What is acid rain?
Acid rain is the term for pollution caused when sulfur and nitrogen dioxides combine with atmospheric moisture. The term ‘acid rain’ is slightly misleading, and would be more accurate if deemed ‘enhanced acid rain’, as rain occurs acidic naturally. Acidity is measured on what is know as the pH scale. Fourteen is the most basic, seven is the most neutral, and zero is the most acidic. Pure rain has a pH level of 7, which is exactly neutral.
The acidity of rain is determined by the pH of pure water in reaction with atmospheric concentrations of carbon dioxide, resulting in carbonic acid. These particles partly dissociate to produce hydrogen ions and bicarbonate ions. A bicarbonate atom is an ion formed by one hydrogen atom, one carbon at atom, and three oxygen atoms, and is very effective in natural waters at neutralizing hydrogen ions and reducing acidity.
The dissociation results in the natural acidity of pure rain, which is moderately acidic at a pH of 5.7. Rain less than 5.7 is considered ‘acid rain’, meaning it has reacted with acidic atmospheric gases other than carbon dioxide, such as sulfur dioxide and nitrogen dioxide. Sulfur dioxide is produced by electric utilities, industrial, commercial and residential heating, smelters, diesel engines and marine and rail transport, which creates sulfuric acid in rain. Nitrogen dioxide will also react with the rain, caused largely by transportation (cars, trucks, planes, etc.) and electric utilities, producing nitric acid.
There is a certain degree of naturally occurring acidity in rain water. This acid is from reaction with alkaline chemicals, found in soils, lakes and stream, and can occasionally occur when a volcano erupts as well. Bacterial action in soils and degasing from oceanic plankton also contribute to the acidity found in rain. More than 90% of the sulfur and 95% of the nitrogen emissions which occur in North America are due to the pollution created by humans.
How Is Acid Rain Formed?
Acid rain consists mainly of acids formed in the atmosphere. It consists of the oxides of sulfur, SO2 and SO3, and of nitrogen NO and NO2. Let us examine the major contributor to acid rain, sulfur oxides. Natural sources which emit sulfur dioxide include volcanoes, sea spray, plankton and rotting vegetation. Despite these natural occurrences, the burning of fossil fuels (such as coal and oil) can be largely blamed for the emissions.
The chemical reactions begin as energy from sunlight, in the form of photons, hit ozone molecules (O3) to form free oxygen (O2), as well as single reactive oxygen atoms (O). The oxygen atoms react with water molecules (H2O), producing electrically charged, negative hydroxyl radicals (HO). These hydroxyl radicals are responsible for oxidizing sulfur dioxide and nitrogen dioxide, which produces sulfuric acid and nitric acid. Some particles will settle to the ground (in the form of acid deposition) or vegetation can absorb some of the SO2 gas directly from the atmosphere. When sulfur dioxide comes in contact with the atmosphere, it oxidizes and forms a sulfate ion.
It becomes sulfuric acid as it joins with hydrogen atoms in the air and falls down to earth. Oxidation occurs most in clouds, especially in heavily polluted air, where other compounds such as ammonia and ozone help to catalyze the reaction, increasing the amount of sulfur dioxide changing to sulfuric acid. Not all of the sulfur dioxide is converted to sulfuric acid, and it is not uncommon for a substantial amount to float up into the atmosphere, move to another area, and return to earth as sulfur dioxide, unconverted.
S (in fossil fuels) + O2 =* SO2 2 SO2 + O2 =* 2 SO3
Much of the sulfur dioxide is converted to sulfur trioxide in the atmosphere SO3 + H2O =* H2SO4
The sulfur trioxide can then dissolve within water to form sulfuric acid Nitric oxide and nitric dioxide are mainly from power plants and exhaust fumes. Similar to sulfur dioxide, reactions are heavily catalyzed in heavily polluted clouds where iron, manganese, ammonia and hydrogen peroxide are present. Also, the formation of nitric acid can trigger further reactions which release new hydroxyl radicals to generate more sulfuric acid. The following is a typical reaction, which is direct combination of nitrogen and oxygen at the high temperature inside a car engine.
N2 + O2 + heat =* 2NO 2NO + O2 =* 2NO2
This nitrogen monoxide immediately reacts with oxygen and forms nitrogen dioxide in the following reaction 3NO2 + H2O =* 2HNO3 (aq) + NO
The nitrogen will then dissolve in water in the atmosphere and produce nitric acid There are several other potential contributors to acid rain. These include oxidation by products of alkene-ozone reactions, oxidation by reactions of NxOy species and oxidation by peroxy radicals. Each of these reactions, however prove to be minor contributors and are rather insignificant.
How Is Acid Rain Harmful?
Environmental Hazards Aquatic Ecosystems Acid rain has an effect on virtually all ecosystems it touches. Perhaps the most prominent, and equally as troubling is the harmful results it produces when in contact with lakes, streams and ponds. Scientists studying the effects of acid rain went to a lake about 135 km away from the Ontario- Manitoba border called Lake 223. This lake, so far north acid rain did not reach it, was extremely healthy, and was a perfect setting to explore the effects of acid rain on aquatic ecosystems. In 1974, scientists began to add sulfuric acid into the lake.
The acid was added very slowly, and it was four years later when they saw a major change. The freshwater shrimp began to die out. Fathead minnows stopped reproducing and began to vanish. As the scientists continued adding acid to Lake 223 in low amounts, large algae mats began to form and crayfish became unhealthy and died. Seven years after the beginning of the experiment, the lake trout stopped reproducing, and most of the fish species, leeches, crawfish and mayflies began to die.
In 1984, the scientists stopped adding the acid. Without the addition of deadly sulfuric acid, the lake slowly began to recover. Some of the fish species began to recover, however some of the scientists estimated it would take one hundred years for the lake to fully recover, even without the addition of any more acid. Fish can still live in a lake with a low acid level, however they will get sick and not grow to proper proportions. Often the fish will not reproduce, and eventually, as the acid level increases, all the fish will die. The acid will also ‘leach’ metals from the bottom of the lake. There are metals contained within the mud and rocks of the lake bottom, however they remain not dangerous as long as they are not released.
The acid will draw out these harmful metals and dissolve them in the water, resulting in the deterioration and disappearance of a species. One of these damaging metals is aluminum, which will coat and burn the gills of the fish as it intakes the polluted water. Some fish found in acidic lakes contain higher levels of mercury in their bodies, which is harmful to humans, resulting in the government telling society to limit the amount of fish they eat from certain lakes and rivers. If the numbers of one species or group of species changes in response to acidification, the ecosystem of the entire body of water is likely to be affected through the predator-prey relationships.
Let us examine how acid rain is dangerous to fish. A freshwater fish’s respiration consists of a ‘trade’ of hydrogen ions (H+) in their blood for sodium ions (Na+) from the water around them. If the concentration of hydrogen ions in the water is increased, which is essentially what happens when pH falls, there are (proportionally) fewer sodium ions. Fish are forced to absorb more hydrogen while finding it harder to obtain sodium.
The acidity of their blood increases, while the salt content drops. An experiment involving brown trout showed that at a pH of 5.2 or lower, this process was fatal to this species, and is likely deadly to many other trout species.
