The Effects of Acids on the Environment
Acidic pollutants can be deposited from the atmosphere to the Earth's surface in wet and dry forms. The common term to describe this process is acid deposition. The term acid precipitation is used to specifically describe wet forms of acid pollution that can be found in rain, sleet, snow, fog, and cloud vapor. An acid can be defined as any substance that when dissolved in water dissociates to yield corrosive hydrogen ions. The acidity of substances dissolved in water is commonly measured in terms of pH (defined as the negative logarithm of the concentration of hydrogen ions). According to this measurement scale solutions with pHs less than 7 are described as being acidic, while a pH greater than 7.0 is considered alkaline. Precipitation normally has a pH between 5.0 to 5.6 because of natural atmospheric reactions involving carbon dioxide. Precipitation is considered to be acidic when its pH falls below 5.6 (which is 25 times more acidic than pure water). Some sites in eastern North America have precipitation with pHs as low as 2.3 or about 1000 times more acidic than natural.
Acid deposition is not a recent phenomena. In the 17th century, scientists noted the ill effects that industry and acidic pollution was having on vegetation and people. However, the term acid rain was not coined until two centuries later when Angus Smith published a book called 'Acid Rain' in 1872. In the 1960s, the problems associated with acid deposition became an international problem when fishermen noticed declines in fish numbers and diversity in many lakes throughout North America and Europe.
(b). Acid Deposition Formation
Acid deposition can form as a result of two processes. In some cases, hydrochloric acid can be expelled directly into the atmosphere. More commonly it is due to secondary pollutants that form from the oxidation of nitrogen oxides (NOx) or sulphur dioxide (SO2) gases that are released into the atmosphere (see Figure *formaciddep.html*). The process of altering these gases into their acid counterparts can take several days, and during this time these pollutants can be transferred hundreds of kilometers from their original source. Acid precipitation formation can also take place at the Earth's surface when nitrogen oxides and sulphur dioxide settle on the landscape and interact with dew or frost.
Emissions of sulphur dioxide are responsible for 60-70 % of the acid deposition that occurs globally. More than 90 % of the sulphur in the atmosphere is of human origin. The main sources of sulphur include:
Coal burning - coal typically contains 2-3 % sulphur so when it is burned sulphur dioxide is liberated.
The smelting of metal sulfide ores to obtain the pure metals. Metals such as zinc, nickel, and copper are all commonly obtained in this manner.
Volcanic eruptions - although this is not a widespread problem, a volcanic eruption can add a lot of sulphur to the atmosphere in...