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Acid Deposition Case Studies


Key facts on United Kingdom

59.7 million (2000)

244 014 km²

London (7.2 million population in Greater London)


Marine West Coast, January (2-7C), July (13-18C)

Land use:
Agriculture 77%, forest 10%, urban 10%

0-800m above sea level

Air Quality:
Generally good in rural areas but ozone levels may be high during summer months. High emissions of acidic pollutants in large urban areas from industry and transport lead to potentially poor air quality.

Acid Deposition:
Amongst the highest acid deposition levels in Europe, particularly in eastern England.

United Kingdom, 2001

Source: The Perry-Castañeda Library Map Collection, The General Libraries, The University of Texas at Austin, Austin, TX 78713-8916, USA

Acidic Pollutant Emissions

In the 1980s, the UK was described by Scandinavian countries as 'the dirty old man of Europe' due to high emissions of sulphur dioxide from industrial sources. Emissions of sulphur dioxide and oxides of nitrogen have since been reduced, resulting in emissions of 1.2 million tonnes of sulphur dioxide and 1.6 million tonnes of nitrogen oxides (as NO2) in the UK during 1999. However, these amounts are still considerable compared to other European countries.

Most of the UK sulphur dioxide comes from power stations (65% in 1999) and other industries (22% in 1999) whilst the largest source of nitrogen oxides is road transport (44% in 1999) and power stations (21% in 1999). Total emissions of sulphurous and nitrogen oxides in 1990 and 1999 and their reductions over this time are shown in Table 3.5.

Table 3.5: Emissions of Acidifying Air Pollutants in UK


Emissions 1990

Emissions 1999

% reduction

Sulphur '000 tonnes




Nitrogen oxides '000 tonnes




Reductions in UK sulphur dioxide have occurred largely through the reduction in use of coal by power stations and the installation of control technologies such as flue gas desulphurisation at Drax power station. Emissions of nitrogen oxides have also reduced from power stations as a result of increasing generation from Combined Cycle Gas Turbine and decreasing use of coal. Emissions from vehicles have slowly been falling since 1989 through increasing use of diesel fuel and the introduction of catalytic converters to all petrol engined cars sold since 1993. The reduction in nitrogen oxides in the UK has however slowed down due to increasing car ownership.

Acidic Pollutant Depositions

Air pollutants deposited on the UK originate not only from the UK but from other countries as well, as shown in Table 3.6.

Table 3.6: The Originating Countries of Sulphur and Nitrogen Deposition on UK

Country of pollution origin

Sulphur deposition on UK,
'000 tonnes 1998
(weight of sulphur)

Nitrogen deposition on UK,
'000 tonnes 1998
(weight of nitrogen)
















Total deposition



Total deposition of sulphur on UK during 1998 was around 0.33 million tonnes whereas the amount emitted in UK was more (1.19 million tonnes sulphur dioxide or 0.59 million tonnes sulphur, 1999). This highlights that the UK emits more sulphur pollution than is deposited in the UK. A significant proportion of sulphurous and nitrous pollutants are hence exported to other countries, mainly Germany, France, Norway, Sweden, the Netherlands and the Russian Federation in addition to the North Sea and Atlantic Ocean. The wind direction is the main factor affecting where UK pollutants are deposited.

The UK generally experiences high levels of acid deposition due to the large quantities of acidic pollutants emitted into the atmosphere each year.

Effects of Acid Deposition on Forests

In 1995, the United Nations Economic Commission for Europe (UNECE) Forest Survey revealed that 22% of UK trees were damaged (with 25% or more leaf or needle loss). Both coniferous and broadleaf trees showed the same degree of damage. Damage cannot be attributed to air pollution alone as many other factors such as climatic conditions, pests, age of tree, and exposure need to be considered. However, air pollution may cause an additional stress to trees and therefore may be a direct or indirect factor affecting tree health.

The commercial forest areas in the UK are mainly located in northern and western Britain where acid deposition is high due to higher levels of precipitation, mist and cloud. These areas of the UK are also particularly sensitive to acid deposition due to their granite-based bedrock offering low buffering capacity. Trees in these areas are more vulnerable to the effects of acid precipitation and air pollution

Effects of Acid Deposition on Water

Acidification studies in UK have shown that freshwater acidification is a serious problem in susceptible parts of the UK. These include central and south west Scotland, the Pennines, parts of Cumbria, central and North Wales and parts of Northern Ireland. Some lakes within these areas are acidified to the extent that they can no longer support many fish species.

Diatom analysis from lake sediments has shown that lake acidification has occurred in some UK freshwaters, with most rapid pH changes occurring since the 1950s. Freshwaters that have been intensively studied include Lyn Brianne in Wales and Loch Fleet in south west Scotland. The sensitivity of freshwaters in the UK has been studied and critical loads have been determined for areas of the UK. Critical loads are the amounts of acidic pollutants that an area can tolerate before damage occurs. Many areas of the UK already exceed such levels, although since the 1970s there is evidence that a recovery has taken place in response to significant reductions in emissions of sulphur dioxide.

Effects of Acid Deposition on Buildings

Acid deposition is known to accelerate normal weathering effects on buildings. Certain stone such as limestone, calcareous sandstone and marble have been shown to deteriorate faster through exposure to acid precipitation. Individual pollutants such as sulphur dioxide have effects on building materials, but pollutants can also act synergistically; nitrogen oxides and ozone may increase the effects of sulphur dioxide attack on certain materials.

On limestone, a black crust may form as pollutants react with the stone. This can accelerate damage as frost and other weather conditions cause this crust to blister and peel.

Many historic monuments and buildings are affected by air pollution in the UK and studies of cathedrals such as Lincoln and St. Paul's highlight that aspect and position are important factors in determining corrosion rates on historical buildings. The cost of damage to buildings is very difficult to estimate but includes costs of restoration to historic buildings which amounts to millions of pounds per year.

Effects of Acid Deposition on Health

The human health effects from acid deposition are not serious in the UK. Indirect effects may occur if ground or surface waters are contaminated by heavy metals leached from soils as a result of acidification. Health effects may occur as a result of the acidic pollutants in acid precipitation.

Back in the 1950s, the famous London smogs highlighted the serious nature of the effects of acidic pollutants on human health. 4,000 excess deaths were attributed to the effects of sulphur dioxide and particulate pollution during the December 1952 smog. Since then, smoke and sulphur dioxide levels have fallen considerably and such smogs are now history.

However, the UK does experience poor air quality episodes under certain climatic conditions which are sometimes classified by the UK Department of the Environment as "poor" air quality. Often such conditions occur during the summer months when photochemical reactions cause nitrogen oxides and hydrocarbons (mainly from traffic sources) to produce ozone. This can cause breathing difficulties amongst, for example, asthmatics. During different meteorological conditions such as temperature inversions, "poor" air quality may also occur when sulphurous and nitrous oxides build up in the atmosphere.

Control and Policy

The UK is committed to reducing sulphur emissions through the 1994 UNECE Protocol 'Further Reduction of Sulphur Emissions'. This protocol requires UK to reduce sulphur emissions by 50% by year 2000, 70% by 2005 and 80% by 2010 (all on 1980 levels). Emissions of sulphur dioxide in the UK have fallen dramatically in the last 20 years and this commitment has been met. The UK is also a Party to the Gothenburg Protocol, designed to Abate Acidification, Eutrophication and Ground-level Ozone. The UK is committed to reducing 1990 emissions of sulphur dioxide by 75% by 2010 and nitrogen oxides by 50% over the same period.

A 1988 Directive for European Union countries has also been introduced; it require all member countries to reduce emissions of sulphur dioxide and nitrogen oxides from Large Combustion Plants (over 50MW in size) by varying percentages. The UK is required to reduce sulphur dioxide by 60% by 2003 and nitrogen oxides by 30% by 1998 (on 1980 levels). The UK is well on course to exceed both of these targets, through new gas-fired power stations which produce smaller quantities of acidic pollutants than conventional coal fired stations, and flue gas desulphurisation equipment fitted to Drax and Ratcliffe on Soar power stations.

All petrol engined vehicles sold in EU countries since January 1993 have to be fitted with a catalytic converter. Catalytic converters reduce nitrogen oxide emissions from vehicles considerably compared with non-catalyst cars. However, there are still many non-catalyst cars in the UK and the increasing number of vehicles registered each year is likely to ensure that vehicular pollutants remain a major source of acidic pollution in the UK.

The introduction of the UK National Air Quality Strategy in March 1997 (and updated in 2000) is intended to achieve new air quality objectives throughout the UK by 2005, and hence improve air quality. The proposed standards and objectives of acid deposition pollutants under this new Strategy are shown in Table 3.7.

Table 3.7: The Proposed Standards and Objectives for Acid Deposition Pollutants
in the UK National Air Quality Strategy


Standard concentration
(parts per billion)

Measured as

Objective to be
achieved by:

nitrogen dioxide

105 ppb
not to be exceeded more than 18 times per year

21 ppb

1 hour mean

annual mean



sulphur dioxide

132 ppb
47 ppb
100 ppb

1 hour mean
24 hour mean
15 minute mean


The National Air Quality Strategy standards and objectives will require reductions in emissions from sources throughout the UK if the targets are to be met. This should form the basis for less acidic pollution in the UK over the coming years.