Two decades ago, few people had heard of acid rain. Then it became a major
political issue. But what is acid rain? Where does it come from? And, more
important, what can we do to minimise its effects?
Acid rain is not a recent discovery. A hundred years ago, Britain’s first air
pollution inspector, Robert Angus Smith, coined the phrase to describe the
polluted rain in his home town of Manchester. He realised that the air in the
city was not only filthy but also acidic, and that it was attacking
vegetation, stone and iron.
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The finding, and the phrase, were forgotten, however, until the 1960s. Then
Scandinavian scientists began to link pollution blown across the sea from
Britain and the European mainland with acidified lakes and streams and the
disappearance of fish from those waters. At first, no one could explain why
acid rain affected rivers when vast amounts of acids occurring naturally in
soils did not. Still less had they established what killed the fish.
Rain is naturally slightly acidic – it reacts with carbon dioxide in the air
to produce weak carbonic acid with a pH of about 5.6. In central Europe, rain
is far more acid with an average pH of around 4.1. Even on the western fringes
of the continent, such as Ireland and Portugal, pH averages 4.9. Rain from
individual storms can have a pH of less than 3, and water droplets in fogs may
be more acid still.
Acid rain has become an important political issue over the past 20 years,
souring relations between the polluters and the polluted: between Britain and
Norway, for instance, and the US and Canada. Air pollution can spread for
thousands of kilometres across land and seas.
Europe, the most concentrated seat of air pollution in the world, still shows
the worst signs of acid damage, but there is evidence of damage from Australia
to Mexico to China. Acid rain is now a global phenomenon. But for many years
the science of acid rain was confused.
Now, the key debate about soil chemistry and the transfer of acid from rain to
rivers is largely over. Research into the atmospheric chemistry behind the
formation of acid rain has yielded important results. But the processes behind
the destruction of Europe’s forests – by far the most serious effect so far
attributed to acid rain – are still far from clear.
In the clouds
A cocktail of chemicals
One of the chief culprits in acid rain is sulphur dioxide, which reaches the
atmosphere by many routes. Sea spray, rotting vegetation, plankton and, in
some places, volcanoes are important natural sources. Round the world, perhaps
30 per cent of the sulphur in the air comes from such sources and 70 per cent
from burning fossil fuels. Over Europe, the proportion from burning fuel is
about 90 per cent.
Once sulphur dioxide (SO2) reaches the atmosphere, it oxidises in
the air, reacting with the hydroxyl ion and leading to the eventual formation
of sulphuric acid (H2SO4).
Not all the sulphur dioxide, however, becomes sulphuric acid. The process of
oxidation can happen in dry air as a “gas phase” reaction but it is usually
slow. A large mass of sulphur dioxide, in the plume from a power-station
chimney for instance, can travel hundreds of kilometres in stable air with
little sign of conversion to acid. Over much of Europe, most of the sulphur
dioxide is absorbed onto vegetation or building materials unconverted.
When sulphur dioxide is incorporated into clouds, however, oxidation by “wet
phase” reactions seems to happen much more quickly. Researchers tracking
plumes from power stations across the cloud-covered Pennines, found most of
the sulphur dioxide converted to acid within a couple of hours. Droplets in
clouds are typically 10 times more acid than the rain that falls from those
clouds.
There are grounds for believing that the whole atmosphere over Europe and the
US is chemically much more reactive than a few decades ago, and that this
accelerates the acidification. In heavily polluted urban air, tiny particles
of metals such as iron and manganese catalyse the reaction. Elsewhere, gases
such as ozone and hydrogen peroxide appear to be critical. One unexpected
finding is that ammonia, given off in large quantities from open slurry tanks
holding wastes from factory farms, is an important catalyst, at least at a
local level.
There are now strict controls on the emission of sulphur dioxide from power
stations in the US and most European countries. There, the rising stars of air
pollution are nitrogen oxides, which are producing acid rain on a scale often
exceeding that of sulphur dioxide.
Nitrogen oxides are given off by power stations (again) and car exhausts, and
include both nitrogen dioxide (NO2) and nitric oxide (NO). In the
air, nitric oxide quickly converts to nitrogen dioxide, which is itself
oxidised and changed into nitric acid (HNO3).
In the soil
Chemicals on the move
MANY of the thin soils that cover granite or sandy rock in northern Europe,
Canada and parts of the US have been acidic ever since they were formed 10000
years ago at the end of the last ice age. Unable to neutralise acids that
arise from natural processes, many of these soils contain the equivalent of
thousands of years of deposition.
When the Scandinavians first claimed that pollution from Britain was causing
acid lakes, critics pointed to this. It was areas with these soils that had
many of the most acid lakes and rivers so, they asked, who needs acid to
explain what is going on?
This theory weakened in the early 1980s when scientists reconstructed detailed
histories of acid lakes. They looked in lake sediments for remains of tiny
organisms called diatoms. These are very sensitive to acidity. It quickly
emerged that lakes in Scandinavia, Scotland and Canada had become acid only
after the height of the Industrial Revolution in the 19th century.
The clincher was a detailed study of soil chemistry. Natural acid in soils is
dominated by carbonic and organic acids. But to transfer acidity to surface
waters that drain the soil requires mobile negative ions to bind to the acid
hydrogen ion. The carbonic and organic acids do not contain such ions.
The strong acids found in acid rain do, however. In particular, the negative
sulphate ion in sulphuric acid is mobile within the soil and efficiently
transfers acidity from soils to surface waters. The nitrate ion, too, would
behave in this way if it were not normally taken up by plants first.
In the water
The poisoning of fish
FISH notably brown trout and salmon, have disappeared from thousands of lakes
and several large rivers in southern Scandinavia since the 1950s. In 1900,
anglers caught 30 000 kilograms of salmon in the seven main rivers in southern
Norway. Since 1970, no salmon have been taken.
Many lochs in Scotland, especially in the Galloway Hills, are also fishless –
as are hundreds more in Canada and parts of the eastern US. The fish died or
failed to reproduce in acid waters, but only rarely is acidity the cause.
The deaths are usually due to poisoning by aluminium. All soils contain
massive amounts of aluminium. Normally, though, it is in insoluble form, bound
to the soil. But, just as the sulphate ion in acid rain transplants acid to
rivers, so it can “unhook” aluminium from its complex compounds and wash it
into streams. There it interferes with the operation of the fish’s gills, so
that they clog with mucus. This reduces the amount of oxygen that reaches the
blood.
The mixture of aluminium and acid in lakes and rivers has a profound effect on
freshwater ecology. Acid lakes are usually crystal clear with luscious carpets
of green algae and moss. All this green is deceptive. When algae and moss
proliferate, they change the “metabolism” of the lake. They slow down the
decomposition of the greenery, providing less energy for the life there. The
result is a changed ecosystem, with fewer species.
Canadian scientists tried dosing a lake deliberately with acid over several
years. The lake initially had a pH of 6.5. At a pH of around 6, shrimps and
minnows were the first organisms to disappear. Trout eat minnows so young
trout soon failed to appear. At 5.6, the external skeletons of crayfish
softened and were soon infested with parasites. Their eggs were overrun by fungi. Soon
there were no crayfish.
In the forest
Trees under attack
THE FIRST signs of the decline of Europe’s forests appeared in parts of the
Alps, in the mid-1970s, when fir trees started to lose their needles. Then, in
what was West Germany, the crowns of Norway spruce began to thin and needles
turn brown.
German foresters, in 1981, decided that the time had come to make their
warnings public in newspapers and magazines. After 1984, the decline
stabilised in Germany. In other countries, the forests continued to
deteriorate, with deciduous trees increasingly affected.
In 1986, a European study classified about 29 per cent of trees in the
Netherlands as moderately or severely defoliated. West Germany had 20 per cent
in this category, and Czechoslovakia and Switzerland 16 per cent. A survey of
selected sites in Britain recorded 29 per cent. Similar figures have been
calculated ever since.
What is doing the damage? In Germany, scientists pointed early on to acid
soils. Soils in parts of Sweden, Germany and Britain are known to have become
more acid in recent decades. Acid waters draining from the soils wash out
nutrients and liberate aluminium, which the roots of trees may take up.
Without essential nutrients, such as magnesium and calcium, trees starve to
death.
Sulphur dioxide also directly damages leaves and needles – it blocks the
stomata on leaves, preventing photosynthesis. Ozone derived from vehicle
exhausts reaches levels each summer that may be toxic to some trees,
especially in conjunction with sulphur dioxide. Shoots seem to develop at the
expense of roots, photosynthesis is disrupted, and chemical processes in
general upset.
The picture slowly emerging from a mass of frequently contradictory research
is one in which acidifying soils and direct attack from air pollutants may
make trees more vulnerable to assault.
Severe frosts may initiate decline. The suggestion, though, is that several
air pollutants, including ozone and sulphur dioxide, reduce the frost-
hardiness of plants. Air pollutants also encourage fungi and pests, such as
the bark beetle, to grow. The ambrosia beetle is attracted by chemicals such
as terpenes given off by trees under stress.
Ammonia, as we have seen, efficiently oxidises sulphur dioxide to create the
sulphate ion. The resulting ammonium sulphate often forms on the surface of
vegetation. Ammonia reduces frost-hardiness; ammonium sulphate, when it
reaches the soil, creates both sulphuric and nitric acid.
Some researchers believe that doses of nitrogen, in the form of nitric acid,
nitrogen oxides or ammonium compounds, reach the soils in some parts of Europe
in such quantities that they “saturate” the soils. We always assume that
nitrogen does nothing but good to plants, but in excess it may stress trees by
forcing them to grow when they are short of nutrients.
In the future
Prospects for a cleanup
BY THE beginning of the next century, the output of sulphur dioxide and
nitrogen oxides from industrialised nations will be much lower than today.
Most countries are installing chemical plants to remove the sulphur from
emissions from new power stations before they reach the atmosphere. A typical
process involves large quantities of limestone (calcium carbonate) to remove
sulphur dioxide and form gypsum (calcium sulphate). Careful design of
combustion processes inside plants will reduce the output of nitrogen oxides
from power stations.
Catalytic converters, which contain platinum catalysts, are being bolted onto
exhaust systems of vehicles to remove hydrocarbons and nitrogen oxides. They
are fitted as standard on cars in many countries, notably the US and Japan,
and reduce the formation of both acid rain and ozone. The use of cars,
however, is rising so rapidly that even stringent controls may do
little more than maintain the status quo.
Even worse is that, if all air pollution were halted tomorrow, many of the
effects outlined here would remain for decades. Some soils would retain their
man-made acidity and reservoirs of sulphur. Acid and aluminium would continue
to poison lakes and streams. A British attempt to model acidity in lochs in
Scotland predicts that even a halving of acid fallout in the hills by the year
2000 would only maintain the acidity of water at their current levels.
An agreement reached between European nations in 1994 ensured an overall
reduction of sulphur dioxide emissions of 70 per cent from 1980 levels by
early next century. Using an assessment of “critical loads” for individual
ecosystems, scientists believe they have ensured that investment in clean-up
technologies will be concentrated where the benefits will be greatest.
Even so, many ecosystems will still be receiving acid fallout well beyond
their threshold, ensuring continued deterioration and little immediate
prospect of reversing the accumulation of acid.
A history of assualt in Britain
Britain has an instructive history of assault by acid rain. Ever since the
chimney became commonplace, ejecting fumes of burning fuel into the outside
air, towns have suffered from air pollution. Smoke has been the most obvious
problem. But sulphur, too, has had a part to play. Coal contains, typically,
between 1 and 3 per cent sulphur which disappears up the chimney as sulphur
dioxide.
In December 1952, a pollution disaster hit London. A deadly “smog” – a cold,
black and sulphurous stew of air – hung over the city for almost a week,
trapped by a blanket of warm air. It was the worst of a long series of
“peasouper” smogs that had hit London. It killed about 4000 people.
Smog irritated bronchial tubes, which became flooded with mucus. People choked
to death or suffered heart attacks as they fought for breath.
At the time, doctors blamed the copious quantities of smoke and sulphur
dioxide that accumulated in the smog. Scientists now believe that the
formation of highly acid particles may have been important. One estimate puts
the pH of London smog of 1952 at 1.6, rather more acid than lemon juice.
After 1952, a public outcry led to legislation banning the burning of
smoky fuels in most towns and cities. This, coupled with the arrival of cheap
North Sea gas, the growing use of electricity and a decision by the government
to build the next generation of power stations outside urban areas, ensured
that towns became vastly cleaner places.
But the government took no specific steps to limit emissions of sulphur
dioxide from power stations burning coal and oil. Britain’s output soared. By
the mid-1970s, chimneys up to 300 metres high were putting more than 5 million
tonnes of sulphur dioxide a year into the air over Europe. Other European
countries emitted similar amounts. The result was that towns were cleaner, but
sulphur dioxide spread in increasing amounts to the remotest corners of the
continent. As later emerged, rainfall everywhere became more and more acid.
The bad side of ozone
Ozone is an enigma. In the upper atmosphere it is a “good thing”. It shields
the Earth from ultraviolet radiation. Close to the ground ozone is a hazard.
It damages plants and many materials from rubber to textiles; it hastens the
formation of acid rain, and may trigger asthma attacks and bronchitis.
Ozone is formed in sunlight by photochemical reactions between nitrogen oxides
and traces of hydrocarbons in the air. Motor vehicles produce both – over
Britain, around two-thirds of the ozone is generated by vehicle exhausts.
Power stations are among the other sources of nitrogen oxides. Hydrocarbons
come from everything from industrial solvents to the methane from ruminating
cattle and leaking North-Sea gas.
Not surprisingly, the amount of background ozone close to the ground has
roughly doubled over Europe in the past three decades.
There is a debate about how best to reduce the formation of ozone.
The amount of the fastest-reacting hydrocarbons, such as alkanes and alkenes
emitted by vehicle exhausts, probably accounts for the peak concentrations of
ozone in the summer. But there are also slow-acting hydrocarbons, such as
methane, in the atmosphere which may take years to react with nitrogen oxide
and create ozone. They are so common that ozone may best be stemmed by
controlling nitrogen oxides.
Ozone is but one of the chemicals produced by reactions between pollutants in
sunlight in industrial areas and which accelerate the formation of acid. One
consequence of this increasingly reactive chemical soup in the atmosphere is
the formation of heat haze, which is usually an aerosol, or `mist’ of
sulphates and nitrates. Such a soup, near Los Angeles, has produced a fog with
a pH of 1.7.
Further reading
Ecological Effects of Deposited Sulphur and Nitrogen Compounds (The Royal
Society) is the collected papers from a meeting at the Royal Society, and
provides one of the best overviews of scientific knowledge in this complex
subject. The text should be accessible to those with the equivalent of A-level
biology or chemistry.