Acid Rain

We have frequently mentioned that oxides of sulfur and nitrogen entering the atmosphere from automobiles, other combustion sources, smelting, and some natural sources dissolve in raindrops and decrease the pH of the rain (Section 2.2). This activity has further effects on weathering reactions, solubility, and biological processes in lakes, rivers, and soils whose pH values are reduced as a consequence. Acidic deposition from the atmosphere may involve both wet deposition (rain, snow, fog, dew, frost) and dry deposition of acidic particulate and gaseous material on surfaces. Dry deposition is dependent on the nature of the surface and the relative humidity (there may be dissolution in a surface film of water), but is comparable to wet deposition in terms of amounts deposited. Vegetation is efficient for collecting dry deposition, which may be as damaging to leaves as the acid rain. Acidity also may come from ammonium sulfate that can be present in rain because of ammonia emitted to the atmosphere reacting with sulfuric acid; a solution of the salt of a weak base and a strong acid will be acidic. While sulfur oxides have been the major acidic component in acid rain, as major sulfur pollution sources have been cleaned up, nitrogen oxides are playing an increasing role. Typically, 60-70% of the acid comes from sulfur dioxide, and most of the rest from nitrogen oxides, but there may be a few percent HCl. Organic acids also may play a part.

The "normal" pH of rainwater in the absence of anthropogenic inputs is uncertain because natural sources of both acidic and basic materials in the atmosphere contribute. However, it is generally agreed that values significantly below 5 represent abnormal conditions, and precipitation with such values is called acid rain. The pH of precipitation over most of western and central North America averages 5 or slightly higher, while values for the eastern third of the continent are typically 4.6 to 4.4 (Figure 11-2) and sometimes less. Similar values less than 5 are found in areas of western Europe (Section 2.2).

Fog particles typically have higher concentrations of dissolved materials than raindrops because they have a higher proportion of the material that made up the condensation nuclei on which they formed. The drops will be diluted as they grow by condensation of more water vapor, although at the same time they can dissolve more gaseous pollutants. Scavenging of particulate and gaseous pollutants in clouds or as raindrops fall is referred to as washout or rainout.

Highly industrialized regions generate considerable amounts of sulfur and nitrogen oxides that are carried long distances by prevailing winds. Acid deposition in the northeastern United States and Canada has been blamed on releases in the industrialized Midwest and is believed to be responsible for acidification of lakes and die-off of forest trees in regions where they are already under stress from other causes. Loss of a large fraction of German forests is ascribed to acid rains from industry in Europe; much of northwest Europe is subject to such deposition as the prevailing winds carry pollutants from the industrialized areas to these regions. Figure 11-3 shows a correlation between forest damage and acid rain levels in Europe.

Part of the widespread distribution of the sulfur and nitrogen oxides that are the cause of the acidic precipitation comes from attempts to reduce local pollution from smelters, power plants, and other sources by employing very tall exhaust stacks. The concentration of pollutants at ground level is inversely proportional to the square of the effective stack height (which may be greater than the stack itself if the exhaust is released with appreciable upward velocity). Local effects thus can be reduced by tall stacks, but at the expense of injecting the pollutants into winds that can carry them long distances.

Ecological effects of acid deposition include disruption of species distribution and food chains in lakes as lower organisms, including plankton and benthic (sediment-dwelling) organisms, disappear, as well as through direct effects on pH-sensitive higher forms. In soils and sediments, aluminum may be solubilized, with potential toxic effects on vegetation. Heavy metals may also be solubilized from minerals, and less strongly tied up in complexes, as protons compete for the ligands or for adsorption sites; humic substances, which have complexing activity, are less soluble. Soil biota are affected. Direct effects on leaves have been reported.

FIGURE 11-2 Approximate distribution of rainwater pH in the United States, based on U.S Geolological Survey data of 1996: light gray, pH 5.2-5.6; medium gray, pH 4.7-5.1; dark gray, pH 4.6-4.8; black, pH 4.3-4.6. Actual data can be found at the agency's Website: http://btdgs.usgs. gov/jgordon/acidrain.htm.

FIGURE 11-2 Approximate distribution of rainwater pH in the United States, based on U.S Geolological Survey data of 1996: light gray, pH 5.2-5.6; medium gray, pH 4.7-5.1; dark gray, pH 4.6-4.8; black, pH 4.3-4.6. Actual data can be found at the agency's Website: http://btdgs.usgs. gov/jgordon/acidrain.htm.

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FIGURE 11-3 Acid rain and forest damage in Europe. From Database for Use in Schools Project, University of Southhampton: http://www.soton.ac.uk/~engenvir/environment/air/acid.how.big. problem.html.
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