“Acid Rain” is a serious environmental issue caused by human interactions with the environment. The burning of fossil fuels, whether from large power plants or small cars, releases chemicals into the atmosphere that combine with water (H20) and create un-naturally acidic precipitation. This acidic precipitation has harmful effects on living, and non-living elements of the environment. Living things such as trees and animals can be killed by acid rain, while historical monuments made of once beautiful marble are being slowly dissolved to little more but a pile of white powder.
Precipitation is naturally acidic due to carbonic acid. Carbonic acid is created by the reaction of carbon dioxide (CO2) and water (H2O). However, “Acid Rain” is created when pH levels of precipitation fall below 5.6. Precipitation with a pH level below 5.6 shows a presence of chemicals that would not be found naturally in such high amounts. There are a variety of chemicals that contribute to acid rain when found in large amounts, with sulfur dioxides and nitrogen oxides being the primary sources. These two chemicals are produced naturally from volcanic eruptions (sulfur dioxides) and lightning (nitrogen oxides), however these natural sources do not produce enough of these chemicals to create the acid rain problems we have today. Instead, sulfur dioxides and nitrogen oxides are released in huge amounts from the combustion of fossil fuels. These two chemicals transform into secondary pollutants, those which are found in the acidic precipitation itself. These include sulfuric (SO2) and nitric (HNO3) acids.
Rain becomes acidic through the chemical reactions with sulfur dioxides and nitrogen oxides.
Sulfur Dioxide
First, the sulfur dioxide (SO2) released from combustion of fossil fuels is oxidized into sulfur trioxide (SO3).
Next, the sulfur trioxide reacts with water in the air to form sulfuric acid.
SO3 +H2O –> H2SO4
Sulfuric Acid is extremely acidic because it can dissociate twice.
H2SO4 + H20 –> H3O + HSO4
HSO4 + H20 –> H3O + SO4
Nitrogen oxides
First, NO from internal combustion engines oxidizes into NO2
2NO + O2 –> 2NO2
Next, NO2 reacts with water to form nitric acid (HNO3)
NO2 +H2O –> HNO3
HNO3 dissociates in water to form H3O and NO3
HNO3 + H2O –> H3O + NO3
These two chemicals are perfectly safe when produced from their naturally occurring sources, however when released by humans in such large amounts they create acid rain. Sulfur dioxides are formed from the combustion of coal and oil. While sulfur dioxides are released primarily through large power plants, nitrogen oxides are released mainly by internal combustion engines, or cars.
The acidity of acid rain is measured using a variety of techniques. Samples of rain can be collected and then measured using a digital pH reader, or indicator strips. Samples of pH are also taken from soil, or water sources like streams or rivers. The measurements are then compared to the pH scale, or the potential hydrogen scale. This scale measures the concentration of hydrogen ions in a solution. Low pH corresponds with a high concentration while a high pH corresponds with a low concentration.
Unpolluted rainwater has a pH of around 5.6. It is slightly acidic due to carbonic acid as well as small amounts of naturally occurring sulfur dioxides and nitrogen oxides. Rain water is considered “Acid Rain” when the pH falls below 5.6. Acid Rain will affect different species at different ranges. Plankton and certain types of algae are very sensitive to their environment and can die if their habitats pH changes by even a small amount. Fish are also highly affected by acid rain, especially because acid rain can run off from other areas and collect in these areas. Different fish species can tolerate different pH levels, but most fish eggs will not be able to develop after the pH reaches a value of 5.
The most harmful aspect of acid rain is the way in which it changes the abiotic factors of an ecosystem in a relatively short period time. This can cause drastic shifts in a food chain and can in turn destroy an entire ecosystem. This is to say that it is not the way in which acid rain directly affects living things that causes there death, rather the removal of the things it depends on for life. For example, certain species of frogs have been known to live in areas with water pH of less than 4.5. This is their environment and they have a tolerance to it, allowing them to thrive despite the low pH level. However take for example a trout living in a lake with a pH of 6. The trout can withstand a pH level as low as 5.5; however the sensitive organic matter it depends on for food can only tolerate a level of 5.8. If acid rain brings the lakes pH level to 5.8, the organic matter, and the trout will die. If the trout dies, anything depending on the trout will die and so on. This is the reason acid rain is so dangerous.
Although it would be nearly impossible to completely solve the problem of acid rain, there are many things we can do to reduce the amount of chemicals we release into the air so that the rain will have a higher, less acidic, pH. One thing we can do is to add scrubbers to smoke stacks of large fossil fuel burning power plants. These scrubbers inject limestone into smokestacks that remove sulfur dioxides to prevent them from entering the atmosphere. Catalytic converters on cars have been required on vehicles for over 20 years, and greatly reduce nitrogen oxide emissions. Also, utilizing green or alternative energy sources will reduce the amount of fossil fuels we need to burn for energy, and therefore reduce the amount of pollutants released into the air. As for reducing the effects of acid rain, bases can be added to affected areas to neutralize the pH and allow life to return again.
