Acid rain is a term which is used to describe a variety of processes which might more accurately be referred to as acidic deposition. Natural rainfall is slightly acidic due to dissolved carbon dioxide, picked up in the atmosphere. Organisms and ecosystems all over the planet have adapted to the slightly acidic nature of normal rain, and thus it poses no environmental problems. It is an increase in the acidity of rain, caused by human activities such as the combustion of fossil fuels, that has turned acid rain into a problem.
Highly acidic rain can damage or destroy aquatic life, forests, crops and buildings, as well as posing a threat to human health. The actual term “acid rain” was first used over one hundred years ago by British chemist Robert Angus Smith. At that time, he realized that smoke and fumes from human activities could change the acidity of precipitation. Unfortunately his awareness was not considered an environmental concern until the 1950’s. Around this time, increased levels of acidity were discovered in lakes in both Canada and Scandinavia.
At first, this was looked at as an interesting situation, rather than a growing problem. Since that time, much research has gone into identifying the sources of acid rain and the damage that it causes. As research continued, the situation reached catastrophe proportions in the late 1970’s. By this time, thousands of lakes in Canada and Scandinavia had been declared dead, devoid of life, while emissions of acid gasses continued to rise. Acid Rain Chemistry As mentioned earlier, the term acid rain is used to describe a variety of different types of acidic deposition.
These include “wet” deposition such as rain, snow and fog, as well as “dry” deposition in the form of acidic gases and dust. The term acid rain is only used to describe deposition which is more acidic than normal. Acidity is measured on the pH scale, a scale which runs from zero to 14, distilled water, which contains no carbon dioxide, has a neutral pH of seven. On the scale, pH levels below seven represent acidic solutions, while those above seven are alkaline. Testing of the pH is usually performed by litmus paper which responds to the spectrum of pH in different ways.
Each decrease of one number on the pH scale represents a tenfold increase in acidity. For example, rain with a pH of four is ten times more acidic than rain with a pH of five. Normal rain is slightly acidic, with a pH of around 5. 5. This acidity is a result of naturally occurring carbon dioxide (CO2), which dissolves into water vapor in the atmosphere. Human activities such as the use of fossil fuels for energy can result in the release of sulfur dioxide (SO2), and nitrous oxides (NOx) into the atmosphere.
These gases can travel for thousands of kilometers before coming back down to earth in the form of dry particles or acid precipitation. In the atmosphere, SO2 and NOx react with water vapor to form weak solutions of nitric and sulfuric acid. The areas of greatest acidity (lowest pH values) are located in the Northeastern United State. This pattern of high acidity is caused by the large number of cities, the dense population, and the concentration of power and industrial plants in the Northeast.
In some regions of New York State, rain with a pH of 2. as been measured. The average pH of rainfall in central Ontario is currently about 4. 2. Around Washington, D. C. , however, the average rain pH is between 4. 2 and 4. 4. In addition, the prevailing wind direction brings storms and pollution to the Northeast from the Midwest, and dust from the soil and rocks in the Northeastern United States is less likely to neutralize acidity in the rain. More than 90% of the SO2 and NOx present in the atmosphere over eastern North America is a result of human activities.
The major sources of sulfur emissions are coal (and to a lesser extent, oil) burning electric power plants, and industries such as ore smelting. Together, these two industries account for more than 70% of the sulfur emissions in North America. About 40% of NOx emissions come from the transportation sector, (cars, trucks, planes etc. ). The rest are emitted from fossil fuel fired power plants and other combustion processes. Once emitted into the atmosphere, acid gases can travel for thousands of kilometers before being deposited back onto the ground.
In fact, about half of the acid rain that falls on Canada originated from United States sources. A similar situation exists in Scandinavia where Sweden, Norway and Finland are exposed to acid rain from Great Britain and other northern European countries. Impacts of Acid Rain Acid rain has a wide variety of environmental and health impacts. The magnitude of these impacts is very dependant upon the type of bedrock and soil in a specific region. Regions where the bedrock and/or soil contains carbonates such as limestone and dolomite are less susceptible to damage by acid rain than areas with igneous bedrock.
This is because the carbonate material acts to neutralize the acidity of the precipitation. Carbonates act as a “buffer”; they tend to keep both surface and groundwater at a constant pH. Areas such as the “Canadian Shield” in Ontario and Quebec are very susceptible to damage by acid rain because the igneous bedrock has no natural buffering capacity and the soils are very shallow. The amount of damage to aquatic and terrestrial ecosystems is therefore dependant upon both the amount of acid deposition, and the type of soil and bedrock. Soils, surface waters such as lakes and rivers, and forests can all be damaged by acid rain.
In Canada, thousands of lakes are found in regions such as the Canadian Shield which are highly sensitive to changes in the pH of precipitation. The actual interactions between aquatic organisms (such as fish, crustaceans, insects and amphibians) and changes in water chemistry are extremely complex. Acidification can hinder the ability of aquatic organisms to reproduce. This is especially true for fish and amphibians that spawn in streams or shallow bays in the early spring. Large influxes of runoff from melting (acid) snow can drastically depress the pH in these areas for short periods in the spring.
This “acid shock” can kill the eggs of many species. Many species of frogs and salamanders, for example, can’t reproduce when the acidity of their breeding habitat goes below a pH of five. In addition to reproductive failures, acidification can reduce the amount of calcium available to vertebrates such as fish, as well as increasing the concentration of toxic heavy metals in surface waters. Both of the above can result in deformed bone structures, and poor growth in fish. The decline of any one member of a food chain impacts on many other species.
Birds such as loons and osprey, which eat fish, can’t survive without their main source of food. Similarly many mammals depend upon aquatic organisms such as crustaceans for their food. Acidification can eventually result in a “dead” lake. Such lakes exhibit very clear water, because there are no aquatic organisms such as plankton to color the water. The affects of acid rain on terrestrial (land based) plants and animals are also both complex and potentially devastating. Soils can be damaged by removing needed nutrients and dissolving toxic heavy metals from the soil.
Metals such as aluminum can get into the roots of plants and prevent the uptake of other important nutrients. Forests in areas with sensitive soils can be severely affected by acidification. Acid rain can damage the foliage of trees (leaves etc. ), and retard their growth. In Quebec, for example, sugar maple forests have been heavily damaged by acid rain. Similarly, there are forests in Poland and Czechoslovakia where 40% of the trees are dead or dying, as a result of acid rain. It has been estimated that acid rain causes $197 million in damage to commercial forests in Canada each year.
Acid rain can also result in human health concerns and damage to buildings. Acid rain can aggravate respiratory ailments such as bronchitis and asthma. Humans may also be affected by drinking water which contains higher levels of toxic metals which have been dissolved from soils and pipes by the increased acidity of drinking water supplies. Construction materials such as limestone, marble and sandstone can also be damaged by acid rain, resulting in eroded buildings and monuments. The value of this damage was estimated to by $830 million in Canada, as of 1985.
