Transcript O 3

This lecture will help you understand:
• Outdoor air pollution
• Chemical reaction in
the atmosphere
• Acid deposition
• Indoor air pollution
• Stratospheric ozone
depletion
Central Case: London’s 1952 “Killer Smog”
• In December 1952 a thick
smog settled on London,
killing 4,000–12,000 people.
• It was caused by weather
conditions that exacerbated
the city’s air pollution from
factories.
• This and other events led
Britain to pass clean air laws.
Donora Smog
• On October 26, 1948, people in Donora,
Pennsylvania woke up to a dense fog that
lasted 5 days.
– The town has a large steel mill that used
high-sulfur coal and a plant that roasted
sulfur-containing ores.
• People complained of difficulty breathing,
stomach pain, headaches, nausea, and choking.
– Within 1 month, 70 people died and 6,000
got sick.
– Mill owners did not think their mills were
responsible.
• State and national laws now control
pollution.
– Culminating in the Clean Air Act (1970)
air
Clean Air Act impacts
Air pollution essentials
• Air pollutants: substances in the atmosphere (gases
and aerosols) that have harmful effects.
• The atmosphere contains many gases.
– N2, O2, Ar, CO2, water vapor
– 40 trace gases: ozone, helium, hydrogen, nitrogen
oxides
– Aerosols: microscopic liquid or solid particles (dust,
pollen, sea salts, etc.) from land and water
• The Industrial Revolution changed the mixture of
atmospheric gases and particles.
Primary air pollutants
Five primary air pollutants
Pollutants
• Three factors determine the level of air pollution.
– The amount of pollutants entering the air
– The amount of space into which the pollution is
added
– Mechanisms that remove pollutants from the air
• Troposphere: the lower atmosphere
– The site and source of weather, water vapor, clouds
– Pollutants are removed within hours or days.
– Pollutants in the upper troposphere can persist for
days.
• Pollutants in the stratosphere are resistant to cleansing.
– Ozone-depleting chemicals (chlorine, bromine)
Processes producing air pollution
• Incomplete combustion of fossil fuels and refuse
– Creating gaseous and particulate products
• Evaporation: creates gaseous and particulate products
• Strong winds: pick up dust and other particles
• Primary pollutants: direct products of combustion and
evaporation
– Particulates, volatile organic compound(VOC)s, CO,
NOx, SO2, lead, air toxics
• Secondary pollutants: reaction products of primary
pollutants in the air
– Ozone, peroxyacetyl, nitrates, sulfuric and nitric
acids
Artificial sources of air pollution
Human-caused air pollution includes:
• Point sources : specific spots where large amounts of pollution
are discharged (factory smokestacks)
• Non-point sources : diffuse, often made up of many small
sources (charcoal fires from thousands of homes)
Primary Pollutants
CO
CO2
SO2 NO NO2
Most hydrocarbons
Secondary Pollutants
SO3
HNO3 H2SO4
Most suspended
particles
H2O2
Most
Sources
Natural
Mobile
Stationary
–
NO3
O3
PANs
and SO24– salts
Primary pollutants
• Power plants: the major source of sulfur dioxide
– Industrial plants: particulates
– Transportation: carbon monoxide, nitrogen oxides
– Burning fossil fuels and wastes: soot, smoke
– Unburned fragments of fuel molecules: VOCs
• Nitrogen oxides (NOx): nitrogen gas is oxidized to
nitric oxide (NO) under high combustion temperatures.
– Nitric oxide and oxygen form nitrogen dioxide (NO2:
photochemical smog) and nitrogen tetroxide (N2O4)
• Coal also contains sulfur and heavy metals.
Secondary pollutants
• Photochemical oxidants: ozone and other reactive
organic compounds formed by nitrogen oxides and
VOCs.
– Sunlight provides the reaction’s energy
• Ozone concentrations in preindustrial times: 10–15 ppb
– Unpolluted, summer air in North America: 20–50 ppb
– Polluted air: 150 ppb or more (very unhealthy)
• Ambient U.S. ozone levels decreased by 20% during
1980–92
– But only a few percent in the 2000s
Major air pollutants
• Carbon oxides:
– CO is a highly toxic gas that forms during the incomplete
combustion of carbon-containing materials.
– 93% of CO2 in the troposphere occurs as a result of the
carbon cycle.
– 7% of CO2 in the troposphere occurs as a result of human
activities (mostly burning fossil fuels).
• Nitrogen oxides and nitric acid:
– Nitrogen oxide (NO) forms when nitrogen and oxygen gas
in air react at the high-combustion temperatures in
automobile engines and coal-burning plants. NO can also
form from lightening and certain soil bacteria.
• NO reacts with air to form NO2.
• NO2 reacts with water vapor in the air to form nitric
acid (HNO3) and nitrate salts (NO3-) which are
components of acid deposition.
• Sulfur dioxide (SO2) and sulfuric acid:
– About one-third of SO2 in the troposphere occurs naturally
through the sulfur cycle.
– Two-thirds come from human sources, mostly combustion
(S + O2  SO2) of sulfur-containing coal and from oil
refining and smelting of sulfide ores.
– SO2 in the atmosphere can be converted to sulfuric acid
(H2SO4) and sulfate salts (SO42-) that return to earth as a
component of acid deposition.
• Suspended particulate matter (SPM):
– Consists of a variety of solid particles and liquid droplets
small and light enough to remain suspended in the air.
– The most harmful forms of SPM are fine particles (PM-10,
with an average diameter < 10 micrometers) and ultrafine
particles (PM-2.5).
– According to the EPA, SPM is responsible for about 60,000
premature deaths a year in the U.S.
• Ozone (O3):
– Is a highly reactive gas that is a major component of
photochemical smog.
– It can cause and aggravate respiratory illness and can aggravate
heart disease.
– Damage plants, rubber in tires, fabrics, and paints.
• Volatile organic compounds (VOCs):
– Most are hydrocarbons emitted by the leaves of many plants
and methane.
– About two thirds of global methane emissions comes from
human sources.
– Other VOCs include industrial solvents such as trichlorethylene
(TCE), benzene, and vinyl chloride.
– Long-term exposure to benzene can cause cancer, blood
disorders, and immune system
• Radon (Rn):
– Is a naturally occurring radioactive gas found in some types of
soil and rock.
– It can seep into homes and buildings sitting above such deposits.
Emission of air pollutants
internal combustion engine
from
four-cycle
Catalytic converter
HCs
+ H2O  H2 + CO
2 NO + 2 H2  N2 + 2 H2O
2 CO
+
O2  2 CO2
HC + 2 O2  CO2 + 2 H2O
The overall reaction for the reduction of NO and the oxidation of CO
2 NO
+
2 CO
Rh, Pt, Pd catalysts
N2

+
2 CO2
The oxidation of a typical gasoline hydrocarbon, octane
2 C8H18
+
25 O2
Pt, Pd catalysts

16 CO2 +
18 H2O
The fate of atmospheric SO2: acid rain
SO2
.
HSO3
SO3
.
+
+
OH
O2
+
H2SO4 (g) +
SO3

H2O

H2O
.
HSO3

+
H2SO4(g)
H2SO4 (aq)

SO2 (g) + H2O (aq)
KH
.
HOO
H2SO3(aq)
= [H2SO3(aq)]/PSO2 (0.1 ppm)
[H2SO3(aq)] = PSO2KH = 1.0 x 10-7 atm x 1.0 M/atm = 1.0 x 10-7 M
H2SO3(aq)
HSO3- (aq) + H+ (aq)
-
+
[HSO3 ] = [H ]
+
Ka = 1.7 x 10-2 = [HSO3 ][H ]
[H2SO3]
=
- 2
[HSO3 ]
1.0 x 10-7
-
[HSO3 ] = 4.1 x 10-5 M
So the ratio of HSO3- to H2SO3 is 410:1 (4.1 x 10-5 /1.0 x 10-7).
Because [HSO3-] = [H+] = 4.1 x 10-5 M
Therefore the pH of the aerosol droplets is 4.4.
Hydroxyl radicals
• Photochemical breakdown of ozone is a major source
of hydroxyl radical.
• They remove anthropogenic pollutants from the air.
– Highly reactive hydrocarbons are rapidly oxidized.
– Nitrogen oxides are oxidized within a day
– Less reactive substances (e.g., CO) take months.
• Atmospheric levels of hydroxyl radicals are determined
by levels of anthropogenic air pollutants.
– Hydroxyl’s cleansing power is used up.
– Pollutants are able to build up.
The hydroxyl radical: Nature’s cleanser
Factors influencing levels of
outdoor air pollution
• Outdoor air pollution can be reduced by:
– settling out, precipitation, sea spray, winds,
and chemical reactions.
• Outdoor air pollution can be increased by:
– urban buildings (slow wind dispersal of
pollutants), mountains (promote temperature
inversions), and high temperatures (promote
photochemical reactions).
Atmospheric cleansing
• Natural air pollutants: volcanoes, fires, dust storms
– Plants emit volatile organic compounds
– Mechanisms in the biosphere remove, assimilate,
and recycle natural pollutants
• Hydroxyl radical (OH): a naturally occurring compound
– Oxidizes many gaseous pollutants to harmless
products brought to land or water by precipitation
• Sea salts: a cleansing agent that helps form raindrops
– Picked up by wind flowing over oceans
• Sunlight: breaks down organic molecules
Impacts of air pollutants
• We are exposed to a mixture of pollutants that varies
over time and place.
– Plants may be so stressed from pollution that they
become vulnerable to drought or insects
• Human health: every one of the primary and secondary
air pollutants is a threat to human health.
• Acute exposure can be life threatening.
– Chronic exposure: long-term exposure that causes
gradual deterioration and premature mortality.
• Some pollutants contribute to lung cancer.
Long-term exposure and chronic effects
• Sulfur dioxide: leads to bronchitis (inflammation of
the bronchi)
• Ozone: leads to inflammation and scarring of the
lungs
• Carbon monoxide: reduces the oxygen-carrying
capacity of the blood and leads to heart disease
• Nitrogen oxides: impair lung function and affect the
immune system
• Particulate matter: respiratory and cardiovascular
pathologies
• Other factors (diet, exercise, genetics) influence
effects
Size of selected particulates that
produce the greatest lung damage
Bronchitis and emphysema
Epithelial cell
Cilia
Nasal cavity
Goblet cell
(secreting
mucus)
Oral cavity
Pharynx (throat)
Mucus
Trachea (windpipe)
Bronchioles
Bronchus
Alveolar
duct
Right
lung
Bronchioles
Alveolar sac
(sectioned)
Alveoli
Chronic obstructive pulmonary disease (COPD)
• Chronic obstructive pulmonary disease (COPD)
– A slowly progressive lung disease that makes it hard
to breathe.
– The 4th leading cause of death: affects 18 million in
the U.S.
– Affects 10% of adults over 40 worldwide.
– From smoking and burning wood or dung for fuel.
• Involves three diseases: emphysema (destruction of the
lung alveoli), bronchitis, and asthma.
Asthma
• Most sensitive to air pollution: small children,
asthmatics, those with chronic pulmonary or heart
disease, the elderly
• Asthma: an immune disorder
– Impaired breathing caused by constricted airways
– Is triggered by allergens (dust, mites, mold, pet
dander)
– Is also triggered by pollution (ozone, particulates,
SO2)
• Causes 500,000 hospitalizations/year
– 1.8 million visits to emergency departments
Strong evidence
• Studies of thousands of adults show strong evidence
of harm caused by fine particulates and sulfur
pollution
– Asthma, chronic bronchitis, cardiovascular problems,
etc.
• Higher concentrations of fine particles correlate with
increased mortality from cardiopulmonary disease and
lung cancer.
– The EPA used these studies to regulate fine particles.
• Fine particles and ozone exceeded standards
– Meeting standards would save $28 billion/year in
avoided health costs, missed work, premature
deaths, etc.
Acute and carcinogenic effects
• Air pollution can kill people already suffering from
heart or respiratory diseases.
– Lethal doses also occur in accidental poisoning.
• Moderate air pollution can change cardiac rhythms
in people with heart disease.
– Triggering fatal heart attacks.
• Diesel: a likely human carcinogen.
• Benzene: clearly correlated with cancer
– In motor fuels, solvents, explosives, smoke,
medicines
– Linked to leukemia, blood disorders, damaged
immunity
Smogs and brown clouds
• Industrial smog: smoke + fog
– An irritating, grayish mix of soot, sulfur compounds,
and water vapor
– In industrialized, cool areas that use coal
– China, India, Korea, eastern European countries
• Photochemical smog: in cities with huge freeway
systems
– A brownish, irritating haze in warm, sunny areas
– Arises during the morning traffic
– Pollutants from vehicle exhaust are acted on by
sunlight
– Nitrogen oxides, volatile organic compounds
Industrial and photochemical smog
Creation of industrial and photochemical smog
Industrial smog
Photochemical smog
Atmospheric brown clouds
• Atmospheric brown cloud (ABC): relatively new
– 1–3 km blanket of pollution over south/central
Asia
• Similar to North Temperate Zone’s aerosol pollution
– But persists year round and has a different make
up.
• ABC: black carbon and soot
– From burning biomass and fossil fuels (coal,
diesel).
• Impacts: dimming over large cities, less rainfall,
heating of air, decreased reflection of snow and ice
– Shrinking glaciers will reduce water sources.
– Weaker Indian monsoons, less crops, health
effects.
Industrial smog
Flue gas desulfurization
• SO2 scrubbing, or Flue Gas Desulfurization processes can
be classified as:
 Throwaway or Regenerative, depending upon
whether the recovered sulfur is discarded or recycled.
 Wet or Dry, depending upon whether the scrubber is
a liquid or a solid.
• Flue Gas Desulfurization Processes
The major flue gas desulfurization (FGD ) processes are :
 Limestone Scrubbing
 Lime Scrubbing
 Dual Alkali Processes
 Lime Spray Drying
 Wellman-Lord Process
Limestone scrubbing
• Limestone slurry is sprayed on the incoming flue
gas. The sulfur dioxide gets absorbed The limestone
and the sulfur dioxide react as follows :
CaCO3 + H2O + 2SO2 ----> Ca+2 + 2HSO3-+ CO2
CaCO3 + 2HSO3-+ Ca+2 ----> 2CaSO3 + CO2 + H2O
Lime scrubbing
• The equipment and the processes are similar to those in
limestone scrubbing Lime Scrubbing offers better utilization
of the reagent. The operation is more flexible. The major
disadvantage is the high cost of lime compared to limestone.
The reactions occurring during lime scrubbing are :
CaO + H2O -----> Ca(OH)2
SO2 + H2O <----> H2SO3
H2SO3 + Ca(OH)2 -----> CaSO3.2 H2O
CaSO3.2 H2O + (1/2)O2 -----> CaSO4.2 H2O
Dual alkali system
 Lime and Limestone scrubbing lead to deposits inside
spray tower.
 The deposits can lead to plugging of the nozzles
through which the scrubbing slurry is sprayed.
 The Dual Alkali system uses two regents to remove
the sulfur dioxide.
 Sodium sulfite / Sodium hydroxide are used for the
absorption of sulfur dioxide inside the spray chamber.
 The resulting sodium salts are soluble in water, so no
deposits are formed.
 The spray water is treated with lime or limestone,
along with make-up sodium hydroxide or sodium
carbonate.
 The sulfite/sulfate ions are precipitated, and the
sodium hydroxide is regenerated.
Lime – spray drying
– Lime Slurry is sprayed into the chamber
– The sulfur dioxide is absorbed by the slurry
– The liquid-to-gas ratio is maintained such that the
spray dries before it reaches the bottom of the
chamber
– The dry solids are carried out with the gas, and are
collected in fabric filtration unit
– This system needs lower maintenance, lower capital
costs, and lower energy usage
Wellman – Lord process
• This process consists of the following subprocesses:
– Flue gas pre-treatment.
– Sulfur dioxide absorption by sodium sulfite
– Purge treatment
– Sodium sulfite regeneration.
– The concentrated sulfur dioxide
processed to a marketable product.
stream
is
The flue gas is pre-treated to remove the particulate.
The sodium sulfite neutralizes the sulfur dioxide :
Na2SO3 + SO2 + H2O -----> 2NaHSO3
Wellman – Lord process
• Some of the Na2SO3 reacts with O2 and the SO3
present in the flue gas to form Na2SO4 and NaHSO3.
• Sodium sulfate does not help in the removal of sulfur
dioxide, and is removed. Part of the bisulfate stream
is chilled to precipitate the remaining bisulfate. The
remaining bisulfate stream is evaporated to release
the sulfur dioxide, and regenerate the bisulfite.
Photochemical smog
Production of hydroxyl radicals
NO2
+ sunlight (less than 320 nm)

NO
+
O
photo-dissociation reaction
O
+
O2

O3
+
NO


O3
O + H2O

O3
NO2
+ O2
O2 +
O
.
2 OH (hydroxyl radical)
By this pathway one NO2 molecule produces two hydroxyl radicals.
Destruction of hydroxyl radicals
.
.
OH +
.
NO2
OH + HOO
.
2 OOH

.

HNO3
 H2O + O2
H2O2 + O2
Secondary air pollutants from HCs
Production of peroxyacetyl nitrate (PAN)
.
.
RCHO + OH  R CO + H2O
.
R CO + O2  RCOOO
.
.
.
RCOOO + NO2  RCOOONO2
When R = CH3, CH3COOONO2
Development of photochemical smog over
the Los Angeles area on a typical warm day
• Cold, cloudy weather in a valley surrounded by
mountains can trap air pollutants (left).
• Areas with sunny climate, light winds, mountains on
three sides and an ocean on the other are susceptible
to inversions.
The environmental damages
• Plants are more sensitive than humans to air pollution.
– Sulfur dioxide from smelters and power plants killed
large areas of vegetation.
• Ozone damages crops, orchards, and forests.
– Ozone enters plants through stomata (pores)
– Symptoms of damage: black flecks, yellow leaves
• Crops vary in their susceptibility to ozone.
– Soybeans, corn, wheat are damaged at ambient
ozone levels.
– Countries lose billions of dollars/year in lower yields.
Ozone impact on crop yields
National ambient air quality standards
Indoor air pollution
• Indoor air pollution usually is a greater threat
to human health than outdoor air pollution.
• According to the EPA, the four most dangerous
indoor air pollutants in developed countries are:
– Tobacco smoke.
– Formaldehyde.
– Radioactive radon-222 gas.
– Very small fine and ultrafine particles.
Indoor air pollution
More disease from indoor
(orange) pollution than
outdoor (red)
In developing nations,
indoor cooking fires are
common, and a major
health risk.
Indoor air pollutants
Chloroform
Para-dichlorobenzene
Tetrachloroethylene
1, 1, 1Trichloroethane
Formaldehyde
Benzo-a-pyrene
Nitrogen Oxides
Styrene
Tobacco Smoke
Asbestos
Radon-222
Carbon Monoxide
Methylene Chloride
Concentrations
of
common
indoor
air
pollutants compared with outdoor concentrations
We can reduce indoor air pollution
• In developed countries:
– Use low-toxicity materials, limit use of plastics and
treated wood, monitor air quality, keep rooms clean
– Provide adequate ventilation
– Limit exposure to known toxicants
– Test homes and offices and use CO detectors
• In
–
–
–
developing countries:
Dry wood before burning
Cook outside
Use less-polluting fuels (natural gas)
Bringing air pollution under control
• By the 1960s it was obvious that pollutants were
overloading natural cleansing processes.
– Unrestricted pollution discharge could not
continue
• The Clean Air Act of 1970 (CAA): passed by Congress
– Amended in 1977 and 1990
– Is administered by the EPA
– The foundation of U.S. air-pollution control efforts
– Ambient standards: levels protecting human
health and the environment
National standards
We can reduce smog
• Regulations require new cars to have catalytic
converters
• Require cleaner industrial facilities
– Close those that can’t improve
• Financial incentives to replace aging vehicles
– Restricting driving
• Vehicle inspection programs (“smog checks”)
• Reduce sulfur in diesel; remove lead in gasoline
• Electronic pollution indicator boards raise awareness
• But increased population and cars can wipe out
advances
Reducing particulates
• Before 1970, major sources of particulates were
industrial stacks and open burning of refuse.
– The CAA prevents both of these
– Disposal of refuse is now through landfilling
– Industries installed filters, electrostatic precipitators,
etc. to reduce stack emissions
• Particulates are still emitted from steel mills, power
plants, cement plants, smelters, construction sites,
Diesel engines, wood-burning stoves, fires
Reducing particulates
• The EPA added new ambient air quality standards
for particulates (PM2.5) in 1997
– Particulates smaller than 2.5 micrometers
(microns) are the most dangerous to the lungs.
– Larger particulates are still regulated.
• 24-hour primary standards were lowered.
– Saving $9–$75 billion/year in health benefits
Managing ozone
• To control ozone: address the VOCs and NOx that form it
• Reducing emissions of VOCs from motor vehicles, point
sources (industries), and area sources (dry cleaners, print
shops, household products)
– Since the CAAA, emissions have declined 24%
• The 1997 revised ozone standard was strongly opposed
by industry, but the Supreme Court upheld the EPA
• Implementation was delayed until 2004
– Health benefits far outweighed compliance costs
• Some don’t think the current ozone standard is strict
enough.
Down with NOx
• The EPA implemented regulations to
reduce NOx emissions from mobile
sources, power plants, industrial
boilers, turbines.
• Tier 2 Standards: emissions from all
SUVs, trucks, passenger vans are held
to the same standard as cars.
– They are not being met due to
older, polluting cars.
• NOx Budget Trading Program (NBP):
the EPA establishes a market-based
“cap and trade” system
– Industries
have
flexibility
in
attaining the targets
– States
can
come
back
to
compliance
Structure of the atmosphere
and ozone concentration
Dobson spectrophotometer
•Dobson spectrophotometer invented in 1924, can infer ozone
concentrations from the ground, and helped scientists detect
ozone depletion.
• 1 DU= 2.7 x 1016 O3 molecules /cm2, layer of ozone that would
be 0.001 cm thick at STP(273 K, 1 atm)
UV radiation with
wavelengths between
0.2 and 0.4 μm
Ultraviolet A Ultraviolet B
Thin layer of
dead cells
Squamous Cell Carcinoma
Hair
Epidermis
Squamous
cells
Basal
layer
Sweat
gland
Melanocyte
cells
Melanoma
Basal Cell Carcinoma
Dermis
Basal
cell
Blood
vessels
Squamous Cell Carcinoma Basal Cell Carcinoma
Melanoma
Radiation and importance of the shield
•
•
•
•
•
Skin cancer (700,000 new cases each year)
Premature skin aging
Eye damage
Cataracts
Blindness
Ozone formation
• The ozone shield: stratospheric ozone that absorbs
most (99%) UV radiation
– The 1% UVB radiation causes skin cancer, and
damages crops and other life forms
• Ozone is formed in the stratosphere:
– UV radiation splits O2 molecules into free oxygen (O)
O2 + UVC → O + O
– Some O combines with O2 to form ozone (O3)
O + O2 → O3
Total ozone (Dobson units)
Ozone cycle
400
October monthly means
350
300
250
200
150
100
19551960196519701975198019851990199520002005
Year
Chapman cycle: Steady-State
Concentration of Ozone
O2 + hv (λ <240 nm)

O
+
O
Rate of reaction 1 = K1 [O2]
O +
O2
+ M

O3
+ M
Rate of reaction 2 = K2 [O][O2][M]
O3
+ hv (λ 240 to 320 nm)
Rate of reaction 3 = K3[O3]
O3 + O

2 O2
Rate of reaction 4 = K4[O][O3]

O2
+
O
Ozone is produced only in the second reaction. Ozone is
destroyed in reactions 3 and 4. The ozone steady state can be
expressed as:
O3 production
O3 destruction
Rate of reaction 2 = Rate of reaction 3 + Rate of reaction 4
K2 [O][O2][M] =
K3[O3] +
K4 [O][O3]
5.1
O production
O reduction
2 (rate of 1) + rate 3 = rate of 2 + rate of 4
substituting
2K1[O2] + K3[O3] = K2[O][O2][M] + K4[O][O3]
5.2
subtraction equation 5.1 from equation 5.2 gives
2 K2[O][O2][M] = 2 K1[O2] + 2 K3[O3]
5.3
2 K2[O][O2][M] = 2 K3[O3]
when K1[O2] ≪K3[O3]
5.4
rearranging
[O] =
K3[O3]
K2[M][O2]
addition of 5.1 and 5.2 gives
5.5
2 K1[O2] = 2 K4[O][O3]
5.6
substitute 5.5 into 5.6
2 K1[O2] = 2 K4K3[O3]2
K2[M][O2]
[O3]/[O2] =
K1K2[M]
K3K4
1/2
= 10-4 (실제 10 ppm 존재)
5.7
→ This result suggests that there are additional pathways for
ozone destruction.
Catalytic destruction of ozone
X
+
O3

XO
XO
+
O

X
+
+
O2
O2
The sum of these two reactions is:
O3
XO
+
+
O 
O

2 O2
X
+
Decomposition
reaction
O2
X acts as catalysts,
OH, NO, Cl-, Br-
.
Hydroxyl radical cycles
• Production reaction
.
O + H2O → 2 OH
.
.
O + CH4 → OH + CH3
.
.
H2O + hν → H + OH
.
• Destruction cycle reaction(40 O3/1 OH)
.
.
OH + O3 → OOH + O2
.
.
OOH + O → OH + O2
Net reaction: O + O3 → 2O2
NO Cycle
N2O
+ hv
 N2
+
O*
Below 30 km, in the stratosphere, the excited state
oxygen reacts with the N2O to produce nitric oxide.
N2O
+
O*

2 NO
.
NO can act as an X catalytic species (105 O3/NOx radical).
NO
.
+
.
O3 
.
.
NO2
+
O2
NO2 + O 
NO + O2
-------------------------------------Net reaction:
O + O3  2O2
Halogens in the atmosphere
• Chlorofluorocarbons (CFCs): halogenated hydrocarbons
– Nonreactive, nonflammable, nontoxic organic
molecules
– Chlorine and fluorine atoms replace some hydrogens.
– Normally gaseous, but liquefy under some pressure
• Used in refrigerators, air conditioners, heat pumps
– Production of plastic foams
– Cleaning computer parts
– Pressuring agent in aerosol cans
Reaction of ozone with CFC
Ultraviolet light hits a chlorofluorocarbon
(CFC) molecule, such as CFCl3, breaking
off a chlorine atom and leaving
CFCl2.
Sun
Cl
Cl
C
F
Cl
UV radiation
Once free, the chlorine atom is off
to attack another ozone molecule
and begin the cycle again.
Cl
Cl
O O
The chlorine atom attacks
an ozone (O3) molecule,
pulling an oxygen atom
O
off it and leaving
O
O
an oxygen
molecule (O2).
Cl
Summary of Reactions
CCl3F + UV Cl. + CCl2F
Cl. + O3 ClO. + O2 Repeated
ClO. + O Cl. + O2
many times
A free oxygen atom pulls
the oxygen atom off
the chlorine monoxide
Cl
molecule to form O2.
O
O
Cl
The chlorine
atom and the
O
oxygen atom join
O
to form a chlorine
O
monoxide molecule
(ClO.).
Chlorine is regenerated after reacting with O3
• Chlorine catalytic cycle: chlorine is regenerated
– Chlorine acts as a catalyst—promoting a chemical
reaction without being used up.
• Every chlorine molecule lasts 40–100 years in the air.
– It can break down 100,000 O3 molecules.
• The EPA banned CFC use in aerosol cans in 1978.
– Manufacturers switched to nondamaging substitutes.
– CFCs were still used in other applications.
• Bromine, a soil fumigant and pesticide, depletes ozone
– It is 60 times as potent as Cl.
Null cycles
Prevent catalytic species from taking part in the catalytic
cycles
NO2 + O3  NO3 + O2
NO3 + hv  NO2 + O
---------------------------------------net reaction: O3 + hv  O2 + O
Some of the NO3 can react to produce N2O5:
NO3
.
NO2
+
+
.
Cl
.
NO2
OH
+
+
CH4
N2O5
+M
M

 HNO3
HCl
+
.
+
CH3
+
M
M
As the atmospheric concentration of ClO.
increases, the concentration of O3 decreases
Ozone-depleting substances production
and presence in the atmosphere
CFCs
HCFCs
Relative abundances of chlorine and bromine
in the stratosphere
The ozone “hole”
• In 1985, scientists noticed a hole (serious thinning) in
the stratospheric ozone layer over the South Pole.
– Ozone levels were 50% lower than normal.
• In summer: gases (NO2, methane) trap Cl, preventing
ozone depletion.
• The Antarctic winter (in June) creates a vortex that traps
stratospheric gases.
• Extremely cold temperatures create stratospheric clouds.
– Cloud surfaces allow chemical reactions to release Cl2.
The Antarctic’s ozone “hole”
• Spring sunlight breaks up the stratospheric clouds.
• UV light attacks molecular chlorine.
– Initiating the chlorine cycle, which destroys ozone.
• Summer (November) breaks up the vortex.
– Ozone-rich air returns to the area.
• Ozone-poor air has spread over the Southern Hemisphere.
– Queensland, Australia: two out of three are expected
to develop skin cancer.
• The ozone hole is now the size of North America.
• The Arctic does have 25% depletion, but no ozone hole.
Ozone depletion in Antarctica
Antarctic ozone depletion
ClONO2 (g)
HOCl (g)
Cl2
+
+
+
.
2 Cl
+
.
ClO
+
ClOOCl
.
HCl (s)  Cl2 (g) +
HCl (s)
 Cl2 (g)
hv

H2O (s)
.
.
.
+
2 Cl
2 O3  2 ClO
ClO
HNO3 (s)
+
2 O2
 ClOOCl
.
+ hv  ClOO
.
.
+ Cl
ClOO
 Cl + O2
----------------------------------------------net reaction: 2 O3 + hv  3 O2
Stratospheric ozone depletion
•
In 1985, the
Antarctica.
“ozone
hole”
was
detected
over
• Ozone levels had declined 40–60% over the previous
decade.
Ozone loss and the extent of the ozone hole
Annual record of ozone hole area
Further ozone depletion
• Worldwide ozone losses of 3%–6% from 2002 to 2007.
• Ozone losses of the 1980s will have caused 12 million
people in the U.S. to develop skin cancer.
– More UVB radiation than ever reached Earth
• In 1987, UN member countries met in Montreal,
Canada.
– The Montreal Protocol: 194 nations agreed to scale
back CFC production 50% by 2000
– Amendments moved the target date for complete
phaseout of CFCs to January 1, 1996
– CFCs are still being used.
International efforts to reduce
stratospheric ozone depletion
• In 1987, 196 nations signed the Montreal Protocol,
which restricted CFC production in half by 1998
• Follow-up agreements deepened cuts, advanced
timetables, and addressed other ozone-depleting
chemicals
– Industry shifted to safer, inexpensive, and efficient
alternatives
• Challenges still face us
– CFCs will remain in the stratosphere for a long time
– Nations can ask for exemptions to the ban
• The Montreal Protocol is one of the biggest
environmental success stories of our time.
• Reasons for success of the Montreal Protocol:
• Government and industry cooperated on finding
solutions (cheap replacement technologies for CFCs,
CF3CH2F), so battles typical to environmental
debates were minimized.
• Protocol
was
implemented
with
“adaptive
management”—ability to fine-tune actions as time
goes on, in response to new data or conditions.
Protecting the ozone layer
International agreements
reduced ozone-depleting
substances
The hole in the ozone
has stopped growing
Final thoughts
• The ozone story is a remarkable episode in human
history.
• The world has shown that it can respond collectively
and effectively to a clearly perceived threat.
• The scientific community played a crucial role.
– Alerting the world
– Researching the threat
• Skeptics
stressed
uncertainties
but
scientific
consensus convinced politicians to act.