Joanna Rentes, 24-November-1997
The acidity of rainfall over parts of the United States, Canada and Europe has increased steadily over the past forty years or so. Researchers believe that this is primarily due to the increased emissions of sulfur and nitrogen oxides which accompany increased economic activity.
The sulfur and nitrogen oxides are oxidized in the air until they are converted to sulfuric and nitric acids. These acids are then captured by raindrops which fall to the earth as acid precipitation. This process is called deposition. We commonly know this as acid rain, but we can have acidic snow or hail and even acidic dust particles falling from the sky.
Acid rain is a human-related phenomena. Since our industries are so fond of burning fossil fuels (coal and oil) they tend to release a lot of sulfur into the air. This sulfur combines with the oxygen already present in the air to form sulfur dioxide (SO2). Also, since we like to drive big fancy cars rather than ride bikes or walk, we cause the formation of nitrogen oxides (NO or NO2 or NO 3, etc) in air from burning gasoline.
In the eastern United States, the primary conversion of sulfur dioxide is through an aqueous phase reaction with hydrogen peroxide (H2O2) that exists in clouds. On the other hand, nitrogen dioxides in the air react with hydroxide (OH) radicals formed photochemically. The result of these chemical reactions is the formation of acids in the atmosphere. These acids are, most notably, sulfuric acid (H2SO4) and nitric acid (HNO3).
Formation of SO2 and nitrogen oxides (NO & NOx):
S(in fuel) + O2 -------> SO2
N2 + O2 -------> 2NO
NO + .5O2 -----> NO2
Formation of hydrogen peroxide
VOC + sunlight + HO2 (in air) -------> H2O2
***VOC = Volatile Organic Compound ***
Formation of acids :
SO2+ OH + O2 (in air) --------------> H2SO4
SO2 + Oxidants (from wet surfaces) --> H2SO4
NOx + sunlight + OH (air) ------------> HNO3
Sometimes, the environment can naturally adapt to acid rain. For example, in locations where there is a large amount of lime occurring naturally in the soil, the soil will have no problem with acid rain. The lime, which is a base, will neutralize the acid in the rain, thus minimizing their effects.
However, in locations where there is not a way to naturally compensate for the acid rain, the acids can cause a lot of harm to things that we care about and enjoy. For example. some animals, like frogs and fish, have difficult time adapting to and reproducing in an acidic environment. Also, the leaves of many plants and trees can be severely damaged by acidic precipitation. It is believed that acid rain leaches calcium and magnesium from the soil. This causes a decrease in in the ratio of calcium to aluminum in the soil, which stimulates the uptake of aluminum by roots. The uptake of aluminum by trees and plants can be destructive. Finally, in cites and towns all over the world, stone structures, such as buildings, ancient ruins, etc are being deteriorated by the corrosive effects of acidic rainfall.
As with many other environmental issues, there has been increasing need to come up with rules and regulations to limit the amount of acid rain producing gases that are emitted into the atmosphere. One of the major legislative acts regarding acid rain is the Title IV of the 1996 amendments to the Clean Air Act (CAA). For more detailed information on these regulations, please use the following link: clean air
With the enactment of new rules and regulations for emission control, engineers have developed various systems to help limit the amounts of SO2 and NOx that are released in to the atmosphere. For brief explanations of different types of control processes for SO2 and NOx, please use the following links or refer to the references listed in my bibliography.
SO2 control technologies
NOx control technologies
If you want to know where I got all the information for this series of pages, feel free to click on the following link to view my bibliography. references
I know that there are many young people who know that there is such a thing as acid rain, but don't really know what it means. Also, I realize that many parts of this page are geared towards helping older folks learn more and are not necessarily aimed at teaching the younger people. So, if you are interested in getting a better explanation in terms that someone with out a degree can understand, please click here and I will see what I can do for you.