Acid rain
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What is acid rain?
Rain is naturally acidic, even in unpolluted areas, due to carbon dioxide
in the atmosphere which dissolves into rain water. As a consequence, rain
has a pH close to 5.4. Acid rain has a pH that is lower than normal 5.4.
Rain can become more acidic when man-made pollutants enter into the atmosphere
and become dissolved into rainwater. The oxides of sulphur (SO2) and nitrogen
(NO2) are the main source of acidifying pollutants, principally derived
from the burning of fossil fuels such as coal and oil as well as ammonia
which is mainly derived from intensive agriculture. However there are
also natural sources, such as from volcanoes, oceans, biological decay
and forest fires.
The increasing demand for electricity and the rise in the number of motor
vehicles in recent decades has meant that emissions of acidifying pollutants
have increased dramatically from human sources, particularly since the
1950s. Emissions of such pollutants are heavily concentrated in the northern
hemisphere, especially in Europe and North America. As a result, precipitation
is generally acidic in these countries.
Most of the SO2 and NO2 produced in Britain now comes from power stations
and large industry. Transport has also contributed to the problem, although,
since catalytic converters have been fitted to new cars the amount of
nitrogen oxides emitted per car has reduced. However, since the volume
of traffic in the UK has increased, levels of NO2 from this source have
not fallen significantly.
Acid rain is a problem because it:
- Can be carried over large distances by winds affecting places far
removed from its source. As lakes acidify, fish and small invertebrates
are killed.
- It dissolves nutrients in the soil, which are then leached out, making
the soil infertile and killing large numbers of trees.
- Attacks the stonework of buildings and certain metal structures, costing
millions of pounds to treat.
It is a ‘transboundary’ problem and as a result the first
international agreement, the Convention on Long Range Transboundary Air
Pollution, was agreed in 1979. Since then a range of protocols have been
agreed to cut emissions, the most recent of which set emissions targets
for sulphur dioxide, nitrogen oxides, ammonia, and volatile organic compounds,
to be met from 2010. The UK is committed to annual emissions ceilings
of 625 ktonnes for sulphur dioxide, 1,181 ktonnes for nitrogen oxides
and 297 ktonnes for ammonia.
Since the UK signed up to protocols on sulphur dioxide reduction, levels
are now substancially lower than they were a decade ago. More details
about what is being done to combat acid rain is available from Defra.
Acid rain in the South West
The incidence of acid rain is less common in the South West than in areas
of high man-made emissions, such as the Midlands. However, due to high
rainfall and naturally acidic granite and peat land, the region’s
upland areas such as Dartmoor, Exmoor and Bodmin can be more susceptible.
Narrator Brook,
an upland stream which drains into Burrator Reservoir on Dartmoor, is
one of the 22 UK acid monitoring network sites and the only one in the
South West. Measurements of water quality in this stream have showed a
reduction of acidity between 2003 and 2004 from pH 5.71 between 1991 –
1996 to pH 6.05 (UK
Acid Waters Monitoring Network, 2005)
National perspective
The by-products of burning fossil fuels such as coal and oil form the
largest source of man-made acidity in the atmosphere. The proportion of
man-made and natural acidity in rainwater is variable throughout Britain.
In the west of Scotland it is estimated that less than half of the acidity
is man-made, while in England up to 90% may be as a result of pollution.
Since 1980, sulphur dioxide emissions in the UK have decreased by 75%,
leading to 78% less dry deposition and 61% less wet deposition.
The
Future of Britain's Upland Waters is a recent study by the UK
Acid Waters Monitoring Network for Defra. This report has shown that
acidic sulphur in Britain's water has generally halved in the last 15
years, with invertebrates, juvenile brown trout, acid sensitive mosses
and native algae showing signs of recovery in around half of 22 sites
monitored.
The Forestry
Commission has conducted detailed research into the impacts of acid
rain on trees and woodland in the UK.
European and international
perspective (where available)
The 2005
State of the European Environment Since the 1980s acid rain has been
significantly reduced in Europe, peaking in the late 1970s and falling
by two-thirds since then.
Overall, emissions of acidifying gases have decreased across Europe by
more than 40% in the EU-15 (the 15 members states of the European Union
in the period prior to enlargement in 2004)
and almost 60 % in the EU-10 (the 10 newest members), and by more than
half in industry and power generation.
However, although many ecosystems throughout Europe are receiving acid
depositions below their critical load, around 10 % are still subject to
deposition above critical loads in 2004. This includes 18 % of forests
in the EU-15 and 35 % of forests in the EU-10.
Action to halt acid rain
More details about what is being done to combat acid rain is available
from Defra.
The Freshwater Umbrella research
programme is looking into the recovery of aquatic ecosystems from
the effects of acidification. Funded by DEFRA and the devolved agencies,
this programme has been running since 1990. It undertakes 3-year applied
science programmes to develop the scientific background to aid DEFRA make
policy decisions concerned with air pollution effects on freshwater systems
in the UK. The main focus of the current Freshwater Umbrella research
programme is the role of nitrogen and its effects on freshwater ecosystems
both as a eutrophier and through the leaching of nitrate from catchment
soils.
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