Acid Rain
Acid rain refers to any precipitation that is distinctly more acidic than normal rainfall. It
can be the result of natural causes – volcanic eruptions in particular – but is also the result
of human activities, such as the burning of fossil fuels. The geographic pattern of acid rain
is changing. Countries that are industrialising rapidly are showing signs of increased acid
rain. In contrast, some of the countries that have traditionally been associated with acid
rain are now seeing signs of reduced emissions, as a result of the decline of polluting
industries and the rise of cleaner fuel technology.
Introduction
Acid rain was first noted by Robert Angus Smith in 1852 in an article called ‘On the air and rain of
Manchester’. Rainwater is normally a weak carbonic acid with a pH of about 5.5. Acid rain is a more
acidic substance, due to the addition of SO2, CO2 and NOx. Any rain below 5.5 is termed ‘acid rain’.
Acid rain includes wet deposition and dry deposition. Wet deposition is the deposition on surfaces of
dissolved substances and particles formed by any form of precipitation. In contrast, dry deposition is the
deposition on surfaces of dry gases or particles in the atmosphere. In Britain, about 6.6 per cent of acid
precipitation falls as dry deposition.
The sulphur cycle
The sulphur cycle is the return movement of sulphur through living matter, air, rock, soil and water.
Atmospheric sulphur (oxides and sulphides) dissolves in rainwater. Once on land or in the oceans, it is
assimilated and metabolised by animals, plants and microorganisms. It returns to the soil and water
bodies in dead organisms. Soil sulphates are reduced and soil sulphides are oxidised. Under reduced
conditions, some sulphur may return to the atmosphere as hydrogen sulphide. Under oxidising
conditions, sulphates may form, including sulphuric acid. Acidification in sulphide-rich soils (e.g. spoil
heaps of mines) is a serious problem. The burning of fossil fuels releases sulphur dioxide, as do
volcanoes. This quickly oxidises and falls as acid rain.
The causes of acid rain
Natural causes
•
Volcanoes are important sources of atmospheric pollution – especially sulphur dioxide and
hydrogen dioxide. Volcanic sulphur emissions can affect soil and water acidity as well as
weather patterns, notably temperature and rainfall. Researchers in Taiwan have found
concentrations between sulphur-rich eruptions and drought in Taiwan. In the USA, following
volcanic eruptions, maximum temperatures are reduced and minimum temperatures increased
(on a diurnal scale).
Figure 1. The five largest continuously emitting volcanoes in terms of sulphur dioxide per day.
Volcano
SO2 (mg/day)
Etna
4000
Bagana
3300
Lascar
2400
Ruiz
1900
Sakura-Jimn
1900
•
Vegetation changes can also influence acidification. Approximately 33 per cent of Britain’s land
surface is upland – mainly in the cooler and wetter northwest of the country. Most soils are
acidic podzols and peats, and the natural vegetation is mainly upland grass, herbs, heather
and bracken. From 1919 onwards, the Forestry Commission acquired large tracts of moorland
and began planting coniferous trees for commercial concerns. Research at Plynlimon in Wales
has shown that streams in forest catchments are generally more acidic than those in grassland
catchments. Upland soils have a limited buffering potential and are therefore very sensitive to
changes in the acidity of precipitation. The large surface area of tree canopies means they take
in a larger proportion of pollutants from the atmosphere. The uptake of nutrients by trees leads
to soil acidity. In addition, tree litter is acidic and not easily broken down – as the Sitka spruce
is not native to Britain, specialised decomposing bacteria are absent. This also leads to an acid
humus layer in the soil.
Human-related causes
The main human-related causes of acid rain are emissions of NOx, SO and SO2 by industry, electricity
generation, households and transport. On a global scale, volcanoes are very important (Figure 2).
Fossil-fuel combustion is the largest anthropogenic source of CO2. Power stations and domestic fuel
combustions are also important. Up to five per cent of global sulphur emissions come from shipping,
although there has been a dramatic decrease in SO2 emissions between 1990 and 2000, especially in
the EU (decreased by 52 per cent) and North America (decreased by 17 per cent).
Transport is a major emitter of SO2. By mass, the most significant emissions are water vapour and CO2.
However, not all the fuel is combusted; a small fraction remains partially oxidised, and hence CO and
volatile hydrocarbon are emitted. Trace impurities are also released, so any sulphur may be oxidised to
sulphur dioxide or converted to sulphate.
At the high combustion temperatures in engines, atmosphere nitrogen is readily oxidised as nitric oxide
(NO) and some nitrogen dioxide (NO2). Once the exhaust gases leave the engine, the NO is converted
to NO2. Even in cars fitted with catalytic converters, some NO has been emitted.
Figure 2. Volcanoes are a major cause of acid rain.
Each year humans extract over 30,000 tonnes of arsenic, mainly for use in pesticides and wood
preservatives. Another 28,400 tonnes of arsenic finds its way back into soils and groundwater in wastes
of various sorts. Once in the soil, arsenic sulphides cause soil acidification, which in turn increases the
mobility of arsenic. Tailings (refuse produced by mining activities) are a major source for arsenic
contamination.
The effects of acid rain
Accelerated weathering of historical buildings in Greece, such as the Parthenon, has been linked with
acid precipitation. St Paul’s Cathedral in London is built of Portland limestone and acid rain has been
linked with the increased weathering of this church. Weathering hollows have appeared where rainwater
has run across the building’s surface. About 0.62 microns of the limestone surface are lost each year,
representing a loss of 1.5 centimetres since St Paul’s was built. Around Guilin, in China, acid rain has
weathered many of the karst towers that rise from the alluvial plain of the Li River.
Acid rain has been linked with changes in biodiversity, such as the death of coniferous forests in
Germany and Sweden. It increases the acidity of soils (though not chalk or limestone), and soil
acidification depletes bases. Where the pH is below 4.2, aluminium is released – this damages root
systems, decreases tree growth, increases the development of abnormal cells and aids the premature
loss of leaves and needles. In Europe and North America, forest soil acidity has increased fivefold since
1950. In Ontario and Quebec, the decline of sugar maples has great economic impact. In Germany, half
of the country’s forests are in decline, partly as a result of acid rain, although other factors also play a
part, such as land use, tree harvesting, drought and fungal attacks.
Acid rain affects the chemical composition of lakes and groundwater. In Sweden, some 90,000 lakes
have been acidified – 4,000 seriously – along with 100,000 kilometres of river. The susceptibility of
groundwater increases with latitude. Higher latitudes have thinner soils and lower rates of weathering.
By 1930, 4,000 lakes had no fish in Sweden, and 80 percent of Norway’s lakes were technically dead. In
the Eastern USA and Canada, over 48,000 lakes were too acidic to support fish.
Health effects of sulphur dioxide (SO2) are associated with the respiratory system including, for
example, asthma. Levels of sensitivity vary but some individuals experience effects at 100 ppb. The very
young and very old are most susceptible to high levels of SO2. High levels of SO2 also have an impact
on plants and animals, and the built environment. Stone, clothing and rubber corrode under high levels
of SO2.
Many of these effects are increasing because acid rain is no longer a local phenomenon. Tall
smokestacks – such as the 381-metre super-stack at Sudbury, Ontario – force the pollutants higher in
the atmosphere, making them airborne for longer and therefore increasing the likelihood of chemical
reactions producing nitric and sulphuric acid. The pollutants are also transported further, so that they are
falling at greater distances from the original source. Long-range transport of atmospheric pollutants
over 500 kilometres is most likely to produce strong acid rain.
According to researchers at the Open University, however, the effects of acid rain are not all negative,
as acid rain can benefit the environment by blocking one of the most powerful greenhouse gases.
Sulphur in acid rain reduces the natural production of methane, responsible for an estimated 22 per cent
of the greenhouse effect that is causing global warming. Bacteria that thrive on sulphur compete with,
and significantly reduce, methane generation from wetlands.
Possible solutions
There are now major acidification concerns in LEDCs, notably China and India. In contrast, in some
MEDCs, environments are beginning to recover as ‘management’ techniques have been put in place.
The cause and effects are difficult to prove, and so governments have resisted taking the required
action. However, modelling and empirical studies eventually produced proof, which, combined with
public pressure, led to action to curb emissions and provide more ‘at source’ technological solutions,
such as flue-gas desulphurisation. Other potential solutions include:
•
•
•
Reducing emissions by using cleaner fuels.
A gradual switch over from fossil fuels to renewable sources of energy.
The use of catalytic converters and the scrubbing of the air coming out of factories – these
reduce the levels of the agents of acid rain.
Early attempts to manage pollution such as acid rain included the idea of ‘dilute and disperse’. However,
localised solutions to local problems are now seen as inefficient and wrong as steps to remedy the acidrain problem may reduce atmospheric sulphur concentrations and, as a result, it may be necessary to
supplement soil sulphur content through the use of fertilizer.
In the 1980s, investigation into critical loads (the amount of deposition of acidic compound that could be
handled by the environment without any harmful effect) was carried out. Critical loads allowed a
quantification of the amount of reduction in emissions needed to protect vulnerable areas.
The ecological impact of volcanoes: a case study of Chances Peak, Soufriere,
Montserrat
In 1994, a team of biologists visited Montserrat. At that time, Chances Peak had some of the finest cloud
forest in the Caribbean, with a high diversity of plant life, including a rich display of tree ferns, insects,
lizards, birds and bats.
By January 1996, vegetation loss from acid rain, gases, heat and dust on the top of Chances Peak and
the surrounding area was severe. The cloud forest had disappeared. Tree ferns were dead, and
vegetation was gradually dying further down the mountain. On the eastern side, the lush forests of the
Tar River Valley were degraded from ash and gases, and finally destroyed by pyroclastic flows. One of
the factors leading to vegetation death was acid rain, caused by volcanic sulphur. The pH of the lake at
the top of Chances Peak was 2.0 (around 1,000 times more acidic than a pH of 5.0). Acid rain affects
plants directly by breaking down lipids in the foliage, and by damaging membranes which can lead to
plant death. Indirectly, acid rain increases the leaching of some nutrients and renders other nutrients
unavailable for uptake by plants.
Figure 3.
Chances Peak in Montserrat.
Acid rain in China
Location
In 2003, acid rain fell on more than 250 cities nationwide and caused direct annual economic losses of
110 billion yuan ($13.3 billion), equal to nearly three per cent of the country’s gross domestic product.
The regional acid-rain pollution is still out of control in some southern cities, especially in the
southwestern areas. With the exception of Chongqing, the average pH value of the central districts was
lower than 5.0 and the acid rain frequency was 70 per cent. The acid rain in southern China was mainly
distributed in the Pearl River delta and central and eastern areas of Guangxi.
Acid rain – blamed on smoke from coal-burning factories and power plants – is spreading, with the
number of cities suffering from severe levels rising last year to 218. In Beijing, the government is pouring
money into moving polluting industries out of the capital in an effort to clean up the city before the
Olympics in 2008.
Acid rain is one of the environmental costs of surging economic growth. Other costs include two-thirds of
the country's household sewage being untreated in 2004, and ‘heavy pollution’ tainting some cities’ air.
Causes
Major causes of acid rain are the rapidly growing number of cars on the roads, and the increasing
consumption of cheap, abundant coal, as the country struggles to cope with energy shortages and meet
power demand. China is the world’s largest source of soot and sulphur dioxide (SO2) emissions from
coal, which fires three-quarters of the country’s power plants. More than 21 tonnes of SO2 were
discharged in China in 2003, a rise of twelve per cent on the previous year. It is estimated that the
country will consume more than 1.8 billion tons of coal in 2005, emitting an additional six million tons of
SO2. The growth of nitrates, due to a swift rise of automobile and coal consumption, plus overuse of
fertilizers, is playing an increasing role in the country’s acid-rain pollution. In short, China’s explosive
economic growth is outpacing environmental protection efforts.
Possible solutions
The Chinese government has made significant efforts and progress in energy saving and consumption
reduction. Energy consumption has gone down year by year over the past two decades. However,
China’s environment has been ravaged by two decades of breakneck growth, and by the pressure of
feeding and housing a population of 1.3 billion. In industry, the rate of smoke and dust removal from
industrial waste gas has been reduced, and the government has taken measures such as the
introduction of levying charges for pollution emissions, and issuing licences for discharging air
pollutants. It has also promoted the adoption of clean coal, energy conservation and desulphurisation
technologies to help with the prevention of acid rain.
Figure 4.
Acid rain is partly the result of economic growth.
Acid rain in India
Location of acid rain and trends
Urban air pollution is probably the most well-known problem created by rapid industrialisation. Air
pollution around major factories, thermal power plants, open mines and quarries has attracted a lot of
attention. Rain over India is much less acidic than most of the other countries in Asia, Europe and North
America. However, it has become more and more acidic over the last few decades.
The pH of rain in India ranges from 5.9 to 8.4, and the average is about 6.7. India seems to be much
better off than the USA (4.15–6.19), Canada (4.23–5.96), Germany (4.05–4.25), Norway (4.10–4.40),
and most other countries. However, there are places in India where things are not so good. Parts of
south Bihar and West Bengal are likely to be the worst affected, along with the southernmost tip of the
Indian peninsula. Occasional rains with a pH of 4.8 have been reported from Chembur in Mumbai and a
pH of 4.5 from Delhi. The more worrying trend is the gradual acidification of the rain in India over the last
couple of decades – the pH has decreased from 7.0 to 6.1 in Delhi, and from 9.1 to 6.3 in Agra.
Causes and impacts
Thermal power plants in India, which generally use coal with relatively high sulphur content (0.5 per cent
to three per cent), are the major source of oxides of sulphur – they release about 2,500 tons per year.
Oxides of nitrogen are produced during high-temperature combustion. The greatest source of nitrogen
oxides is road vehicles.
India has been rather lucky to have predominantly alkaline-rich soils. For example, in the Thar Desert in
the northwest of India, the aerosols from coastal areas help reduce the acidity to a considerable extent.
Higher temperatures prevalent in India also contribute towards transforming the oxides of sulphur to
sulphates and oxides of nitrogen to nitrates. India also does not have natural sources of sulphur
emission like volcanoes. These factors have kept the acid rain in check so far. However, the emissions
from the increasing number of power plants, industries, fossil-fuel burning and vehicles have gradually
begun to overcome the natural checks. In 1990, none of the ecosystems in India was threatened by acid
rain. However, if steps are not taken to control emissions, by the year 2020 about 85 per cent of the
ecosystems will be threatened by acid rain.
Figure 5.
A building weathered by acid rain.
Possible solutions
India’s solutions are similar to that of many other countries: the use of cleaner fuels, a gradual switching
to renewable energy and the use of catalytic converters. In addition, a 66–130 million-hectare wasteland
should provide enough ground for growing biomass and using renewable sources of energy in a
sustainable manner.
Conclusion
Acid rain is both a natural and man-made pollutant. Its impacts are very diverse, affecting soils, water,
vegetation, buildings (Figure 5) and people. There has even been a claim that acid rain may reduce
global warming. The problem of acid rain is changing – or at least changing its location. Areas that are
seeing an increasing problem are those that are industrialising rapidly, notably China and India. In
contrast, in some MEDCs the problem has been reduced because of the reduced emission of pollutants
and attempts to manage the acid-rain problem. Natural sources, such as volcanoes, are very important
locally and attempts to manage them are difficult.