Environmental Engineering Reference
In-Depth Information
annual pH of rain over Britain between 1978
and 1980 was between 4.5 and 4.2 (Mason
1990). Remarkably high levels of acidity have
been recorded on a number of occasions on both
sides of the Atlantic. In April 1974, for example,
rain falling at Pitlochry, Scotland had a pH
measured at 2.4 (Last and Nicholson 1982), and
a value of 2.7 was reported from western Norway
a few weeks later (Sage 1980). At Dorset, north
of Toronto, Ontario, snow with a pH of 2.97
fell in the winter of 1976-77 (Howard and Perley
1991), and an extreme value of pH 1.5, some
11,000 times more acid than normal, was
recorded for rain falling in West Virginia in 1979
(LaBastille 1981). Although these values are
exceptional, the pattern is not. Acid deposition
tends to be episodic, with a large proportion of
the acidity at a particular site arriving in only a
few days of heavy precipitation (Last 1989).
Annual averages also mask large seasonal
variations. Summer precipitation, for example,
is often more acid than that in the winter,
although emissions are generally less in the
summer.
The quality of the rain is determined by a
series of chemical processes set in motion when
acidic materials are released into the
atmosphere. Some of the SO
2
and NO
X
emitted
will return to the surface quite quickly, and
close to their source, as dry deposition. The
remainder will be carried up into the
atmosphere, to be converted into sulphuric and
nitric acid, which will eventually return to
earth as acid rain (see Figure 4.2). The
processes involved are fundamentally simple.
Oxidation converts the gases into acids, in
either a gas or liquid phase reaction. The latter
is more effective. The conversion of SO
2
into
sulphuric acid in the gas phase is 16 per cent
per hour in summer and 3 per cent per hour in
winter. Equivalent conversion rates in the
liquid phase are 100 per cent per hour in
summer and 20 per cent per hour in winter
(Mason 1990). Despite the relatively slow
conversion to acid in the gas phase, it is the
main source of acid rain when clouds and rain
are absent, or when humidity is low.
The rate at which the chemical reactions take
place will also depend upon such variables as the
concentration of heavy metals in the airborne
particulate matter, the presence of ammonia and
the intensity of sunlight. Airborne particles of
manganese and iron, for example, act as catalysts
to speed up the conversion of SO
2
to sulphuric
acid and sulphates. Natural ammonia may have
similar effects (Ontario: Ministry of the
Environment 1980). Sunshine provides the energy
for the production of photo-oxidants—such as
ozone (O
3
), hydrogen peroxide (H
2
O
2
) and the
hydroxyl radical (OH)—from other pollutants
in the atmosphere, and these oxygen-rich
compounds facilitate the oxidation of SO
2
and
the NO
X
to sulphuric and nitric acid respectively
(Cocks and Kallend 1988). The role of the
photochemical component in the conversion
process may account for the greater acidity of
summer rainfall in many areas (Mason 1990).
In the presence of water, these acids, and the other
chemicals in the atmosphere, will dissociate into
positively or negatively charged particles called
ions. For example, sulphuric acid in solution is a
mixture of positively charged hydrogen ions
(cations) and negatively charged sulphate ions
(anions). It is these solutions, or 'cocktails of
ions', as Park (1987) calls them, that constitute
acid rain.
Whatever the complexities involved in the
formation of acid rain, the time scale is crucial.
The longer the original emissions remain in the
atmosphere, the more likely it is that the reactions
will be completed, and the sulphuric and nitric
acids produced. Long Range Transportation of
Atmospheric Pollution (LRTAP)—transportation
in excess of 500 km—is one of the mechanisms
by which this is accomplished.
Air pollution remained mainly a local problem
in the past. The effects were greatest in the
immediate vicinity of the sources, and much of
the effort of environmental groups in the 1960s
and 1970s was expended in attempts to change
that situation. Unfortunately, some of the changes
inadvertently contributed to the problem of acid
rain. One such was the tall stacks policy. In an
attempt to achieve the reduction in ground level
