Choi Man Ho
Chan Kwun Keung
Acid Rain and
Photochemical Smog
What is Acid Rain?
• Unpolluted rainwater is slightly acidic
(pH 5.6) because of the carbon dioxide from
air dissolved in it. CO2 + H2O => H2CO3
• Rainwater with a pH as low as 2.5 has been
recorded in some parts of the world. They
are commonly known as acid rain.
What is Acid Rain?
• "Acid rain" is also a broad term used to
describe several ways that acids fall out of
the atmosphere.
• A more precise term is acid deposition,
which has two parts: wet and dry.
• Wet deposition: acidic rain, fog, and snow.
• Dry deposition: acidic gases and particles
Causes of Acid Rain
• Sulphur dioxide (SO2) and nitrogen
oxides (NOx) are the primary causes of acid
rain.
• Acid rain occurs when these gases react in
the atmosphere with water, oxygen, and
other chemicals to form various acidic
compounds.
• Sunlight increases the rate of most of
these reactions. The result is a mild solution
of sulfuric acid and nitric acid.
Causes of Acid Rain
Causes of Acid Rain
Formation of Sulphuric Acid
(H2SO4)
• Sulphur dioxide (from the burning of fossil
fuels in power plants and industrial plants)
is the primary cause of acid rain.
• In the atmosphere, sulphur dioxide is slowly
oxidized to sulphur trioxide which
dissolves readily in water droplets to form
sulphuric (vi) acid.
• 2SO2+O2 => 2SO3
• SO3 + H2O => H2SO4
Formation of Sulphuric Acid
(H2SO4)
• The actual pathways are more complex.
• The formation of SO3 from SO2 is
influenced by the prevailing atmospheric
conditions: sunlight, temperature,
humidity, and the presence of
hydrocarbons, nitrogen oxides and
particulates in the atmosphere.
Formation of Sulphuric Acid
(H2SO4)
• Sulphuric (iv) acid is also formed when
SO2 dissolves in rainwater:
• SO2 + H2O => H2SO3
Formation of Nitric Acid (HNO3)
• Nitrogen oxides (from the burning of fossil
fuels in automobiles and power plants) also
cause the formation of acid rain.
• When released to the atmosphere, nitrogen
monoxide combines with atmospheric
oxygen to form nitrogen dioxide:
• 2NO + O2 --> 2NO2
Formation of Nitric Acid (HNO3)
• In a series of complex reaction, nitrogen
dioxide combines with oxygen and water
vapour to form nitric (v) acid.
• 4NO2 + 2H2O + O2 --> 4HNO3
Environmental Problems
• (i)In water of pH less than 4.5, calcium
metabolism in fresh water fish will be
affected, leading to poor health and
stunted growth.
• As a result, diversity and population of
some fresh water species will be reduced.
Environmental Problems
• (ii)In soil of pH less than 4.5, absorption of
cations by plants will be affected, resulting
in death of plants.
• (iii)Inflow of acidic water containing
poisonous metal ions from soil will kill the
fish and water plants
• (iv)Acid rain corrodes metals and
accelerates the deterioration of building,
rock and statue.
How do we Measure Acid Rain?
• Acid rain's pH (and the chemicals that cause
acid rain) is monitored by two networks,
both supported by EPA.
• The National Atmospheric Deposition
Program measures wet deposition, and its
Web site features maps of rainfall pH and
other important precipitation chemistry
measurements.
How do we Measure Acid Rain?
• The Clean Air Status and Trends
Network (CASTNET) measures dry
deposition. Its Web site features information
about the data it collects, the measuring
sites, and the kinds of equipment it uses.
How do we Reduce Acid Rain?
• Understand acid deposition's causes and
effects
• Clean up smokestacks and exhaust pipes
• Use alternative energy sources
• Restore a damaged environment
• Be green
How do we Reduce Acid Rain?
• EPA's Acid Rain Program limits, or
"caps," sulphur dioxide (SO2) emissions
from power plants at 8.95 million tons
annually, allows those plants to trade SO2
allowances, and reduces nitrogen oxide
emission rates.
What is Photochemical Smog?
• Photochemical smog is a mixture of
pollutants which includes particulates,
nitrogen oxides, ozones, aldehydes,
peroxyacetyl nitrate (PAN), unreacted
hydrocarbons, etc.
• A brownish haze and painful eyes are
often indicators of photochemical smog.
Nitrogen dioxide is responsible for the
brownish colour of the haze.
Causes of Photochemical Smog
• The reactions that lead to the formation of
photochemical smog are irritated by
sunlight and involve hydrocarbons and
nitrogen oxides emitted from automobiles,
the combination of sunlight, the catalysis
by particulates and the abundant
pollutants present in modern cities provide
favourable conditions for smog formation.
Causes of Photochemical Smog
• Nitrogen dioxide from automobile exhaust
first absorbs sunlight and breaks down
into nitrogen monoxide and reactive
oxygen atom:
• NO2 => NO + O
Causes of Photochemical Smog
• The oxygen atom reacts with other
components of automobile exhaust (e.g.
unburnt hydrocarbons) and those of the
atmosphere (e.g. oxygen) in a series of
complex reactions to produce a variety of
lachrymatory and toxic chemicals (e.g.
peroxyacetyl nitrate).
Causes of Photochemical Smog
• O + O2 => O3
• O + hydrocarbons => aldehydes
O3 + hydrocarbons => aldehydes
• NO2 + O2 + hydrocarbons => lachrymatory
substances, including peroxyacetyl nitrate
(PAN): CHCOOONO2
Formation of Nitrogen Oxides
(NOx)
• The most natural way of forming nitrogen
oxides [NOx, where x may be 1 or 2] is
lightning.
• Atmospheric nitrogen reacts with nearby
oxygen to form nitrogen monoxide.
• N2 + O2 => 2NO
Formation of Nitrogen Oxides
(NOx)
• Nitrogen monoxide further reacts with
oxygen to form nitrogen dioxide.
• 2NO + O2 => NO2
• But this only generate limited amount of
nitrogen oxides.
• The pollution caused can be accounted for
the engines of vehicles and furnaces. Due
to their high temperature, these gases are
evolved easily. The great number of cars on
Hong Kong roads deepens the problem.
Formation of Ozone (O3)
• Ozone [O3(g)] is a pale blue gas (exists in
the stratosphere of our atmosphere as the
ozone layer).
• It is formed from atmospheric oxygen by
the absorption of ultraviolet radiation of
the right energy (wavelength 250 nm).
O2 => 2O
O + O2 => O3
Formation of Ozone (O3)
• Ozone itself undergoes photodissociation
with 215-295 nm ultraviolet radiation.
O3 => O2 + O
O+O3=>2O2
• Thus, ozone is constantly created and
destroyed. The above reaction is responsible
for the vital screening effect of ozone.
Formation of Ozone (O3)
• Ozone can also be formed in the lower part
of the atmosphere.
• It is formed by reactions between nitrogen
oxides and hydrocarbons under sunlight,
or by electric sparks which occur in car
engines or electrical appliances like
photocopiers.
• In nature, the gas can be generated during
lightning.
Formation of Hydrocarbons
• Hydrocarbons have both the elements
carbon and hydrogen. Motors of cars do not
always burn the fuel completely. There is
tiny amounts of unburnt hydrocarbons in
car exhaust.
• Petrols and organic solvents are left
unattended and exposed to air. Since those
solvents are volatile in nature, these
hydrocarbons pose danger to our health.
Formation of Particulates
• Particulates make up smoke.
• They may be ashes from burning of fuels.
• If fossil fuels (like coal and oil) are burnt,
the tiny particules formed are mainly soot
(carbon) from incomplete combustion.
• Incineration plants, factories and diesel
vehicles are sources of emission.
Environmental Problems
• (i)It can cause headaches, eye, nose and
throat irritations, impaired lung function,
coughing and wheezing.
• (ii)It can cause rubbers and fabrics to
deteriorate.
• (iii)It can cause damage plants, leading to
the loss of crops.
Another Smog
Characteristics
Industrial smog
Photochemical
smog
Typical city
London
Los Angeles
Climate
Cool and humid
Warm and dry
Pollutants
SOx, particulates
NOx, O3,
aldehydes, PAN,
etc.
Another Smog
Characteristics
Industrial smog
Photochemical
smog
Major sources
Major effects on
humans
Industrial and
Motor vehicles
household
burning of coal
oil
Irritation of lungs Eye irritation
and throat
Times when
worst episodes
occur
Winter months
(especially in
early morning)
Summer months
(maximum effect
around noon)
Cases of Smog (photochemical
smog )
• China's Pearl River Delta region is
starting to suffer photochemical smog
due to heavy air pollution in the cities.
• Emissions from factories, power
plants and the rapid growth of vehicles
in cities are providing the raw
materials for photochemical smog.
• The first case of photochemical smog
happened in Los Angles as early as in
1943.
Cases of Smog (photochemical
smog )
Pearl River
Delta region
Hong Kong
Cases of Smog (Industrial smog)
• On 9 December 1952, foggy conditions
developed over London.
• Being very cold, most houses kept fires
burning, with coal as the major fuel.
• The smoke from these fires mixed with the
fog and was unable to disperse, resulting in
a smog which persisted for 4 days.
Cases of Smog (Industrial smog)
• The pH of air during the Great London
Smog was as low as 1.6. During this period
some 4000 more people died than would
expected at this time of the year. Most of
these additional deaths were due to
respiratory disorders.
Cases of Smog (Industrial smog)
Before
After smog
New York City
How do we Reduce
Photochemical smog?
• To address the problem of photochemical
smog, we have to reduce the emissions of
NOx and VOC.
• Reduce the emissions from motor vehicles.
Sources of Information
• Chem notes by Ng sir
• lhs.hkcampus.net/~lhs-chem/acid
rain.files/frame.htm
• www.epa.gov/airmarkets/acidrain
• http://hk.geocities.com/xavier114fch/03/03b.htm
• http://resources.emb.gov.hk/envired/text/hkissue/e_m1_1_3.htm
• http://en.chinabroadcast.cn/143/2004-113/116@164576.htm
The End
Thanks for paying attention