Acid Deposition Information

Acids, pH, and the formation of acid rain

Acidification of freshwater

The natural acidity of rain

 

Acids, pH, and the formation of acid rain

Acids are aqueous solutions with a concentration of hydronium ion (H30+) greater than that of pure water. The concentrations are measured using the pH scale. Acid deposition can reach the surface in the form of rain, snow, fog, or dust (which is known as dry deposition.) Rain with a pH below 5.6 is commonly called acid rain.

The main gases responsible for acid deposition are sulfur dioxide (SO2) and nitrogen oxides (NOx, x = 1,2, or 3). Sulfur dioxide is released during the combustion of coal with a high sulfur content. Most nitrogen oxides are products of automobile exhaust.

The gases react to produce acid according to the following reactions:

 

 

The main sources of acid rain are SO2 from combustion of fossil fuels and NOx from automobile exhaust.

 

SO2 + 2OH- à H2SO4

H2SO4 + H20 à H30+ + HSO4-

HSO4-  + H20 à H30+ + SO42-

  Sulfur dioxide molecule reacts with hydroxide ion to produce sulfuric acid.

 The two hydrogen atoms in the sulfuric acid molecule react with water to create hydronium ions, leading to acid rain.
NO2 + OH- à HNO3

HNO3 + H20 à H30+ + NO3-

 

  Nitrogen dioxide molecule reacts with hydroxide ion to produce nitric acid.

 The hydrogen atom in the nitric acid molecule reacts with water to create a hydronium ions, leading to acid rain.

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Acidification of Freshwater

Acid rain reaches freshwater environments through direct precipitation, runoff, and drainage.

 

Why the Northeast?

Heavy industry in the Ohio River Valley states is responsible for large amounts of acid rain. Prevailing west-to-east winds carry the acid rain clouds towards the Northeast. The precipitation often occurs in remote areas, particularly New York’s Adirondack Mountains, Vermont's Green Mountains, and much of northeastern Canada.

 

 

 

 

A computer generated model of acid rain concentration.

  

 

 

The pH of rainfall in certain areas of the Northeast is now well below 5.

  

 

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Natural Acidity of Rain

It is important to note that rain is naturally acidic. This is due to the reaction of naturally occurring atmospheric carbon dioxide with water vapor to produce carbonic acid. The reaction is as follows:

CO2 + H20 à H2CO3 (carbonic acid)

"Pure" rain has a pH measure of 5.6 of 5.7, though this measurement may vary slightly depending on location. Natural, unpolluted lakes show pH levels between 6 and 8, which is relatively neutral on the pH scale and far less acidic than the rainwater feeding the lake. This is due to to the natural acid-neutralizing capacity (ANC) of lakes. ANC is highest in lakes surrounded by limestone or dolomite rock beds. These rocks serve as acid buffers, neutralizing the acid and leaving pure water as a by-product. However, the increased acidity of rain in the Northeast has greatly lowered the acid-neutralizing capacity of many lakes in the region. A highly reduced or almost absent ANC is an omnipresent characteristic of dead and severely affected acid lakes. Many lakes in the Northeast now show pH levels of less than 5.

* Since pH is a logarithmic scale, a drop of 1 in pH equals a tenfold increase in acidity. For example, a pH of 5.0 is ten times more acidic than a pH of 6.0, and one hundred times more acidic than a pH of 7.0.

 

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