Page 184 - Fundamentals of Air Pollution

150 10. Effects on the Atmosphere, Soil, and Water Bodies

In the simplest case, CO 2 dissolves in raindrops, forming carbonic acid.
At a temperature of 20°C, the raindrops have a pH of 5.6, the value often
labeled as that of clean or natural rainwater. It represents the baseline for
comparing the pH of rainwater which may be altered by SO 2 or NO X oxida-
tion products. Figure 10-11 illustrates the pH scale with the pH of common
items and the pH range observed in rainwater. The pH of rainwater can
vary from 5.6 due to the presence of H 2SO 4 and HNO 3 dissolved or formed
in the droplets. These strong acids dissociate and release hydrogen ions,
resulting in more acidic droplets. Basic compounds can also influence the
+
2+
2
pH. Calcium (Ca ), magnesium (Mg ^, and ammonium (NH 4 ) ions help
+
to neutralize the rain droplet and shift the overall H toward the basic end
of the scale. The overall pH of any given droplet is a combination of the
effects of carbonic acid, sulfuric and nitric acids, and any neutralizers such
as ammonia.
The principal elements of acidic deposition are shown in Fig. 10-12. Dry
deposition occurs when it is not raining. Gaseous SO 2, NO 2, and HNO 3
and acid aerosols are deposited when they come in contact with and stick
to the surfaces of water bodies, vegetation, soil, and other materials. If the
surfaces are moist or liquid, the gases can go directly into solution; the
acids formed are identical to those that fall in the form of acid rain. SO 2
and NO 2 can undergo oxidation, forming acids in the liquid surfaces if
oxidizers are present. During cloud formation, when rain droplets are
created, fine particles or acid droplets can act as seed nuclei for water to
condense. This is one process by which sulfuric acid is incorporated in the
droplets. While the droplets are in the cloud, additional gaseous SO 2 and
NO 2 impinge on them and are absorbed. These absorbed gases can be
oxidized by dissolved H 2O 2 or other oxidizers, lowering the pH of the
raindrop. As the raindrop falls beneath the cloud, additional acidic gases
and aerosol particles may be incorporated in it, also affecting its pH.
The United States has established a National Acid Deposition Program
(NADP), and Canada has established the CANSAP program, which consists

Fig. 10-11. The pH scale is a measure of hydrogen ion concentration. The pH of common
substances is shown with various values along the scale. The Adirondack Lakes are located
in the state of New York and are considered to be receptors of acidic deposition. Source:
U.S. Environmental Protection Agency, "Acid Rain—Research Summary," EPA-600/8-79-028,
Cincinnati, 1979.