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What goes up
The atmosphere acts as a mixing pot for a host of substances. Air pollution has
made rain more acidic in many areas. This first caused concern in the 1960's.
Acid rain has detrimental effects on - lakes and streams with their aquatic populations,
forestation, contamination of ground water and corrosion of man-made structures.
It is also a contributing factor to the deterioration of coastal waters.
Acid rain describes two
kinds of deposition, dry and wet. Dry deposition is the fall of acid dust particulates.
This usually occurs near to the source of the emission as direct particulate fallout
from industrial stacks and ventilation systems. Exhaust fumes from vehicles also
contain heavy particles.
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Wet deposition comes as
rain, mist, cloud cover, snow and hail. Tiny droplets of water make up clouds.
They continuously capture suspended particles and soluble gases. Sulphur dioxide
and oxides of nitrogen are converted in the cloud droplets as sulphuric acid and
nitric acid. Carbon particles in smoke help this process. They adsorb - not absorb,
these chemicals onto their particles. This makes a little mixing pan bringing
the oxides together with water to produce the acids.
Nitric oxide converts to
nitric acid and ozone. The nitric acid falls as acid rain and together with the
ozone attacks foliage. The quantities of nitric oxides are increasing owing to
increased car use.
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Pure water is defined as
neutral with a ph of 7. ph stands for the "power of h", or the amount
of h+ ions. The ph scale is between 1 and 14. Blood is slightly alkaline ph 7.5
and ammonia is very alkaline ph 12. Milk is slightly acidic ph 6.5, tomato juice
is slightly more acidic, vinegar is even more acidic ph 3, and lemon juice still
more acidic ph 2. With no pollution at all, normal rainwater is on the acid side
of this scale, not the alkaline side. Normal rainwater is less acidic than tomato
juice, but more acidic than milk. With pollution, rain can be as acidic as vinegar
or lemon juice.
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Sulphur and nitrogen oxides
are also produced through natural processes such as volcanic activity and the
action of soil organisms. This is the natural cycle of minerals and elements.
however, industrial pollution in the air creates much more damaging amounts of
acid in the rain. The main sources are from power stations. Oil and coal burning
produces not only carbon dioxide. They also produce sulphur dioxide and oxides
of nitrogen, depending on the purities of the original materials.
Must come down
Acid rain may fall a long way from its source. It does not respect national boundaries.
Emissions from the UK have a direct effect on the forests of other EU countries.
The UK contributes 26% of the acidic sulphur deposited in the Netherlands, 23%
in Norway and 12% in Sweden.
The higher the altitude the more susceptible vegetation becomes to acid rain.
Soil and vegetation enveloped in acidic clouds and mists are directly exposed
to the extreme acidic cloud-base. hence many forested areas in high altitude regions
suffer to a greater extent than lowland regions.
Forests in the EU, particularly those in Germany, Norway and Sweden have suffered
serious decline beyond from stress conditions caused by naturally occurring drought
or severe storms. In Germany this has been called "Waldsterben" (forest
death). Coniferous forests seem to suffer most. Dead timber is the dramatic evidence
for the loss of tree vigour. There is an historical link with dead timber being
found in regions where there are chemical plants that emit sulphur dioxide, oxides
of nitrogen or fluoride compounds. The forests that are now dying are far from
sources of pollution and are exposed to smaller doses than those previously proved
to injure trees.
Acid rain may be a stress factor rather than a direct lethal cocktail. Stresses
naturally withstood by trees in isolation, such as age, disease, floods, winds,
occasional shortages of water and essential minerals may be compounded by this
extra stress factor. Acid and other pollutants add to the stress of the whole
ecosystem by contributing to thin soil, low temperatures and changes in climatic
conditions.
Acid rain also leaches out nutrients from pine needles faster than the roots can
replace them. An additional pollutant, ozone, aggravates nutrient leaching by
degrading the waxy coating of the needles. This leads to chlorophyll damage inhibiting
the action of photosynthesis. Conifers in winter go through a stage called "cold
hardening" whereby the tree withdraws water from the needles in order to
prevent freezing in winter. This is triggered by a signal that comes from the
roots in the form of decreased nitrogen supply. With the fall of acid rain adding
to the nitrogen supply the natural reduction is counteracted, and the tree fails
to "cold -harden" therefore suffering extreme stress during the winter
months, eventually leading to its death. The increase in the nitrogen supply acts
as a fertiliser for the tree and does not give the signal "it is winter".
All historic buildings suffer from damage and decay over a period of time. The
activity of acid rain and the dry deposition of pollutants add to this. Porous
stones such as limestone act as a sponge for the acid rain, SO2 penetrates the
pores of the rock and chemical reactions take place causing the rocks to change
character and crumble. Most damage to buildings occurs in built up areas where
there are more emissions from industry. Other materials affected by acid deposition
are paint, metal, marble, glass and concrete.
To Ground
The geology of the region affects whether the acidity can be neutralised. Soils
rich in limestone can directly neutralize the acid. When acid rain reaches the
earth it undergoes another cycle of changes, both physical and chemical, that
change the groundwater that enters streams and lakes.
In slightly acidic soils other processes can reduce the effects of acid deposition.
The soil and vegetation can retain some of the sulphuric and nitric acids. The
acidic properties may be "buffered", rather than neutralised. If precipitation
is acidic, acid-buffering chemicals can eventually become depleted. The buffering
effect will no longer occur, and nature's ability to maintain balance will have
been destroyed.
Where acid rain falls on frozen ground, granite or fully saturated rock, no change
occurs. It runs off directly into watercourses and lakes, causing several changes.
An acidified lake can easily be recognised. Where the ph is below 6 there is high
sulphate and an increase in aluminium, calcium and magnesium ions. Around ph 6.0,
crustaceans, insects, and some plankton species begin to disappear. At ph 5.0,
major changes in the plankton community occur and less desirable species of mosses
and plankton may begin to invade. Below ph 5.0 the water is largely devoid of
fish, the bottom is covered with undecayed material and mosses may dominate the
shore areas. Birds who feed from the lake life will suffer.
Critical loads are the levels of acidity that various environments can withstand
before suffering serious harm. Different environments can withstand different
levels. 8% of UK land will still be receiving SO2 in excess of critical loads
by the year 2005.
The need to reduce the ambient burden of nitrogen and sulphur compounds in the
atmosphere, in order to cut down the acidity of rain, has been recognised for
some years. The overall conditions in some areas improved as a result of the 'Clean
Air Acts' of 1956 and 1968 which helped to control smoke, grit and dust emissions.
The UK persisted with a "high stack" policy in the 1960's and 1970's
for power stations. This meant building taller chimneys and arguing that the air
would disperse emissions rather than reduce pollutants at source. The Central
Electricity Generating Board (CEGB) carried out a survey of the extent of corrosion
due to acid rain in the UK in the 1970s. They found the corrosion was more extensive
than originally expected. What did they do? Reduce pollutants at source? Put in
more scrubbers to clean the emissions? No, they galvanised their pylons thicker
to protect against the corrosion.
The key to controlling acid rain is the reduction of emissions from fossil fuel
powered generating plants, coal in particular, and the reduction of the use of
oil in the transport sector and using catalytic converters to reduce nitrogen
oxide emissions.
For an Acid Rain 'Hot List', go to
http://www.miamisci.org/ph/hexpand2.html
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