Acid Rain

From Chempedia

Acid Rain: What Causes It and What Is Being Done?


Acid rain is a modern age concept born from concerns about the relatively high acidity of our nation’s rain. Though it is normal for rain to be slightly acidic due to its carbon dioxide content, the most acidic rain in the United States had a pH of 4.3 in 2003. Rain this acidic is harmful to the environment in a variety of ways and it crosses both state and national borders. Thinking with chemistry can help us understand this alarming acidity which invades our ecosystem and, moreover, it can bring about ways to curb its progression.


Acid rain is a term that refers to acids falling out of the upper atmosphere; in chemistry the preferred term is acid deposition, which is then categorized into wet and dry components.[1] Wet deposition refers to acidic rain, the form most people are familiar with, in addition to both fog and snow. Acidic gases and particles account for what is known as dry deposition. Strong winds blow the acid deposition hundreds of miles across state and national borders; for instance, sensitive ecosystems in Canada are affected by the acids in the air from the United States.[2] Acidic particles and gases collect on buildings, cars, homes and trees. Acidic solutions are often formed when dry depositions wash from these surfaces during heavy rainstorms and create a solution that is more acidic than rain alone.

We hear all the time in the news that acid rain comes from the burning of fossil fuels, particularly in industrial facilities. When you hear this, it is most frequently associated with SO2 emissions. Ore smelting, power generators burning coal and natural gas processing are the main contributors. For example, in the year 2002, the measured output of SO2 emissions for the United States was 14.8 million tons.[3]

Image:acidrain_1.png

This graph illustrates where the 14.8 million tons of SO2 emissions from the United States are coming from.[4]



Scientists have determined that acid rain is primarily caused by sulfur dioxide (SO2) and nitrogen oxides (NOx. When these pollutant gases react with water and oxygen in the atmosphere, compounds of sulfuric acid (H2SO4), nitric acid (HNO3) and ammonium nitrate (NH4NO3)are formed.[5] See the diagram below of both the sources and receptors of acid rain. 

Image:Acidrain_2.gif

It is estimated that half of the acid deposition in the atmosphere lingers close to the earth and eventually falls down as acid rain. Before the rain falls, the acid compounds contribute to smog, which has well known detrimental effects to the people who breathe it and are forced to look at it in such areas as Los Angeles and Houston.[6] The rain also causes the acidification of lakes and streams and soils as well as the decay of buildings and forests. The lakes and streams that are most affected are those with a low buffering capacity, or the ability to neutralize acidic compounds. According to the National Surface Water Survey, "acid rain has been determined to cause acidity in seventy-five percent of the acidic lakes and about fifty percent of the acidic streams."[7]


Acidification leads bodies of water toward a completely barren state in which no fish or plant life can be sustained. The rate of acidification is significantly increased when there is heavy rainfall or melting snow, a phenomenon known as episodic acidification. This increased rate is the cause of death for many fish populations. However, bodies of water are not the only things affected by acid rain, forestry is also in danger.


Acidic accumulation in the forest leads to many problems with the growth of trees, mainly by stripping bark and a natural defense against insects and weather. These acidic deposits also strip the forest soil of important minerals for the trees to survive and reproduce, thus having an impact on future generations of trees. Since acid deposition has become such a widespread issue, many public and private agencies at the local, state, national and world level are studying the effects of acid rain and trying to find solutions for this pandemic problem.


Monitoring atmospheric deposition and surface water analysis is essential in addressing the acid rain problem.[8] With a stringent monitoring system in place, scientists will have a clear initiative for further technological innovations to prevent acid rain. From a chemistry perspective we can understand what some of these innovations are.


Given that acid rain is caused primarily from the releasing of NOx and SO2 into the atmosphere, researchers are studying ways to reduce these chemicals from the emission of burned fossil fuels.[9] The primary focus is on automobiles and coal-fired electric utility boilers since they produce the most NOx and SO2.[10]


Catalytic converters were developed for automobiles on their exhaust pipes to control the amount of NOx emissions.[11] A catalytic converter treats the exhaust before it leaves the tail pipe and removes a great amount of NOx and other pollutants. Inside a catalytic converter are basically two stages of cleansing. The first is a reduction catalyst which uses platinum and rhodium. When an NO or NO2 molecule comes in contact with the catalyst, it splits the nitrogen atom from the molecule and the oxygen is free to leave the exhaust pipe. The nitrogen atom is held in the catalytic converter where it bonds with other nitrogen atoms that have been split to form N2. This safe form of nitrogen can then leave the tail pipe.

2NO => N2+02 or 2NO2 => N2 + 2O2


The second part of the catalytic converter is an oxidation catalyst, which takes hydrocarbons and carbon monoxide that is created by the emissions and converts them into carbon dioxide by burning them over a platinum and palladium catalyst.[12] Another solution to control the emissions of pollutants into the air is the use of scrubbers in power plants. These scrubbers use lime (calcium hydroxide) to react with sulfur dioxide, thus producing a calcium sulfate solid.

CaOH + SO2 => CaS

The calcium sulfate solid can then be pumped into a storage pond. This system usually removes 80-90 percent of the SO2.


The acid rain solutions outlined in this report reflect but a glimpse of the landscape. Other solutions include using low sulfur coal, fluidized bed combustion and limestone injection using a multistage burner.[13] In addition, the use of alternative energy sources such as solar and wind power can help generate energy without having to release acid rain-causing pollutants into the air. Scientists are continuing to think creatively and find innovative solutions, working for the production of energy in a non-polluting and sustainable way.


Researched and written by Mike Bendewald, Ryan Allen, Danielle Berry, and Nick Allardyce.





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Footnotes

  1. ^ www.epa.gov/acidrain/#what. October 14, 2005
  2. ^ The 1999 review of acid rain science programs in Canada: a report prepared to meet the requirements of the Canada-Wide Acid Rain Strategy for Post 2000. p.7
  3. ^ www.ec.gc.ca/acidrain/acidfact.html. October 14, 2005
  4. ^ The Journal of the Operational Research Society Vol. 37, No.9 pp.838
  5. ^ www.epa.gov/acidrain/#what. October 14, 2005 and www.ec.gc.ca./acidrain/acidfact.html. October 14, 2005 (pie chart)
  6. ^ United States Congress. House. Committee on Science. “Acid rain: the state of the science and research needs for the future.” May 3, 2001. p. 20.
  7. ^ http://www.sciencemaster.com/jump/earth/acid_rain.php. October 4, 2005
  8. ^ United States Congress. House. Committee on Science. “Acid rain: the state of the science and research needs for the future.” May 3, 2001. p. 14.
  9. ^ What is Acid Rain and What Causes it? 6 Aug 2002. Almanac of Policy issues. 22 Sept. 2005. www.policyalmanac.org/environment/archive/acid_rain.shtml
  10. ^ Acid Rain. 15 Jan 1999. Acid Rain Solutions. 22 Sept. 2005. http://www.geocities.com/CapeCanaveral/Hall/9111/DOC.HTML
  11. ^ Acid Rain Solutions. 2003. Virtual Chembook. 22 Sept. 2005. http://www.elmhurst.edu/~chm/vchembook/197acidrainsoln.html
  12. ^ How Catalytic Converters Work. 2000. How Stuff Works. 22 Sept. 2005. http://auto.howstuffworks.com/catalytic converter.htm/printable
  13. ^ Acid Rain Solutions. 2003. Virtual Chembook. 22 Sept. 2005. http://www.elmhurst.edu/~chm/vchembook/197acidrainsoln.html
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