioxide from the atmosphere. But the rain that is now failing in many parts of the industrialized world, even in regions remote from industry, is often much more acidic. The average pH of rain in the northeastern United States has decreased over the past years and is now 4.3. Rain with a pH as low as 1.5 has been recorded at Wheeling, West Virginia. The Los Angeles basin routinely has fogs made up of suspended water droplets with a pH of 2.2 – 4.0.
CAUSES Acid rain is caused by oxides of sulfur and nitrogen. These are present in the atmosphere as a result of combustion processes.
- Sulfur Coal and oil typically contain 1%-3% sulfur. This is converted into sulfur dioxide when coal and oil are burned. Sulfur dioxide slowly changes as it reacts with atmospheric oxygen. Then it dissolves in water droplets to form a dilute solution of sulfuric acid.
Of course, some sulfur dioxide arises from natural sources. Scientists have estimated that the eruption of Mount St. Helens in the state of Washington in 1980 blew out approximately 400, 000 tons of sulfur dioxide. But that is only about 1.5% of the estimated total sulfur dioxide from manmade sources in the United States for the same year.
- Nitrogen Nitrogen monoxide is formed during electrical storms, but it is also formed in combustion processes, particularly in internal combustion engines. Nitrogen monoxide reacts with atmospheric oxygen to give nitrogen dioxide. This in turn reacts with water in clouds and raindrops to give a solution of nitric acid.
OUTCOME Some of the harmful effects of acid rain are now well established. Because marble is soluble in acid, Greek and Italian monuments that have withstood many centuries of natural weathering are now rapidly deteriorating. There are even more serious harmful ecological effects. In limestone areas, acid rain is largely neutralized. However, in other regions this does not happen. The lakes and rivers in these areas have consequently become acidified. It is estimated that 20,000 lakes in Sweden have now become too acidic for fish and other life. The same effects are being observed in lakes in the United States and Canada.
Acid rain dissolves aluminum compounds from the soil and washes them into lakes where they poison the fish. Plant life is also affected because acid rain kills microorganisms in the soil that are responsible for nitrogen fixation. Acid rain also dissolves and washes away essential magnesium, calcium, and potassium compounds. Moreover, acid rain can dissolve the waxy coating that protects leaves from fungi and bacteria. The recent deterioration of some coniferous forests in Germany and Canada has been attributed to the effects of acid rain.
What can be done to combat the effects of acid rain? A local and limited solution used in Sweden is to add lime to lakes to neutralize the acid. However, this is expensive and must be repeated annually. In the past, very high smoke stacks were built at smelting plants to disperse the sulfur dioxide as high into the atmosphere as possible. This certainly benefitted the vegetation for miles around the smelting plants which in many cases had been completely killed off.
However, sulfur dioxide may be carried in air currents many hundreds of miles to be deposited, often in another country, as acid rain. The Swedes blame the British for their acid rain, and controversy rages between Canada and the United States over the acid rain which both countries believe originates on the other side of the border.
The obvious solution to the problem is to drastically reduce emissions of sulfur and nitrogen oxides. Sulfur dioxide could be removed from the gases which are produced by combustion and smelting processes. Removal is achieved by passing these gases over lime. The sulphur dioxide reacts with the lime to produce a solid.
Other solutions include removing sulfur from oil and coal or using only low sulfur content fuels. Or we could convert to alternative energy sources such as solar power or nuclear power. Most solutions are expensive or they bring problems of their own – such as disposal of nuclear wastes. Because these solutions are expensive and are often unpopular with certain interest groups, governments are reluctant to act. As an excuse to delay, they use the fact that neither all the effects of acid rain, nor all the details of its formation are yet well understood. Although scientists will continue to search for a better understanding of the phenomenon of acid rain and for better and less expensive solutions to the problems that it causes, we already know enough about its harmful effects to warrant drastic steps to reduce emissions of the oxides of sulfur and nitrogen.