Stratospheric Ozone Depletion

Environmental Chemistry
Part 2 Atmospheric Chemistry
2.6 Stratospheric Ozone Depletion

Ozone in the atmosphere

The ozone layer

Ultraviolet protection by ozone
Ozone absorbs UV light in the solar irradiation that is
harmful to life

Ultraviolet protection by ozone
The overlap of ground level radiation with the sunburn
sensitivity curve would be much greater without the filtering
effects of the ozone layer.

Express ozone abundance
• Total column ozone is the total amount of ozone
integrated from the surface to the top of the
atmosphere.
• Dobson Units (DU) is used to express the total column
ozone, named after G.M.B. Dobson, a scientist who
conducted pioneering measurements of the
stratosphere in the 1920s and 1930s.
• One DU is the thickness, measured in units of
hundredths of a millimeter (0.01 mm), that the ozone
column would occupy at standard temperature and
pressure (273 K and 1 atm)

Typical ozone column values
• Total ozone column value ranges from 290 to
310 DU over the globe.
• If all the atmosphere's ozone were brought
down to the earth's surface at standard
pressure and temperature, it would produce a
layer of about 3mm thick.
• Ozone depletion: when sum of ozone over
height is lower than 2/3 of the normal value,
we say "ozone depletion" occurs.

What is ozone?
Ozone is a stable molecule
composed of three oxygen atoms. O
While stable, it is highly reactive. The Greek word ozein
means “to smell” and O 3 has a strong pungent odor.
Electric discharges in air often produce significant
quantities of O 3 and you may have smelled O 3 near these
sources.
O
O

Ozone formation and destruction in the
stratosphere

Chapman Theory
a) O 2 + hv ( 2O
b) O+O 2 +M -> O 3 +M
c) O 3 + hv (

Prediction by Chapman theory vs. Observation
Using Chapman theory

There must be other O 3 destruction pathways
Catalytic ozone destruction
Net reaction
X + O 3 = XO + O 2
XO + O = X + O 2
O + O 3 = 2 O 2
X is a regenerated in the process – act as a catalyst.
The chain reaction continues until X is removed by some
side reaction.

The important catalysts for stratospheric
O 3 destruction
• Hydroxy radical (OH)
. .
OH + O3 = HO2 + O2
HO 2
. + O = . OH + O2
Net: O + O 3 = 2O 2
• Chlorine and bromine (Cl and Br)
Cl . + O 3 = ClO . + O 2
ClO . + O = Cl . + O 2
Net: O + O 3 = 2O 2
• Nitric oxide (NO)
NO + O 3 = NO 2 + O 2
NO 2 + O = NO + O 2
Net: O + O 3 = 2 O 2
HOx cycle
ClOx cycle
NOx cycle

Hydroxy radical
• Accounts for nearly one-half of the total ozone
destruction in the lower stratosphere (16-20 km).
• Sources
O3 + hv (

Chlorine atom
Sources:
Photolysis of Cl-containing compounds in the stratosphere.
CFCl3 + hv (185-210nm) CFCl .
2 + Cl .
CF2 Cl2 + hv (185-210nm) CF2 Cl . + Cl .
Subsequent reactions of CFCl2 and CF2 Cl more Cl atoms
The principal Cl-containing species are:
CF2 Cl2 , CFCl3 , CFCl2 , CF2 Cl, CCl4 , CH3 CCl3 , CF2 HCl, CH3 Cl
Sources for Cl-containing compounds (need to be long-
lived in the troposphere)
•Man-made: e.g. CFCs
•Natural: e.g. methyl chloride from biomass burning.

Chlorofluorocarbons (CFCs)
• CFCs is the abbreviated form of ChloroFluoroCarbons, a
collective name given to a series of compounds
containing chlorine, fluorine and carbon atoms.
Examples: CFCl 3 , CF 2 Cl 2 , and CF 2 ClCFCl 2 .
• Related names
– HCFCs: Hydrochloroflorocarbons, halocarbons
containing hydrogen atoms in addition to chlorine,
fluorine and carbon atoms.
– HFCs: hydroflorocarbons, halocarbons containing
atoms of hydrogen in addition to fluorine and carbon
atoms.
– Perhalocarbons: halocarbons in which every available
carbon bond contains a haloatoms.
– Halons: bromine-containing halocarbons, especially
used as fire extinguishing agents.

Chlorine atom (Continued)
Termination reactions for Cl
Cl . + CH 4
ClO . + NO 2 + M
Reservoir species
CH 3 . + HCl
Stable in the stratosphere
Removed from air by precipitation
when it migrates to the troposphere
ClONO 2 + M
Relatively unreactive but can regenerate
reactive species upon suitable conditions
ClONO 2 + hvClO + NO 2
ClONO 2 + hvCl + NO 3

Nitric oxide
• NO is produced abundantly in the troposphere,
but all of it is converted into NO2 HNO3 (removed through precipitation)
• NO in the stratosphere produced from nitrous
oxide (N2 O), which is much less reactive than NO.
N2 O + hv N2 + O (90%)
N2 O + O 2 NO (~10%)
• Removal processes:
NO2 + . OH HNO3 ClO . + NO2 ClONO2 Inhibit the HOx
and ClOx cycles

The two-sided effect of NOx
• NOx provides a catalytic chain mechanism for
O 3 destruction.
• NOx inhibit the HOx and ClOx cycles for O 3
destruction by removing radical species in the
two cycles.
• The relative magnitude of the two effects is
altitude dependent.
– >25 km, the net effect is to destruct O 3 .
– (NOx accounts for >50% of total ozone destruction
in the middle and upper troposphere.)
– In the lower stratosphere, the net effect is to protect
O 3 from destruction.

The catalytic destruction reactions described so far,
together with the Chapman cycle, account for the
observed average levels of stratospheric ozone, they
are unable to account for the ozone hole over
Antarctica.
The ozone depletion in the Antarctica is limited both
regionally and seasonally. The depletion is too great
and too sudden. These observations can not be
explained by catalytic O 3 destruction by ClOx alone.

Numbering system for CFCs and HCFCs
CFC-XYZ
1) Z = number of fluorine atoms.
2) Y =1 + number of hydrogen atoms.
3) X = number of carbon atoms -1
When X=0 (i.e., only one carbon compound), it is omitted.
4) The number of chlorine atoms in the compound is found by
subtracting the sum of fluorine and hydrogen atoms from the
total number of atoms that can be connected to the carbon
atoms.
5) Examples:
CCl2F2 (CFC-12, refrigerant)
CCl3F (CFC-11, blowing agent)
CHClF2 (CFC-22, refrigerant, blowing agent)
C2Cl2F4 (CFC-114)

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 (HNO 3 . 3H2 O)
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 ClONO 2 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
N2 O5 HCl(g) HCl(s)
HCl(s) + N 2 O 5
ClNO 2 + HNO 3 (s)
• Pathway 3: ClONO 2 (g)HOCl (g)
ClONO 2 + H 2 O (s)
HOCl + HNO 3 (s)
The gas phase reactions between HCl and N 2 O 5 ,
between ClONO 2 and H 2 O 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 O 3
• 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
3O 2

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
ClONO 2

Evidence linking ClO generation and O 3 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 N2 O5 ) 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
ClONO 2

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 O 3 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 HNO 3 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 HNO 3
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 steadyozone
destruction that results from per unit mass of
species i emitted per year relative to that for a unit
mass emission of CFC-11
ODP i
O
O
3
3
i
CFC 11

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 O 3 .
• Distribution of halogen atoms, Cl, Br, and F, contained
within the molecule
– Molecule for molecule, F

What controls a compound’s lifetime in the
troposphere?
Reaction with OH radical
Lifetime in the troposphere
i
k OH
1
[ OH
k OH reaction constant
[OH] tropospheric average
OH concentration
]

ODPs of Selected Compounds
Compound ODP
CFC-11 (CFCl 3 ) 1.0
CFC-113 (CCl 2 FClF 2 ) 0.8
CCl 4
CFBr 3
1.20
12
CH3 CCl3 0.12
HCFC-22 (CF2 HCl) 0.055
CH 3 Cl 0.02
CH 3 Br 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