Geoscience Reference
In-Depth Information
Acid rain
HUMAN IMPACT
Climatology has a number of misleading terms. We have already mentioned that the greenhouse effect of the
atmosphere should really be called the 'enhanced greenhouse effect' because it supplements the natural processes
operating. Similarly the term acid rain is used to indicate precipitation which is more acidic than normal - but even
pure rainfall is acidic, with a pH of about 5
ยท
6. Neutral water would have a pH of 7.
Pure rainwater is slightly acidic because it absorbs some of the carbon dioxide from the atmosphere to form dilute
carbonic acid. Levels of pollutants in the atmosphere have increased greatly as a result of human activities, particularly
the burning of fossil fuels. Large quantities of sulphur and nitrogen oxides are added to the atmosphere (
Figure 5.2
).
These gases react with water vapour and sunlight to produce nitrates and sulphates. Some of the pollutants are
deposited directly from the atmosphere as dry deposition and some are absorbed into the precipitation process and
reach the ground as rain or wet deposition. It is the wet deposition which is measured as acid rain, with pH values
as low as 2, but both processes add acidity to the ground surface.
The source of much of this pollution is industry and urban areas. However, it is not just urban areas which receive
acid rain. The gases released are dispersed by the winds, and levels of high rainfall acidity extend over considerable
areas. Over Europe the highest levels of acidity are to the centre and east, with mean values below 4. It is believed
that heavy industry and lignite burning are the main contributors to this peak. As the prevailing winds in Europe are
from the west, it is not surprising that the most acid rain occurs towards the east (
Figure 5.3
).
Wind is not the only
factor in dispersal. Atmospheric stability will determine whether pollutants remain concentrated or are dispersed.
The westerly winds tend to be relatively unstable and so allow greater vertical mixing; south-easterly winds, though
less common, are often relatively stable, and pollution concentration can remain high. Much of Scandinavia's
pollution is brought from central Europe in this way. Pollution which may start as a local problem can cross national
boundaries to become a regional issue.
The effect of this increased acidity has been debated. It has been argued that the biosphere, human health and
building materials can suffer from its effects. What does seem clear is that the ecological effects of acid rain will
depend upon the ability of an ecosystem to neutralize the incoming acid. This ability is known as its buffering capacity
and it depends largely upon the amount of calcium or magnesium in the bedrock. Levels of these minerals are generally
low in much of the recently glaciated areas of northern Europe, so lakes in that area have been particularly affected
by acid rain. Fish stocks have dropped dramatically, but other aquatic organisms have been affected. The causes are
complex. One factor is believed to be the release of aluminium as a result of acid water reacting with heavy metal
cations in the soil. Aluminium can affect fish by obstructing their gills.
The other area of impact of acid rain is on vegetation. Acid rain will increase soil acidity, decrease nutrient availability,
mobilize toxic metals like aluminium and affect micro-organisms. It is not surprising that it has been held responsible
for many changes in the terrestrial ecosystem. Perhaps the most drastic effect has been on forests. Monitoring of
forests in Europe since 1986 has shown evidence of increasing damage, especially to deciduous species. The damage
takes the form of thinning of the crown, the shedding of leaves or needles and decreased resistance to disease, drought
and frost. Pollution is not the only factor involved but is believed to be important. Many central European forests have
shown damage. Norwegian forests have shown an improvement in vitality following a decline in the 1990s.
Efforts to combat the problem can be made at source or in the environment affected. Some attempts have been
made to reduce sulphur emissions at power stations, such as at Drax in the United Kingdom. Unfortunately they
cost money, which makes the electricity more expensive to generate. Greater use is being made of natural gas as
a source of energy, though this has more to do with economics than with environmental considerations. Alternatively
soil or water acidity can be neutralized by adding lime. It has been argued that it is cheaper to add lime than to adopt
expensive systems of reducing sulphur emission. Some success has been achieved in Scandinavia, where over 3,000
lakes have been limed. Reduced acidity is followed by the recovery of the biota; lower organisms reappear first,
succeeded by amphibians and fish. Further liming has to take place every few years as long as the acid rain input
continues.
