Precipitation Chemistry and Acid Rain

Before we can know that precipitation has been subject to pollution, we must know what its natural composition is. This will depend on a variety of natural contributions and effects that are discussed in this chapter. Let us first examine the question What is the pH of natural rain To answer this question we will follow a method that applies to calculations of equilibria in natural waters, generally. The first question we ask is what information is given or assumed In this case we will assume 25°C, pure water, and an atmospheric CO2 pressure of co, jq-3.5 jjjg second question to ask is what chemical equilibria apply to the problem In this case the equilibria are 

We can combine these equilibria expressions, eliminating H2CO3, and solving for [H+], to obtain  

Third is the charge-balance equation, which describes the fact that the total of the equivalents (eq) or milliequivalents (meq) of cations in a given volume or weight of water must equal the same sum for the anions. The charge-balance equation in this example is 

In general problems in solution chemistry there is also a fourth kind of equation called a mass-balance equation. Mass-balance expressions relate the total concentration of a species reported in the chemical analysis to concentrations of the several forms of that species in solution. For example, 

It turns out, however, that we will not need this or any other mass-balance equation to solve our rain pH problem. For rain, we know the pH is generally below about 6, so that mOH and mCO are negligible relative to mH and mHCOj. As a rule of thumb, any species whose concentration is less than 1% (two orders of magnitude) of another species concentration present is negligible in a charge-balance or mass-balance calculation. (How do we know these two species are negligible at this pH ) With this simplification, and ignoring activity coefficients in such a dilute water, we find expression (8.8) reduces to simply mH = mHCOj. 

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Water chemistry Fate of inorganic and organic pollutants in natural waters Analytical chemistry of natural waters and trace contaminants Trace metal-particulate matter interactions Structure-activity relationships for organic compounds Aquatic colloid chemistry Precipitation chemistry/acid rain... 

Shown in Fig. 9.9 are water-composition ranges for some humid-climate streams (in New Jersey), a dilute, freshwater lake (Lake Huron) and lake-bottom muds from the Great Lakes (Sutherland 1970), and deep-soil moisture from Pennsylvania (Sears 1976 Sears and Langmuir 1982). Lake Huron and the Delaware River are dilute, humid-climate waters. They both plot near the kaolinite-gibbsite boundary. Their composition can be described as water dominated. In other words, their chemistries are controlled chiefly by dilution with fresh rainfall and runoff, not by reactions with geological materials. In a study of acid rain (water-dominated) control of soil moisture and ground-water chemistry of a sandy aquifer in Denmark, Hansen and Postma (1995) found that pore waters were close to equilibrium with gibbsite and supersaturated with kaolinite (Fig. 9.9). Precipitation pH = 4.34 at the site, and log([K+]/lH+]) = -0.95. 

After Japan, the most extensive chemistry data set in the Asian region is for China. The Institute of Environmental Chemistry initiated a survey of precipitation chemistry in some cities in the late 1970s, and nationwide surveys have been reported since 1982. The results of these studies show that acid rain occurs in many parts of China, especially in the southwest (Zhao and Sun, 1986 Galloway et al, 1987 Zhao and Xiong, 1988, Naritaetal, 1997). 

Although the annual mean pHs do not differ sitewise, pH of individual precipitation showed a very large fluctuation, ranging from 3.4. to 8.0. Acidic precipitation appeared episodically about 4% of the precipitation events showed pH of less than 4.0. About 70% of the precipitation had a pH below 5.6 and can be considered as acid rains. On the other hand, about 2% of the precipitation events had pH above 7.0, suggesting strong inputs of alkaline species to rainwater in this region. The input of Yellow sand is one of the important alkaline sources that effect the rainwater chemistry over South Korea. 

The effect of industrial society on the environment is especially apparent in the problem of acid rain the underlying chemistry applies several principles of ionic equilibria. Acidic precipitation—rain, snow, fog, or dry deposits on particles—has been recorded in all parts of North America, the Amazon basin, Europe, including Russia, much of Asia, and even at the North and South Poles. Three major substances are involved ... 

Figure 19.12 Formation of acidic precipitation. A complex interplay of human activities, atmospheric chemistry, and environmental distribution leads to acidic precipitation and its harmful effects. Car exhaust and electrical utility waste gases contain lower oxides of nitrogen and sulfur. These are oxidized in the atmosphere by O2 (or O3, not shown) to higher oxides (NO2, SO3), which react with moisture to form acidic rain, snow, and fog. In contact with acidic precipitation, many lakes become acidified, whereas limestone-bounded lakes form a carbonate buffer that prevents acidification.

Chemistry is for a large part conducted in solutions involving ions and such solutions are ubiquitous in nature. Oceans are vast aqueous solutions of salts, consisting mainly of sodium chloride, but other salts and minor components are also present in ocean water. Lakes, rivers, and brackish water are dilute solutions of ions and are essential to survival, since they provide drinking water and water for irrigation. Rain and other precipitates may remove ionic species from the atmosphere that arrived there as spray from oceans and seas or from human activities, for example, acid rain. Physiological fluids consist mostly of water in which colloidal substances, but also ions essential to their function, are dissolved. 

FIGU RE 1.1 Illustration of the definition of environmental chemistry exemplified by the life cycle of a typical pollutant, sulfur dioxide. Sulfur present in fuel, almost always coal, is oxidized to gaseous sulfur dioxide, which is emitted to the atmosphere with stack gas. Sulfur dioxide is an air pollutant that may affect human respiration and may be phytotoxic (toxic to plants). Of greater importance is the oxidation of sulfur dioxide in the atmosphere to sulfuric acid, the main ingredient of acid rain. Acidic precipitation may adversely affect plants, materials, and water, where excessive acidity may kill fish. Eventually, the sulfuric acid or sulfate salts end up in water or in soil. 

As precipitation chemistry is intimately related to atmospheric composition, air quality becomes a starting point to understand the acidity of rain water. An evaluation of atmospheric SQi content in Brazilian natural, industrial and urban areas is presented in Table 1. 

Gertler, A. W., D. F. Miller, D. Lamb, and U. Katz, Studies of Sulfur Dioxide and Nitrogen Dioxide Reactions in Haze and Cloud, in Chemistry of Particles, Fogs, and Rain (J. L. Durham, Ed.), Acid Precipitation Series, Vol. 2, pp. 131-160 (J. I. Teasley, Series Ed.), Butterworth, Stoneham, MA, 1984. 

The chemistry of rain acidified by the sulfur oxides is complicated because the sulfur may be deposited in different forms. It may either precipitate as H2S03(aq) or it may be first oxidized to S03(g) and precipitate as H2S04(aq). Deposition may occur either in the aqueous form (wet deposition) or in association with particulates (dry deposition), in which case much more of the sulfur will deposit in the form of sulfite or sulfate ions than the free acid (see Chapter 3). 

Laboratory chemical analysis of particulate material, following the collection of samples, typically includes a selection of the listed parameters in Table 3. The reasons for monitoring these constituents are also listed in the table. These reasons are normally related to surveillance of certain materials which are identified with major sources, and are believed to be related to adverse health and ecological impacts. Recent efforts in rain chemistry require analysis for the species listed in Table 4. These parameters reflect interests concerning causes and sources of acidity in precipitation. Certain key trace metal constituents provide information on sources. 

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