Environmental Engineering Reference
In-Depth Information
Figure 4.2
Schematic representation of the formation, distribution and impact of acid rain
Source:
Compiled from information in Park (1987); Miller (1984); LaBastille (1981)
the relative proportions of these gases close to
the North American values (Mason 1990).
Acid precipitation produced by human
activities differs from natural acid precipitation
not only in its origins, but also in its quality.
Anthropogenically produced acid rain tends to
be many times more acidic than the natural
variety, for example. The acidity of a solution is
indicated by its hydrogen ion concentration or
pH (potential hydrogen); the lower the pH, the
higher the acidity. A chemically neutral solution,
such as distilled water, has a value of 7, with
increasingly alkaline solutions ranging from 7 to
14, and increasingly acidic solutions ranging from
7 down to 0 (see Figure 4.1). Since this pH scale
is logarithmic, a change of one point represents
a tenfold increase or decrease in the hydrogen
ion concentration, while a two-point change
represents a one hundredfold increase or
decrease. A solution with a pH of 4.0 is ten times
more acidic than one of pH 5.0; a solution of
pH 3.0 is one hundred times more acidic than
one of pH 5.0.
The difference between 'normal' and 'acid'
rain is commonly of the order of 1.0 to 1.5 points.
In North America, for example, naturally acid
rain has a pH of about 5.6, whereas
measurements of rain falling in southern Ontario,
Canada, frequently provide values in the range
of 4.5 to 4.0 (Ontario: Ministry of the
Environment 1980). To put these values in
perspective it should be noted that vinegar has a
pH of 2.7 and milk a pH of 6.6 (see Figure 4.1).
Thus, Ontario rain is about 100 times more acidic
than milk, but 100 times less acidic than vinegar.
Similar values for background levels of acidic rain
are indicated by studies in Europe. The Central
Electricity Generating Board (CEGB) in Britain
has argued for pH 5.0 as the normal level for
naturally acid rain (Park 1987), but the average
