The Earth’s ecosystem is fragile and interconnected. Whatever happens to one aspect of the environment may affect some of the other parts. The abiotic and biotic components work together to make up the environment we live in. When we place air pollutants into the atmosphere, such as sulfur dioxide and nitric oxide, they do not just “go away”. They are carried by air currents over great distances and can chemically react with other chemicals in the air. They can form compounds like nitrogen dioxide, sulfuric acid and nitric acid. These chemicals are able to dissolve in rainwater, snow, sleet, fog and dew. When this polluted water falls to Earth, it is called “acid precipitation.”
Regular rainwater is slightly acidic (pH 5.1) naturally. This is due to the presence of carbon dioxide in the atmosphere, which dissolves in the rainwater to form a weak carbonic acid. Natural rainwater’s slight acidity is advantageous because this allows other minerals that plants need to dissolve. However, sometimes the pH is lowered enough to cause grave damage to trees, crops, buildings and even human health among others.
According to the Environmental Protection Agency, “Acid Rain is a broad term used to describe several ways that acids fall out of the atmosphere. A more precise term is acid deposition, which has two parts: wet and dry.” (www.epa.gov/airmarkets/acidrain) This unit will deal mainly with wet deposition and its effect on the environment.
“Wet deposition refers to acidic rain, fog, and snow. As this acidic water flows over and through the ground, it affects a variety of plants and animals. The strength of the effects depends on many factors, including how acidic the water is, the chemistry and buffering capacity of the soils involved, and the types of fish, trees, and other living things that rely on the water.” (epa.gov/airmarkets/acidrain)
Even though some acidity in precipitation is normal, acid rain is a human-related phenomenon. It is created when power-production companies and industries burn fossil fuels, such as coal and oil. This releases sulfur into the air which combines with oxygen to form sulfur dioxide (SO2). These power plants also give off nitrogen oxides (NOX). When these gases are allowed to dissolve in rainwater they form sulfuric acid and nitric acid. Most of the sulfur dioxide and nitrogen oxides released into the atmosphere are from anthropogenic sources with about “2/3 of all SO2 and ¼ of NOX coming from electrical power generation that relies on burning fossil fuels like coal” (epa.gov/airmarkets)
Acid rain is measured on a pH scale. On this scale, pure water is a neutral 7. Any substance measuring lower than seven on the scale is an acid. The lower the substance is on the acid side of the pH scale, the stronger it is. Any substance over seven on the pH scale is called an alkali. The alkalis range from over 7 to14, with the strongest alkalis in the highest part of the range. (See figure 1) Acids and alkalis can ‘cancel’ each other out if they are combined and become closer to neutral. When soils and water resist changes in their pH this is called their buffering capacity.
The abbreviation pH stands for the “power of hydrogen” and it is a measure of the number of hydrogen ions in a given volume of solution. Acids release hydrogen ions in a solution and bases remove hydrogen from solution therefore; the low pH numbers have the highest power of hydrogen, and the higher numbers have the lowest power of hydrogen.
The pH scale is a logarithmic scale. This means that a pH of 6.0 is ten times more acidic than a substance with a pH of 7.0, a substance of pH 5.0 is one hundred times more acidic than a substance with a pH of 7.0, a substance of pH 4.0 is one thousand times more acidic than a substance of pH 7.0, and so on.
Because of the chemically opposite nature of acids and bases, they can neutralize each other. Neutrals have a balance of proton donors and proton acceptors. Thus if you mix equal amounts of proton donors (acids) with equal amounts of proton acceptors (bases) you will produce a neutral solution.
“Buffers are special chemicals that can change a pH of a solution toward a different, specific, predetermined pH” (Acid Rain, Lawrence Hall of Science). For instance you can have a buffer of pH 9, that when mixed with an acid solution of pH 4, would raise the pH, making it more basic. If enough buffer were added then the solution would remain at a pH of 9.
When the sulfur dioxide and nitrogen oxides get into the atmosphere and dissolve in rainwater they enter the hydrological cycle. When the water vapor condenses to form clouds these become acidic as well. Eventually when the clouds become saturated and release water as rain, the acid comes pouring down. But clouds know no boundaries and are moved by wind currents across many miles before they disperse into rainfall. So therefore it seldom happens that the point where the harmful gases are emitted and the point at which the precipitation falls are ever the same spot. Therefore the problem of acid rain is a regional and global issue, not solely a local one.
“The air pollutants that contribute to the formation of acid rain have long been released by natural processes such as volcanoes, and the activity of soil bacteria and other organisms. However, it has only been with the Industrial Revolution, including the invention of the combustion engine and the extensive use of fossil fuels, that these gaseous pollutants have been produced in great enough amounts to significantly affect the acidity of rain.” (Acid Rain, Lawrence Hall of Science)
Oxides of sulfur and nitrogen are the principal air pollutants contributing to acid rain. These are mostly produced from fossil fuel combustion in power plants, industry, and vehicles. Sulfur oxides come from mainly coal burning industry and power plants. Nitrogen oxides (NO and NO2) are found mainly in vehicular emissions. The combustion of fossil fuels increase the levels of carbon dioxide in the atmosphere, thereby increasing the amount of carbonic acid formed. However, carbonic acid accounts for less than 5% of the acid in acid rain.
SOx - (combines with oxygen to produce) --> SO2 (reacts with H2O) -->H2SO4 (sulfuric acid)
NOx - (combines with oxygen to produce) --> NO2 (reacts with H2O) --> HNO3 (Nitric Acid)
CO2 + H2O --> H2CO3 (carbonic acid)
Tables one through four shows the relative amounts of sulfur and nitrogen dioxide emissions in the United States and Canada. These table are used in the lesson plans to make graphs. By being able to visualize the data the students will be able to make fair comparisons between the two nations and see just where the main sources of acid rain pollutants originate.
Although the problem of acid rain was not considered to be a serious environmental issue until the 1970’s,effects of acid deposition were observed well before that time. “In 1306 King Edward I of England issued a proclamation banning the use of sea coal in London due to the smoke it cause.” (Clean Air Act Right Frame) According to the Environmental Protection Agency acid rain was first observed in the 19th century when “some people noticed that forests located downwind of large industrial areas showed signs of deterioration.” (Acid Rain in New England) In 1872 an English Scientist named Robert Angus Smith observed that certain precipitation could cause damage to plants and materials and he called this “Acid Rain”.
It was during the 1970’s that scientists began to observe an increase in acidity in many lakes and streams. Simultaneously, there was also research into problems associated with the transport of air pollutants such as sulfur dioxide. Scientists eventually saw the link between the two. They realized that many power plants used coal that contained high amounts of sulfur as fuel. It was this sulfur that was being transported via winds and dissolving in rainwater to produce acid rain.