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NOx emissions have exceeded SO2 emissions since about 1980, but they too have
fallen with the implementation of the Clean Air Act
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The combined reductions of
SO2 and NOx emissions during this period led to a significant drop in acid
deposition, as well as sulfate (SO42) and nitrate (NO3) deposition
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As a result of actions and agreements such as those described above, acid
deposition in both Europe and eastern North America has been significantly
reduced
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S
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Similar trends were also reflected in data collected at
measuring stations located across the eastern United States, which reported a
decrease of approximately 40 percent in H+ concentration between 1994 and
2008
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Despite significant reductions in acid deposition, some European and North
American ecosystems impaired by acid deposition have been slow to recover
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As a result, these soils are even more susceptible to
continued acid deposition, even at reduced levels
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In contrast to Europe and North America, acid deposition is increasing in other
parts of the world
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However, the introduction of stringent
emission controls in China in 2007 produced a 75 percent decline in the country’s
SO2 emissions by 2019, whereas India’s SO2 emissions continued to increase
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The sulfuric and
nitric acids formed in the atmosphere fall to the ground mixed with rain, snow, fog,
or hail
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The acidic particles and gases may deposit to surfaces
(water bodies, vegetation, buildings) quickly or may react during atmospheric
transport to form larger particles that can be harmful to human health
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The amount of acidity in the atmosphere that deposits to earth through dry
deposition depends on the amount of rainfall an area receives
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Chemistry of Acid Rain:
Acid rain is a popular expression for the more scientific term acid deposition,
which refers to the many ways in which acidity can move from
the atmosphere to Earth’s surface
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Acid deposition also includes the dry deposition of acidic particles
and gases, which can affect landscapes during dry periods
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Acidity is a measure of the concentration of hydrogen ions (H+) in a solution
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Substances are
considered acidic below a pH of 7, and each unit of pH below 7 is 10 times more
acidic, or has 10 times more H+, than the unit above it
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0 has a concentration of 10 microequivalents of H+ per litre, whereas
rainwater with a pH of 4
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Normal rainwater is weakly acidic because of the absorption of carbon
dioxide (CO2) from the atmosphere—a process that produces carbonic acid—and
from organic acids generated from biological activity
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Other
natural sources of acidification include the production of nitrogen oxides from the
conversion of atmospheric molecular nitrogen (N2) by lightning and the conversion
of organic nitrogen by wildfires
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2
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In
the United States, electric utilities produce nearly 70 percent of SO 2 and about 20
percent of NOx emissions
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In the atmosphere, sulfuric and
nitric acids are generated when SO2 and NOx, respectively, react with water
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In such areas
the pH of precipitation can average 4
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5 annually, and the pH of individual
rain events can sometimes drop below 3
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In addition, cloud water and fog in
polluted areas may be many times more acidic than rain falling over the same
region
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In addition to acid deposition, NO x emissions along
with hydrocarbon emissions
are
key
ingredients
in
groundlevel ozone (photochemical smog) formation, which is one of the most widespread
forms of air pollution
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Coal combustion is the leading
source of atmospheric mercury, which also enters ecosystems by wet and dry
deposition
...
) Acid
deposition of nitrogen derived from NOx emissions creates additional
environmental problems
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This eutrophication (or over-enrichment) causes the overgrowth
of plants and algae
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Eutrophication is considered to be a major environmental problem in lake, coastal
marine, and estuarine ecosystems worldwide
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Sulphur and Nitrogen particles which get mixed with
water are found in two ways either man-made i
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According to the Royal Society of Chemistry, which considers him the “father of
acid rain,” the word acid rain was invented in 1852 by Scottish chemist Robert
Angus Smith
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The regular clean rain we experience, even though it is not clean i
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The reaction occurring is
H2O (l) + CO2 (g) ⇌ H2CO3 (aq)
The pH value of regular rainwater is around 5
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The
oxides of nitrogen and sulphur are blown away by the wind along with the dust
particles
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Acid rain is essentially a byproduct of human activities which emit
oxides of nitrogen and sulphur in the atmosphere
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Sulphur dioxide and nitrogen dioxide undergo oxidation, and then they react with
water resulting in the formation of sulphuric acid and nitric acid respectively
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i
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At pH lower than 5 most fish eggs will not hatch and lower pH can kill adult fish
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Acid rain has
eliminated insect life and some fish species, including the brook trout in some
lakes, streams, and creeks in geographically sensitive areas, such as the
Adirondack Mountains of the United States
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e
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The United States Environmental Protection Agency's (EPA) website
states: "Of the lakes and streams surveyed, acid rain caused acidity in 75% of the
acidic lakes and about 50% of the acidic streams
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e
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ii
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Some microbes
are unable to tolerate changes to low pH and are killed
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The hydronium ions of acid rain also mobilize toxins, such as aluminium, and
leach away essential nutrients and minerals such as magnesium
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iii
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Most minor cases which do not result in fatality of plant life can be
attributed to the plants being less susceptible to acidic conditions and/or the acid
rain being less potent
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Acidic water enters the
plant and causes important plant minerals to dissolve and get carried away; which
ultimately causes the plant to die of lack of minerals for nutrition
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Likewise, acid
rain that falls on soil and on plant leaves causes drying of the waxy leaf cuticle,
which ultimately causes rapid water loss from the plant to the outside atmosphere
and eventually results in death of the plant
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If the leaves are green
and look healthy, the soil pH is normal and acceptable for plant life
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Moreover, a plant suffering from
soil acidification cannot photosynthesize; the acid-water-induced process of drying
out of the plant can destroy chloroplast organelles
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iv
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High altitude
forests are especially vulnerable as they are often surrounded by clouds and fog
which are more acidic than rain
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In
cultivated areas, limestone may also be added to increase the ability of the soil to
keep the pH stable, but this tactic is largely unusable in the case of wilderness
lands
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v
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Acid rain can cause
the ocean's pH to fall, known as ocean acidification, making it more difficult for
different coastal species to create their exoskeletons that they need to survive
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Coral's limestone skeleton is particularly sensitive to pH decreases, because
the calcium carbonate, a core component of the limestone skeleton, dissolves in
acidic (low pH) solutions
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vi
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The acid in the rainwater is too
dilute to have direct adverse effects
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Increased amounts
of fine particulate matter in the air contribute to heart and lung problems,
including asthma and bronchitis
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Other adverse effects
Acid rain can damage buildings, historic monuments, and statues, especially those
made of rocks, such as limestone and marble, that contain large amounts of
calcium carbonate
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CaCO3 (s) + H2SO4 (aq) ⇌ CaSO4 (s) + CO2 (g) + H2O (l)
The effects of this are commonly seen on old gravestones, where acid rain can
cause the inscriptions to become completely illegible
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Solution:
There are several solutions to stopping manmade acid rain
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This can be done by restricting the use of fossil fuels and focusing on
more sustainable energy sources

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