Acid Precipitation: Causes, Effects, and Implications

Acid Precipitation: Causes, Effects, and Implications

Contents

1 Introduction ................................................................................. 3

2 What is Acid Precipitation?................................................................ 3

2.1 Definition ........................................................................... 3

2.2 Common Pollutants and Reactions Involved ................................... 4

2.3 Sources of Acid Precipitation Pollutants ....................................... 5

3 The Harmful Effects of Acid Precipitation ............................................. 6

3.1 Effects on Aquatic Systems ...................................................... 6

3.2 Effects on Forests and Other Terrestrial Ecosystems ........................ 7

3.3 Effects on Man-Made Structures and Materials .............................. 8

3.4 Effects on Human Health ......................................................... 9

4 Global Implications of the Acid Precipitation Problem ................................ 10

4.1 Air Pollution Migration ........................................................... 10

4.2 The International Challenge ........................................................... 11

5 Conclusion .................................................................................... 11

6 References .................................................................................... 13

Acid Precipitation: Causes, Effects, and Implications

1 Introduction

Air pollution is a well-recognized environmental problem throughout the world.

Studies have shown, time and time again, that the environmental and health-based impacts of air pollution are both detrimental and undeniable. One of the most important forms of air pollution for our society as a whole to recognize and deal with in an expedient manner is acid precipitation. Because of the very nature of this type of air pollution, the direct and indirect effects of acid precipitation are both far-reaching and difficult to get under control. Understanding more about the causes, effects, and implications of acid precipitation and its impacts on our daily lives is the first step that we all need to take in adopting a more proactive approach to solving this problem.

2 What is Acid Precipitation?

2.1 Definition. According to Sayala (2001), the definition of acid precipitation is "any rainfall/precipitation that has an acidity level beyond what is expected in non-polluted rainfall [or] any precipitation that has a pH value of less than 5.6" (p. 6). This is because theoretically, a pH of 5.6 is representative of the naturally-occurring atmospheric concentrations of carbon dioxide normally present in "pure" rainfall; nearly all rainfall is acidic to some degree because of an imbalance between hydrogen and hydroxyl ions, but a pH of 5.6 appears to be the benchmark in the definition of acid rain, give or take (Schnabel, Lamb, Pionke, & Genito, 2000, p.1). It is important to note, however, that rainfall is only one of the physical forms that acid precipitation can take. Acids can be deposited in our environment in the form of fog, snow, mist, dew, and even general particulate fallout carried on prevailing winds; in other words, acids in the atmosphere can be deposited in the form of both wet acidic solutions and dry acidic particles in the air (Cunningham & Saigo, 1999, chap. 18, pp. 398-399). In general, acid deposition in aqueous form is what the term acid precipitation refers to, and acid deposition in dry or gaseous form is what the term dry deposition refers to (Manahan, 2000, chap. 14, p. 413). This paper will deal primarily with the aqueous form of acid deposition.

2.2 Common pollutants and reactions involved. There are several pollutants that can contribute to the formation of acid precipitation. These include sulfur dioxide, nitrogen oxides, chlorine, hydrogen chloride, hydrogen sulfide, carbonyl sulfide, and carbon dioxide (Sayala, 2001, p. 6). With regard to the distribution of these pollutants, it can be noted that "sulfates account for about two-thirds of the acid deposition in eastern North America and most of Europe, while nitrates contribute most of the remaining one-third. In urban areas...nitric acid is equal to or slightly greater than sulfuric acids in the air" (Cunningham & Saigo, 1999, chap. 18, p. 399). However, perhaps more important than the chemicals themselves are the ways in which they interact with other substances in the atmosphere and the environment in general. This concept is poignantly illustrated by Manahan (2000):

Although acid rain can originate from the direct emission of strong acids, such as HCl gas or sulfuric acid mist, most of it is a secondary air pollutant produced by the atmospheric oxidation of acid-forming gases...As the result of such reactions the chemical properties (acidity, ability to react with other substances) and physical properties (volatility, solubility) of acidic atmospheric pollutants are altered drastically. (chap. 14, p. 414)

Thus, it can be seen that secondary pollutants, so altered in physical and chemical constituency by these atmospheric processes, can play altogether different and many times very unexpected roles in the transport and deposition of acid precipitation. This reinforces Sayala's assertion that "chemical reactions such as oxidation and hydrolysis play a major role in the formation of acid rain" (p. 6). Sayala also notes that some of these reactions, such as the formation of nitric acid as a secondary pollutant in the atmosphere, can be made to occur even faster by the presence of catalysts, such as iron, manganese, ammonia, and hydrogen peroxide. However, it is important to remember that regardless of the level of acidity that these chemical reactions induce in the atmosphere, "the rate of wet deposition is...governed by a complicated set of meteorological and microscale processes working in