Acid Rain

Acid Rain

Acid rain is a rain or other form of precipitation that is unusually acidic, which means that it possesses elevated levels of hydrogen ions (H+) which is the reason for it’s low pH.

The chemicals that make acidic rain acidic are sulfur dioxide and nitrogen oxides, which react with water, oxygen and other chemicals/acidic pollutants in the atmosphere which results in sulfurous acid and nitric or nitrous acids, and thus results in acid rain.

This can be shown in the equations:

SO2 (g) + H2O (l) > H2SO3 (aq)

2NO2 (g) + H2O (l) > HNO3 (aq) + HNO2 (aq)

The pH scale

The pH scale is a numerical scale that measures the acidity or alkalinity of aqueous solutions. The pH of a solution is a measure of its hydrogen ion (H+) concentration and this its acid-base characteristics. Solution with a high hydrogen ion concentration is an acidic solution and has a low pH level. Basic solutions have a low hydrogen ion concentration and have a high pH level. A neutral solution has a pH of 7, such as pure water. The pH scale generally ranges from 0 to 14, with 0 being a very strong acid and 14 being a very strong base. Each level of the pH scale increases by a factor of 10, meaning that a solution with a pH of 4 would be 10x more acidic that a solution with a pH of 5, and a solution with a pH of 10 would be 100x more basic than a solution with a pH of 8. Common examples of acidic solutions are rain water (pH 6), vinegar (pH 3) and car battery acid (pH 0). Common examples of basic solutions are sea water (pH 8), laundry detergent (pH 11) and oven cleaner (pH 14).

The pH of normal rain is slightly acidic because carbon dioxide dissolves into the water which forms a weak carbonic acid. This gives the resulting mixture a pH of 5.6 at the typical atmospheric concentrations of CO2. The formation of carbonic acid can be shown in the equation:

CO2 (g) + H2O (l) > H2CO3 (aq)[pic 1]

Effect on the soil and trees

Acid rain has a detrimental effect upon trees and soil. It causes slower growth, injury or even death of forests. Acid rain often works with a combination of other factors that causes damage in these areas, including air pollutants, disease and freezing weather. While acid doesn’t usually directly kill trees, it weakens them by limiting the nutrients available to them from the soil and it exposes them to toxic substances which are released through the soil. It is often a combination of these which damage and kill the trees. Even if the soil is able to buffer/neutralise the acidity of the rain, trees in some forests high above sea level can be exposed to acidic clouds and fog which is even more acidic than the rainfall. When the leaves or needles of the tree are exposed long enough, essential nutrients are stripped from the tree. This loss of nutrients opens the trees up to the environmental factors discussed earlier such as freezing weather.

When soil absorbs acid rain, it will neutralise either some or all of the acidity of the rain depending upon the soil and its buffering capacity. The soils buffering capacity usually depends upon the thickness of the soil and its composition, along with the bedrock underneath the soil. This is why it can be seen that acid rain can have a huge impact on an area’s vegetation or appear to have none at all.

Effect on aquatic systems

When acid rain flows into or falls upon the aquatic systems themselves, the aquatic systems and the soil will buffer the acid rain. If the acidity surpasses the buffering capacity of the aquatic system and soil then the aquatic systems, such as lakes and streams, will become acidic. In these areas aluminium will be released into the aquatic systems from soil, a metal which is highly toxic to many aquatic organisms.