The Ozone Hole
The discovery of the ozone hole
? The British Antarctic Survey has been monitoring,for
many years,the total column ozone levels at its base
at Halley Bay in the Antarctica.
? Monitoring data indicate that column ozone levels
have been decreasing since 1977.
? This observation was later confirmed by satellite data
(TOMS-Total Ozone Mapping Spectrometer)
– Initially satellite data were assumed to be wrong
with values lower than 190DU
October ozone hole over Antarctic
Features of the ozone hole
? Ozone depletion occurs at altitudes between 10
and 20 km
– If O3 depletion resulted from the ClOx cycle,the
depletion would occur at middle and lower latitude
and altitudes between 35 and 45 km.
– The ClOx cycle requires O atom,but in the polar
stratosphere,the low sun elevation results in
essentially no photodissociation of O2,
– The above observation could not be explained by
the ClOx destruction mechanism alone.
? Depletion occurs in the Antarctic spring
Special Features of Polar Meteorology
? During the winter polar night,sunlight does not reach the
south pole,
? A strong circumpolar wind develops in the middle to lower
stratosphere; These strong winds are known as the 'polar
vortex',
? In the winter and early spring,the polar vortex is
extremely stable,sealing off air in the vortex from that
outside,
? The exceptional stability of the vortex in Antarctic is the
result of the almost symmetric distribution of ocean
around Antarctica.
? The air within the polar vortex can get very cold.
? Once the air temperature gets to below about -80C (193K),
Polar Stratospheric Clouds (or PSCs for short) are formed.
Polar vortex
? The polar vortex is a persistent large-scale
cyclonic circulation pattern in the middle and
upper troposphere and the stratosphere,
centered generally in the polar regions of
each hemisphere,
? The polar vortex is not a surface pattern,It
tends to be well expressed at upper levels of
the atmosphere (> 5 km).
Polar Stratospheric Clouds (PSCs)
? PSCs first form as nitric acid trihydrate (HNO3.3H2O)
once temperature drops to 195K.
? As the temperature gets colder,larger droplets of
water-ice with nitric acid dissolved in them can form.
? PSCs occur at heights of 15-20km.
Why do PSCs occur at heights of 15-20 km?
? The long polar night produces temperature as
low as 183 k (-90oC) at heights of 15 to 20
km.
? The stratosphere contains a natural aerosol
layer at altitudes of 12 to 30 km.
PSCs promote the conversion of inorganic Cl and
Cl reservoir species to active Cl
Pathway 1, HCl(g) ? Cl2 (g)
? Absorption of gaseous HCl by PSCs occurs very
efficiently
HCl(g) ? HCl(s)
? Heterogeneous reaction of gaseous ClONO2 with HCl
on the PSC particles
HCl(s) + ClONO2 ? HNO3 (s) + Cl2
where s denotes the PSC surface
Note,The gas phase reaction between HCl and ClONO2 is
extremely slow,
PSCs promote the conversion of inorganic Cl and
Cl reservoir species to active Cl (Continued)
? Pathway 2,HCl(g)?ClNO2 (g) in the presence of
N2O5
HCl(g) ? HCl(s)
HCl(s) + N2O5 ? ClNO2 + HNO3 (s)
? Pathway 3,ClONO2(g)?HOCl (g)
ClONO2 + H2O (s) ? HOCl + HNO3 (s)
The gas phase reactions between HCl and N2O5,
between ClONO2 and H2O are too slow to be important.
Why PSCs promote the conversion of inorganic
Cl and Cl reservoir species to active Cl?
1,PSCs concentrate the reactant molecules.
2,Formation of HNO3 is assisted by hydrogen
bonding to the water molecules in the PSC particles.
Active Cl species can rapidly yield Cl
atoms when light is available
? Active Cl species include Cl2,HOCl,and ClNO2
? Active Cl species readily photolyze to yield Cl atoms
when daylight returns in the springtime.
Cl2 + hv ? 2Cl
HOCl + hv ? HO + Cl
ClNO2 + hv ? Cl + NO2
Polar ClOx cycle to remove O3
? Polar regions,lack of O atom because of low sun
elevation?The ordinary ClOx cycle is not operative
since it requires the presence of O atom.
? Under polar atmospheric conditions,the reaction
sequence to remove O3 is as follows
Cl + O3 ?ClO + O2
ClO + ClO ? ClO-OCl
ClO-OCl + hv? ClOO + Cl
ClOO + hv ? Cl + O2
2 [Cl + O3 ? ClO + O2]
Net of the last FOUR reactions,2O3 + hv ? 3O2
How does the polar ClOx cycle stop?
? The chain reaction is stopped when the ice particles
melt,releasing adsorbed HNO3.
– HNO3 + hv ?,OH + NO2
? NO2 sequesters ClO.,which shuts down the polar
ClOx chain reaction
– NO2 +,ClO ? ClONO2
Evidence linking ClO generation and O3 loss
ClO mixing ratios in the high-latitude stratosphere are
several orders of magnitude higher than those in the mid-
latitude stratosphere.
Denitrification by PSCs enhances polar ClOx
cycle
? PSCs removes gaseous N species (denitrification)
– Major process,formation of nitric acid trihydrate (NAT)
PSCs
– Minor process,Formation of HNO3 from gaseous N
species (e.g,ClONO2 and N2O5) and subsequent
retention of HNO3(s).
– As PSCs particles grow larger over the winter,they sink
to lower altitudes,falling out of the stratosphere.
Denitrification by PSCs enhances polar
ClOx cycle (Continued)
? If HNO3 is not removed from the stratosphere,it
releases NO2 back to the stratosphere upon
photolysis.
HNO3 + hv ? OH + NO2
? The consequence of released NO2 is to tie up active
chlorine as ClONO2 and make the ClOx polar cycle
less efficient.
ClO + NO2 ? ClONO2
Summary of the roles played by PSCs
? Provide surface for the conversion of inactive
Cl species into active species
? Provide the media for removal of gaseous N
species
Reaction sequence responsible for Antarctic ozone hole
Schematic of photochemical and dynamical features of polar
ozone depletion
Summary,Ingredients for the Antarctica
ozone hole formation
? Cold temperatures; cold enough for the
formation of Polar Stratospheric Clouds,
– Polar winter leading to the formation of the polar
vortex which isolates the air within it,
– As the vortex air is isolated,the cold
temperatures persist,
– This allows the growth of PSCs and subsequent
sink to lower altitude,therefore removal of
gaseous N species.
? Sunlight (to initiate O3 depletion reaction
sequence).
Does ozone hole occur in the north pole
(Arctic)?
? The Arctic winter stratosphere is generally warmer than
the Antarctic by ~10k.
– Caused by the water mass covering the Arctic.
? The warmer temperature results in less PSCs and
shorter presence time.
? The less abundant and less persistent PSCs dramatically
reduce the extent of denitrification.
– PSCs in the Arctic does not have sufficient time to settle
out of the stratosphere.
– PSCs releases their HNO3 back to the stratosphere,
making ClOx polar cycle less efficient.
? Conclusion,Ozone depletion is less dramatic in the
Arctic compared with the Antarctic,
Summary on ozone hole
? Massive ozone loss requires both very cold temperature
(to form PSCs) and sunlight (to photolyze reactive
chlorine to produce Cl atoms),
? Denitrification is required to prevent reformation of
reservoir species once photolysis ensures,
? Denitrification occurs when PSCs containing HNO3
settling out of the stratosphere,
? The massive springtime loss of ozone in the Antarctic
stratosphere (the Ozone hole) is conclusively linked to
anthropogenic halogens,
? Virtually all inorganic chlorine is converted into active
chlorine every winter in both the Antarctic and Arctic
stratosphere as a result of heterogeneous reactions of
reservoir species on polar stratospheric clouds (PSCs).
Summary on ozone hole (Continued)
? The most important difference between the
Antarctic and the Arctic stratosphere is the
extent of denitrification that occurs,
? Because of generally warmer temperatures in
the Arctic,PSCs tend not persist until the onset
of sunlight,releasing their nitric acid back into
the vapor phase,
? As a result,ozone depletion is generally less
dramatic in the Arctic than the Antarctic.
Ozone depletion potential (ODP)
? ODP is used to facilitate comparison of harmfulness
to the ozone layer by different chemicals.
? ODP of a compound is defined as the total steady-
ozone destruction that results from per unit mass of
species i emitted per year relative to that for a unit
mass emission of CFC-11
113
3
?
?
?
?
CF C
i
O
O
O D P i
What influences ODP?
? Lifetime in the troposphere
– The more effective the tropospheric removal processes,
the less of the compound that will survive to reach the
stratosphere.
? Altitude at which a compound is broken down in the
stratosphere
– Ozone is more abundant in the lower stratosphere
– Substitution of F atoms for Cl atoms makes a compound
break down at higher altitude ? less efficient in
destroying O3.
? Distribution of halogen atoms,Cl,Br,and F,contained
within the molecule
– Molecule for molecule,F<Cl<Br in ozone destruction
? Chemistry subsequent to its dissociation
What controls a compound’s lifetime in the
troposphere?
Reaction with OH radical
][
1
OHk OHi
??
Lifetime in the troposphere
kOH reaction constant
[OH] tropospheric average
OH concentration
ODPs of Selected Compounds
Compound ODP
CFC-11 (CFCl3) 1.0
CFC-113 (CCl2FClF2) 0.8
CCl4 1.20
CFBr3 12
CH3CCl3 0.12
HCFC-22 (CF2HCl) 0.055
CH3Cl 0.02
CH3Br 0.64
CFC substitutes
? The main strategy has been to explore the suitability
of hydrochlorofluorocarbons
– The Cl and/or F substituents lend HCFCs some of the
desirable properties of CFCs (e.g,low reactivity,fire
suppression,good insulating and solvent
characteristics,boiling point suitable for use in
refrigerator cycles)
– The presence of C-H bond reduces the tropospheric
lifetime significantly
? HCFCs are only transitional CFC substitutes
The discovery of the ozone hole
? The British Antarctic Survey has been monitoring,for
many years,the total column ozone levels at its base
at Halley Bay in the Antarctica.
? Monitoring data indicate that column ozone levels
have been decreasing since 1977.
? This observation was later confirmed by satellite data
(TOMS-Total Ozone Mapping Spectrometer)
– Initially satellite data were assumed to be wrong
with values lower than 190DU
October ozone hole over Antarctic
Features of the ozone hole
? Ozone depletion occurs at altitudes between 10
and 20 km
– If O3 depletion resulted from the ClOx cycle,the
depletion would occur at middle and lower latitude
and altitudes between 35 and 45 km.
– The ClOx cycle requires O atom,but in the polar
stratosphere,the low sun elevation results in
essentially no photodissociation of O2,
– The above observation could not be explained by
the ClOx destruction mechanism alone.
? Depletion occurs in the Antarctic spring
Special Features of Polar Meteorology
? During the winter polar night,sunlight does not reach the
south pole,
? A strong circumpolar wind develops in the middle to lower
stratosphere; These strong winds are known as the 'polar
vortex',
? In the winter and early spring,the polar vortex is
extremely stable,sealing off air in the vortex from that
outside,
? The exceptional stability of the vortex in Antarctic is the
result of the almost symmetric distribution of ocean
around Antarctica.
? The air within the polar vortex can get very cold.
? Once the air temperature gets to below about -80C (193K),
Polar Stratospheric Clouds (or PSCs for short) are formed.
Polar vortex
? The polar vortex is a persistent large-scale
cyclonic circulation pattern in the middle and
upper troposphere and the stratosphere,
centered generally in the polar regions of
each hemisphere,
? The polar vortex is not a surface pattern,It
tends to be well expressed at upper levels of
the atmosphere (> 5 km).
Polar Stratospheric Clouds (PSCs)
? PSCs first form as nitric acid trihydrate (HNO3.3H2O)
once temperature drops to 195K.
? As the temperature gets colder,larger droplets of
water-ice with nitric acid dissolved in them can form.
? PSCs occur at heights of 15-20km.
Why do PSCs occur at heights of 15-20 km?
? The long polar night produces temperature as
low as 183 k (-90oC) at heights of 15 to 20
km.
? The stratosphere contains a natural aerosol
layer at altitudes of 12 to 30 km.
PSCs promote the conversion of inorganic Cl and
Cl reservoir species to active Cl
Pathway 1, HCl(g) ? Cl2 (g)
? Absorption of gaseous HCl by PSCs occurs very
efficiently
HCl(g) ? HCl(s)
? Heterogeneous reaction of gaseous ClONO2 with HCl
on the PSC particles
HCl(s) + ClONO2 ? HNO3 (s) + Cl2
where s denotes the PSC surface
Note,The gas phase reaction between HCl and ClONO2 is
extremely slow,
PSCs promote the conversion of inorganic Cl and
Cl reservoir species to active Cl (Continued)
? Pathway 2,HCl(g)?ClNO2 (g) in the presence of
N2O5
HCl(g) ? HCl(s)
HCl(s) + N2O5 ? ClNO2 + HNO3 (s)
? Pathway 3,ClONO2(g)?HOCl (g)
ClONO2 + H2O (s) ? HOCl + HNO3 (s)
The gas phase reactions between HCl and N2O5,
between ClONO2 and H2O are too slow to be important.
Why PSCs promote the conversion of inorganic
Cl and Cl reservoir species to active Cl?
1,PSCs concentrate the reactant molecules.
2,Formation of HNO3 is assisted by hydrogen
bonding to the water molecules in the PSC particles.
Active Cl species can rapidly yield Cl
atoms when light is available
? Active Cl species include Cl2,HOCl,and ClNO2
? Active Cl species readily photolyze to yield Cl atoms
when daylight returns in the springtime.
Cl2 + hv ? 2Cl
HOCl + hv ? HO + Cl
ClNO2 + hv ? Cl + NO2
Polar ClOx cycle to remove O3
? Polar regions,lack of O atom because of low sun
elevation?The ordinary ClOx cycle is not operative
since it requires the presence of O atom.
? Under polar atmospheric conditions,the reaction
sequence to remove O3 is as follows
Cl + O3 ?ClO + O2
ClO + ClO ? ClO-OCl
ClO-OCl + hv? ClOO + Cl
ClOO + hv ? Cl + O2
2 [Cl + O3 ? ClO + O2]
Net of the last FOUR reactions,2O3 + hv ? 3O2
How does the polar ClOx cycle stop?
? The chain reaction is stopped when the ice particles
melt,releasing adsorbed HNO3.
– HNO3 + hv ?,OH + NO2
? NO2 sequesters ClO.,which shuts down the polar
ClOx chain reaction
– NO2 +,ClO ? ClONO2
Evidence linking ClO generation and O3 loss
ClO mixing ratios in the high-latitude stratosphere are
several orders of magnitude higher than those in the mid-
latitude stratosphere.
Denitrification by PSCs enhances polar ClOx
cycle
? PSCs removes gaseous N species (denitrification)
– Major process,formation of nitric acid trihydrate (NAT)
PSCs
– Minor process,Formation of HNO3 from gaseous N
species (e.g,ClONO2 and N2O5) and subsequent
retention of HNO3(s).
– As PSCs particles grow larger over the winter,they sink
to lower altitudes,falling out of the stratosphere.
Denitrification by PSCs enhances polar
ClOx cycle (Continued)
? If HNO3 is not removed from the stratosphere,it
releases NO2 back to the stratosphere upon
photolysis.
HNO3 + hv ? OH + NO2
? The consequence of released NO2 is to tie up active
chlorine as ClONO2 and make the ClOx polar cycle
less efficient.
ClO + NO2 ? ClONO2
Summary of the roles played by PSCs
? Provide surface for the conversion of inactive
Cl species into active species
? Provide the media for removal of gaseous N
species
Reaction sequence responsible for Antarctic ozone hole
Schematic of photochemical and dynamical features of polar
ozone depletion
Summary,Ingredients for the Antarctica
ozone hole formation
? Cold temperatures; cold enough for the
formation of Polar Stratospheric Clouds,
– Polar winter leading to the formation of the polar
vortex which isolates the air within it,
– As the vortex air is isolated,the cold
temperatures persist,
– This allows the growth of PSCs and subsequent
sink to lower altitude,therefore removal of
gaseous N species.
? Sunlight (to initiate O3 depletion reaction
sequence).
Does ozone hole occur in the north pole
(Arctic)?
? The Arctic winter stratosphere is generally warmer than
the Antarctic by ~10k.
– Caused by the water mass covering the Arctic.
? The warmer temperature results in less PSCs and
shorter presence time.
? The less abundant and less persistent PSCs dramatically
reduce the extent of denitrification.
– PSCs in the Arctic does not have sufficient time to settle
out of the stratosphere.
– PSCs releases their HNO3 back to the stratosphere,
making ClOx polar cycle less efficient.
? Conclusion,Ozone depletion is less dramatic in the
Arctic compared with the Antarctic,
Summary on ozone hole
? Massive ozone loss requires both very cold temperature
(to form PSCs) and sunlight (to photolyze reactive
chlorine to produce Cl atoms),
? Denitrification is required to prevent reformation of
reservoir species once photolysis ensures,
? Denitrification occurs when PSCs containing HNO3
settling out of the stratosphere,
? The massive springtime loss of ozone in the Antarctic
stratosphere (the Ozone hole) is conclusively linked to
anthropogenic halogens,
? Virtually all inorganic chlorine is converted into active
chlorine every winter in both the Antarctic and Arctic
stratosphere as a result of heterogeneous reactions of
reservoir species on polar stratospheric clouds (PSCs).
Summary on ozone hole (Continued)
? The most important difference between the
Antarctic and the Arctic stratosphere is the
extent of denitrification that occurs,
? Because of generally warmer temperatures in
the Arctic,PSCs tend not persist until the onset
of sunlight,releasing their nitric acid back into
the vapor phase,
? As a result,ozone depletion is generally less
dramatic in the Arctic than the Antarctic.
Ozone depletion potential (ODP)
? ODP is used to facilitate comparison of harmfulness
to the ozone layer by different chemicals.
? ODP of a compound is defined as the total steady-
ozone destruction that results from per unit mass of
species i emitted per year relative to that for a unit
mass emission of CFC-11
113
3
?
?
?
?
CF C
i
O
O
O D P i
What influences ODP?
? Lifetime in the troposphere
– The more effective the tropospheric removal processes,
the less of the compound that will survive to reach the
stratosphere.
? Altitude at which a compound is broken down in the
stratosphere
– Ozone is more abundant in the lower stratosphere
– Substitution of F atoms for Cl atoms makes a compound
break down at higher altitude ? less efficient in
destroying O3.
? Distribution of halogen atoms,Cl,Br,and F,contained
within the molecule
– Molecule for molecule,F<Cl<Br in ozone destruction
? Chemistry subsequent to its dissociation
What controls a compound’s lifetime in the
troposphere?
Reaction with OH radical
][
1
OHk OHi
??
Lifetime in the troposphere
kOH reaction constant
[OH] tropospheric average
OH concentration
ODPs of Selected Compounds
Compound ODP
CFC-11 (CFCl3) 1.0
CFC-113 (CCl2FClF2) 0.8
CCl4 1.20
CFBr3 12
CH3CCl3 0.12
HCFC-22 (CF2HCl) 0.055
CH3Cl 0.02
CH3Br 0.64
CFC substitutes
? The main strategy has been to explore the suitability
of hydrochlorofluorocarbons
– The Cl and/or F substituents lend HCFCs some of the
desirable properties of CFCs (e.g,low reactivity,fire
suppression,good insulating and solvent
characteristics,boiling point suitable for use in
refrigerator cycles)
– The presence of C-H bond reduces the tropospheric
lifetime significantly
? HCFCs are only transitional CFC substitutes
