Acid Deposition - Air Pollution and Global Warming: History, Science, and Solutions - page 224

Throughout the twentieth century, acid deposition
problems continued to plague cities in the UK and
municipalities in other countries where coal and chem-
ical burning occurred. The fatal pollution episodes in
London, discussed in Chapter 4, included contributions
from acidic compounds in smoke.
Acid deposition became an issue of international
interest in the 1950s and 1960s, when a relationship was
found between sulfur emissions in continental Europe
and acidification of Scandinavian lakes. These stud-
ies led to a 1972 United Nations Conference on the
Human Environment in Stockholm that called for an
international effort to reduce acidification. Since then,
numerous studies on acid deposition have been carried
out, and, as a result of these studies, national gov-
ernments have intervened to control acid deposition
problems. Such efforts are discussed at the end of this
chapter. First, the basic science of acid deposition is
discussed.

10.2. Causes of Acidity
In Chapter 5, pH was defined as

log 10 [H + ] pH =− (10.3)

Figure 10.2. Robert Angus Smith (1817-1884).
Reproduced courtesy of the Library and Information
Centre, Royal Society of Chemistry.

where [H + ]isthe molarity (mol L − 1 )ofH + in a solu-
tion containing a solvent and one or more solutes. The
pH scale, shown in Figure 10.3 for a limited range,
varies from less than 0 (a lot of H + and very acidic) to
greater than 14 (very little H + and very basic or alka-
line). Neutral pH, which is the pH of distilled water, is
7.0. At this pH, the molarity of H + is 10 − 7 mol L − 1 .A
pH of 4 means that the molarity of H + is 10 − 4 mol L − 1 .
Thus, a pH of 4 is 1,000 times more acidic (contains
1,000 times more H + ions) than is water at a pH of 7. A
pH of 2 corresponds to an H +

The features of the place are chimneys, furnaces, steam
jets, smoke clouds and coal mines. (Blatchford, 1899,
p. 15)

The first British Alkali Act inspector in the UK was
Scottish chemist Robert Angus Smith (1817-1884;
Figure 10.2). He was charged with ensuring that the
industry reduced HCl(g) emissions by 95 percent.
Smith was also a field experimentalist. In 1872, he
published Air and Rain: The Beginnings of a Chemical
Climatology ,inwhich he discussed results of the first
monitoring network for air pollution in Great Britain.
As part of the analysis, he recorded the gas-phase mix-
ing ratios of molecular oxygen and carbon dioxide, and
measured the composition of chloride, sulfate, nitrate,
and ammonium in rainwater in the British Isles. In his
book, Smith introduced the term acid rain to describe
the high sulfate concentrations in rain near coal-burning
facilities.

molarity 100,000 times

that of distilled water.
The quantity of H + is related to that of OH − by the
equilibrium relationship,

H +
Hydrogen
ion

OH −
Hydroxide
ion

H 2 O(aq)
Liquid
water

+ (10.4)

The equilibrium constant for this relationship is approx-
imately 10 − 14 mol 2 L − 2 ,meaning that the product of
[H + ] and [OH − ]must always equal approximately
10 − 14 mol 2 L − 2

for water to be in equilibrium with

H + and OH − .