Acid rain is caused by the transport and deposition of sulfur dioxide (SO2), nitrogen oxides (NOx), and other acid compounds from the atmosphere. These compounds are emitted from industrial activity and transported long distances before eventually falling to the ground as acid rain, damaging forests, soils, lakes, and buildings. Acid rain has lowered the pH of many lakes and streams in Europe and North America, killing fish and other aquatic life. While efforts have been made to reduce SO2 and NOx emissions to combat acid rain, many ecosystems will require a long time to recover from existing damage.

1. 18.1 INTRODUCTION
With the rise of global industrialisation, increasing acidity in natural water and soils has become a problem.
This acidity is associated with the transport and subsequent deposition of SO2, NOx and other acid
Oxidation products. The phenomenon is called "acid rain". Thus much concern was raised in the international
platform about the future strategies for combating industrial emission of pollutants, regionally and locally.
The pH relationship ofa number ofsubstances are given in the following scale:
Distilled Human
water Blood Ammonia Lime
pH0 12 3 4 5 6 7 8 9 10 11 12 13 14 pH
ACIDIC NEUTRAL ALKALINNE
Battery and Acid rain Normal Milk Sea water
rain

2. suifuric Acid:
SOx OxIDES
NOx Nitric Acid:::
Particlees
ACID RAIN
L 10 km 100 km 1,000 kmn
-Short range- Long range-
amamsabesssmae
Acid precipitation
Fig. 18.1: Schematic outline of acid rainn

3. It has been mentioned in the preceding section that much of the NO, and SO. enterio
atmosphere are converted into HNO, and H,SO, respectively. The detailed photochemicalreacti
the atmosphere are summarized: tochemical reactionsin
NO +Og NO, +
O
(31)
NO +O NO, +O2
NO + N0, > N,Os (32)
(33)
NO +H,O>2HNO (34)
HNO is removed as a precipitate or as particulate nitrates after reaction with bases (NH.
particulate lime).
SO+0, + H,O (HC, NO,) HSO4 (35)
Soot particles
(metal oxide)
The presence of hydrocarbons and NO, steps up the oxidation rate of the reaction. In water
droplets, ions such as Mn(I), Fe(II), Ni(II) and Cu([I) catalyse the oxidation reaction. Soot particles
are also known to be strongly involved in catalysing the oxidation of SO,.
HNO and HSO combine with HCI from HCI emission (both by natural and anthropogenic
sources) to generate acidic precipitation which is widely known as acid rain. Acid rain is now a
major pollution problem in some areas.
Acid rain
causes exiensive damage to building and sculptural materials of marble, limestone
slate, mortar, etc. These materials become pitted and weakened mechanically as the soluble sulphates
are leached out by rainwater.
(36)
CaCO + H,SO, CaSO,+ CO, + H,O
Limestone

4. 18.5 ACID RAIN AND TERRESTRIAL ENVIRONMENT
Terrestrial ecosystem takes much longer period to show the effects of acid rain than aquatic ecosystem. As
a result, the nature and magnitudes of the impact of acid precipitation on the terrestrial environment has
been recognised only in recent decades. Acid precipitation damages the forests and othervegetation cover
in a number of ways. The details are given in Fig. 18.5. The changes include crown dieback, necrosis of
foliages, defoliation, soil acidification and various other related processes. As a result, vegetation cover or
TOrest declines on long term basis. Mobilisation of toxic metals, such as aluminium, cadmium, zinc, mercury,
ead and copper and iron is another feature which accompanies soil acidification. Thus detrimental effect of
these toxic metals were seen with times in these affected areas.
18.6 ACID RAIN AND THE BUILD ENVIRONMENT
Acid rain also contributes to deterioration of the build environment-particularly in limestone and marble
rock buildings. Several monuments thus get affected with time by continued acid precipitation in these
areas. Crystals of calcium and magnesium sulphate begin to form on or beneath the surface of the stone, as
corrosion effect induced by acid rain. By attacking the fabric of buildings, acid rain causes physical and
economic damage, but it does more than that, it also threatens the world's cultural heritage (e.g. the
Parthenon, the Sistine Chapel. the Taj Mahal).

5. 18.7 ACID RAIN AND HUMAN HEALTH
The famous London Smog of 1952 devcloped as a result of meteorological conditions which allowed the
build up of pollutants within the urban atmosphere. Smoke-produced by domestic fires, power stations
and coal burning industries-was the most obvious pollutant, but the most dangerous was sulphuric acid,
flowing free in aerosol form or attached to the smoke particles. Drawn deep into the lungs, the sulfuric acid
caused or aggravated breathing problems and many of the 4,000 deaths attributed to the smog were
brought about by the effect of sulfuric acid on human respiratory system. Moreover, elevated atmospheric
acidity continues to cause chronic respiratory problems in these areas.
Heavy metals such as copper, cadmium, zinc, and mercury, etc. liberated from soil and bedrock by
acid rain may eventually reach the human body via plants and animals in the food chain or through
drinking water supplies. These changes pose threat to human health of the concerned region.

6. In 1958, rain in Europe showed a pH of 5.0, but by 1962 it was 4.5 in the Netherlands. Sweden
experienced rainfall at pH 4.5 in 1966. The acid rain damaged leaves of trees and plants and retarded
the growth of Swedish forests. It may be noted that these forests are important natural resources foor
production of wood pulp, paper and board. In 1979, it vwas estimated that 20,000 lakes of Sweden
were suffering loss of flora and fauna-the fish mortality was increasing. The pollution sources were
emission from UK, the Ruhr and Germany. The sulphur emission rate for Europe was up to 70
metric tonnes per year. H,SO4 and particles of Cd and Pb are deposited on the winter snows andd
when these melt, the pollutants enter the rivers and lakes. This occurs at a time when fish spawning
and hatching occur thereby destroying the fish eggs.
It is intriguing that while 33-Nation UN Conference on acid rain was in session at Stockholm
(July, 1982), the venue received heavy downpour of acid rain for the entire week. The conference
was designed to focus world attention on acid rain, the most potent ecologicai threat to Scandinavia
and Canada, for which UK and USA are responsible.
The acid rain that fell during the conference was depositing Sulphur at the rate of 3.5 gm
every year (safe level is 0.5 gm per year). An estimated 25% of this rain came from UK which
deposited on Sweden in the year 40,000 tonnes of Sulphur in smoke from heavy industry carried by
prevailing winds across the North Sea.
Elsewhere Sweden's 85,000 lakes were slowly being killed by the deluge-plant and fish life
was
damaged and 4000 lakes were completely dead.
In Canada, trees and lakes are also being killed by acid rain, 60% of which originates from
USA.

7. Table 10.3 Analysis of typical acid rain sample (pH =
4.25)
Cation Concentration Anion Concentration
mole/lx 105 mole/l x 10
H 56
NH 10 so 51
Ca2
Na
NO 20
CI 12
Mg2
K
Total 83
2
Total 83
The actual values, however, are subjected to variation depending on the time and location of
collection of the acid-rainwater sample. The trend, however, remains the same-H,SO4 is the major
contributor to acid precipitation, HNO, ranks second and HCI the third in this respect.
Acid precipitation shows a correlation with the prior movement of the airmass over major
sources of anthropogenic sulphur and nitrogen oxide emissions. This has been demonstrated in
sOuthern Norway and Sweden, which receive a heavy load of air pollution from densely-populated
industrial areas of Europe, including UK.

8. Acid Deposition
Leaf fal
.Gas aerosol pesticides
Browning of
Leaf
Reducing of,
Leaf-
Tree weakened
by lack of
nutrients &
accumulation of
toxicants
Damage ofCanopPy
&trunk
Top Soil Acidity
Reduce seedling
regenerator
Soil
acidification
Nutrient washed
Damage of fine roots
& hairs
away
Raduced
decomposition Reduceintake of
Release of water & nutrients
poisonous
metals
Fig. 18.5: Typical symptoms of damage in coniferous t. 3s subject to acid rain

9. 18.9 MITIGATION OF ACID RAIN PROBLEMS
Although the cause-and-effect relationship between emissions ofSO and NO, and
acid rain damage is
not universally accepted, most of the solutions proposed for the problem involvethe disruptionof that
relationship. On the whole, the reduction of acid-forming gases is all that is required to slow down and
eventually stop the damage being caused by the acidification of the environment. Among the acid-

10. forming SO, emission, control is the most important aspect of the problem-solution. A number of measures
were suggested for minimizing S0, emission into the atmosphere-these include desulphurificationof
fuel, flue gas emission control technology adoption and SO, recovery as HSO4 etc. Several European
governments paid considerable attention on this aspect for minimising acid precipitation. On the whole,
progress towards the large scale abatement of acid emissions has been slow and methods for controlling
NO, lag behind those for dealing with SO. Of course, emissions of SO, are beginning to decline in many
areas. But lakes and forests already damaged require considerable time to recover back to the past conditions.
A number of ameliorative measures were also undertaken for speedy recovery of these affected areas too.