Environmental Engineering Reference
In-Depth Information
Emission sources
In cloud transformation
In air transformation
Scavenging
Wet deposition
Dry deposition
Figure 9.8
Acid deposition schematic, showing transformation and deposition paths.
9.2.6
Acid Deposition
Acid deposition is popularly termed acid rain. Acid deposition is a more appropriate term because
acidic matter can be deposited on the ground not only as rain but also in other kinds of precipitation—
for example, snow, hail, and fog—and in dry form. The deposition by precipitation is called
wet
deposition; the direct impaction on land and water of acidic gaseous molecules and acidic aerosols
(particles) is called
dry
deposition. Acid deposition is a secondary pollutant, because it is a result
of transformation of primary emitted pollutants.
In the late seventeenth century, Robert Boyle recognized the presence of “nitrous and salino-
sulfurous spirits” in the air and rain around industrial cities of England. In 1853, Robert Angus
Smith, an English chemist, published a report on the chemistry of rain in and around the city of
Manchester. He later coined the term “acid rain.” In the 1960s and 1970s it was first noticed in
Scandinavia, then in the northeastern United States and southeastern Canada, that some lakes with
a very low buffering capacity (i.e., lakes with low alkalinity), were slowly being acidified with a
pH reaching as low as 5.5-6. As most aquatic organisms cannot survive in that kind of acid water,
these lakes became devoid of life. Because the acidity of rain precipitation in Europe and in North
America was measured to reach a pH value as low as 3, and frequently in the pH 4-5 range, it was
soon realized that lake acidification must be a consequence of acid rain. Lakes act like a mildly
alkaline solution in a beaker. When the solution is “titrated” over the years with acid deposition, it
becomes acidic.
6
A schematic of acid deposition is presented in Figure 9.8. Power plant, industrial, commercial,
residential, and mobile sources emit the precursors of acid deposition, namely, sulfur and nitrogen
oxides (SO
x
and NO
x
). The precursors are advected by winds and are dispersed by turbulent
diffusion. During transport in the air, the precursors react with various oxidants present in the air
and water molecules to form sulfuric and nitric acid (H
2
SO
4
and HNO
3
). The acids are deposited
on land and water in the dry and wet form. This causes deleterious environmental effects that were
listed previously in Table 9.4.
6
pH is the negative logarithm of the hydrogen ion concentration, [H
+
] moles per liter, in an aqueous solution;
that is, pH
log [H
+
]. Neutral water has a hydrogen ion concentration of 10
−
7
moles per liter; thus its pH is
7. A one-tenth molar concentration of hydrochloric acid has a hydrogen ion concentration of 10
−
1
moles per
liter; thus its pH is 1. Lemon juice has an approximate pH of 3. Carbonic acid has a pH of 5.6. Thus, raindrops
in contact with atmospheric carbon dioxide have a slightly acidic pH between 5 and 6 even without addition
of other acidic species, such as H
2
SO
4
and HNO
3
. On the other hand, raindrops in contact with atmospheric
alkaline aerosols, such as CaCO
3
, CaO, MgCO
3
, and others, may actually have a pH greater than 7.
=−
