Environmental Management: Principles and practice

Sulphur dioxide and acid deposition
Pollution from thermal power stations and industry can affect the environment at
considerable distances off-site, one impact being acidification. Acid deposition can
be in the form of snow, mist or cloud droplets (wet deposition), or as dust, aerosols
or gases, especially sulphur dioxide (SO
2
) (dry deposition). Uncontaminated
precipitation is usually slightly acid (pH above about 5.6). Acid deposition is generally
recognized when pH falls below 5.1 (Elsworth, 1984:5). (Note that pH 4 is 10,000
times more acid than pH 8.)
Acid deposition may:
♦
damage plants and animals directly;
♦
alter soil chemistry or structure;
♦
alter plant metabolism;
♦
alter metabolism or species diversity of soil micro-organisms, leading to change
in fertility or soil chemistry;
♦
damage man-made and natural structures;

---

CHAPTER ELEVEN
218
♦
mobilize compounds in soils, waste dumps and water (notably, phosphates,
heavy metals, aluminium).
The impact varies: some localities may be exposed to prevailing winds, others
get localized storms of acid rain, some receive sudden snowmelt carrying the
accumulated deposition of a whole winter. Some soils or water bodies can withstand
more acidification than others (certain soils may become more fertile): they are said
to ‘buffer’ the pollution, through alkaline material within or reaching them from
underlying basic (alkaline) rocks. In temperate and colder environments soils over
slow-weathering, non-alkaline bedrocks are more likely to be affected; in warmer
climates already acidic, aluminium-rich soils are vulnerable. Soils which receive a
dressing of ammonium-rich fertilizer may suffer acidification whether or not there is
acid deposition (Eriksson, 1989; Kennedy, 1992). Agricultural practices deserve as
much attention as causes of acidification as acid deposition has attracted.
Volcanic eruptions, sea spray, weathering of gypsum and gas emissions from
forests, grasslands and marine plankton can lead to natural acid deposition.
Anthropogenic acid deposition has been recognized in the UK since the 1850s, but
the significance was only realized after diatom analysis of lake sediments in the
1970s showed serious acidification of water bodies. In the Pennine uplands of the
UK, blanket peat’s acidification since the 1750s has damaged plants vital for continued
peat formation, notably Sphagnum spp. Land has eroded and the moorland is now
dominated by only two species: Eriophorum vaginatum and Vaccinium myrtilius
(Usher and Thompson, 1988).
During the 1960s acidification of Scandinavian water bodies was linked to acid
deposition from Europe and the UK, and in Germany die-back of conifers was noted
by the 1960s. At the 1972 UN Conference on the Human Environment in Stockholm,
concern was voiced, but was met with some scepticism. By the mid-1980s precipitation
of pH 3.0 was not uncommon in central Europe. Five years later western Europe, parts
of North America, and several other countries were suffering serious damage to acid-
sensitive plants and animals, and increased maintenance costs for infrastructure.
Gradually the problem was accepted as real. Acidification may make pollutants in soil
more mobile and hazardous, and cause aquatic systems to suffer mercury methylation
(release of harmful levels of mercury from sediment or bedrock).
Most years volcanoes vent less SO
2
than the UK’s power stations did in 1987,
but some eruptions release huge amounts, and affect winter temperatures for a few
years. Human SO
2
emissions have more significance in terms of acid deposition
than climatic cooling. Elsworth (1984:6) suggested that roughly 70 per cent of acid
deposition was due to SO
2
pollution (much produced by combustion of coal), and
roughly 30 per cent due to nitrogen compounds—nitrogen dioxide (NO
2
) and nitric
oxide (N
2
O) mainly. Greenland ice cores show a two- to threefold increase in sulphate
and nitrate deposition during the last ca. 100 years, mainly attributable to acid
deposition. By 1988 about half of the sulphur in the Earth’s atmosphere could be
attributed to human activity. The distribution is not uniform: over Europe the
anthropogenic component would probably have been about 85 per cent and over the
USA about 90 per cent (Rodhe and Herrera, 1988:11).

---

POLLUTION AND WASTE MANAGEMENT
219
Even in regions which generate little pollution, wildlife, agriculture and
buildings can suffer acid deposition—an infringement of the polluter-pays principle.
Until recently, acid deposition was a problem for Europe and northeastern America.
It is now spreading because of increasing combustion of coal by industries in
developing countries (Park, 1987:xii; Rodhe et al., 1992).
Northern polar regions receive acid deposition mainly in the spring (visible as
atmospheric ‘Arctic haze’ and as soot particles in the snow), along with aerosols,
dust, pesticides, heavy metals and radioactivity (Heintzenberg, 1989). The sources
are Eurasia, Europe and North America. There is concern that this haze will trap
solar radiation and warm the Arctic enough to cause problems. Another difficulty is
that slow-growing Arctic lichen and mosses may accumulate pollution and die, or
grazing animals get heavy doses of pollutants (Soroos, 1993). Tundra vegetation
appears to be vulnerable to acid deposition damage.
It is possible to map areas of vulnerable soil, vegetation and water bodies, and
to superimpose forecasts of future acid deposition. Large areas of Southeast Asia,
Asia, Africa and Latin America have soils already acidic and with high concentrations
of aluminium and other heavy metals which mobilize to damage plants if the soil is
further acidified. Upland cloud forests which intercept precipitation are vulnerable
to acidification, as are epiphytic plants and acidic tropical rivers.
By the time there are obvious signs of acid deposition there will have been
damage to sensitive ecosystems. How acid deposition damages vegetation can be
difficult to unravel and impacts vary even from plant to plant of the same species.
Plants may not be damaged directly: it may be that symbiotic bacteria or fungi and
other soil micro-organisms are affected and a plant then has less support in its quest
for nutrients or resistance to disease and pests. Vulnerability of trees may be affected
by position, altitude, soil, moisture availability, etc. (Park, 1987:110).
Conifers suffer first, possibly because they often grow in exposed positions
and trap pollution effectively so are under stress and vulnerable. Broad-leaved
trees appear less susceptible, although in Europe and North America they are
increasingly-showing die-back. In Europe beech (Fraxinus spp.) and oaks (Quercus
spp.) are generally the first broad-leaved trees to show damage. The process can
be slow, taking up to 40 years, so is probably under way in many areas without
having become manifest. In 1988 the cost of acid deposition to Scottish foresters
was estimated to be roughly UK£25 million (Milne, 1988:56). By the mid-1980s
probably over half of Germany’s coniferous forests were showing signs of die-
back, and about 560,000 ha were ‘devastated’ (Elsworth, 1984:18). In addition to
serious forest damage and loss of fisheries, if acid precipitation is not checked,
soil may suffer which would damage agriculture and wildlife. Wellburn (1988:52)
reported that cereals and grasses might benefit from slight acid deposition (but
once levels rose above 60 ppmv SO
2
productivity fell). Some wild and crop species
suffer, including: rye, salad vegetables, barley, oats, wheat, tomatoes, apples and
pears. McCormick (1988:5) estimated the value of crop losses to acid deposition
in Europe at US$500 million a year.
With acidification environmental managers must deal with a threat that is
episodic, complex and insidious. There is a need for sensitive, effective monitoring.

---

CHAPTER ELEVEN
220
For this the presence or absence of lichen species have been used as indicators of
increased acidity. In general lichen species diversity decreases as SO
2
levels rise.