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

O

O

While stable, it is highly reactive. The Greek word ozein means “to smell” and O3 has a strong pungent odor. Electric discharges in air often produce significant quantities of O3 and you may have smelled O3 near these sources.

Ozone formation and destruction in the stratosphere

Chapman Theory

a) O2+ hv (<242nm) -> 2O

b) O+O2+M -> O3+M

c) O3 + hv (<320nm) O +O2

d) O + O32O2

Where M is a random air molecule (O2 or N2)

Chapman theory describes how sunlight converts the various forms of oxygen from one to another, explains why the highest content of ozone occur in the layer between 15 and 50 km, termed the ozone layer

formation

Destruction

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dc

ba

kk

Mkk

O

OSteady-state

O3 concentration

Prediction by Chapman theory vs. Observation

Using Chapman theory

There must be other O3 destruction pathways Catalytic ozone destruction

X + O3 = XO + O2XO + O = X + O2O + O3 = 2 O2Net reaction

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 O3 destruction

• Hydroxy radical (OH).OH + O3 = HO2. + O2

HO2. + O = .OH + O2Net: O + O3 = 2 O2

• Chlorine and bromine (Cl and Br)

Cl. + O3 = ClO. + O2

ClO. + O = Cl. + O2Net: O + O3 = 2 O2

• Nitric oxide (NO)NO + O3 = NO2 + O2NO2 + O = NO + O2Net: O + O3 = 2 O2

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).

• SourcesO3 + hv (<325nm) = O2 + O1D (2%)

= O2 + O3P (98%)

O1D + H2O = 2 .OH (major)

O1D + CH4 = .OH +CH3. (minor)• Termination reaction

.OH + NO2 HNO3

Chlorine atom Sources: Photolysis of Cl-containing compounds in the

stratosphere.CFCl3 + hv (185-210nm) CFCl2. + Cl. CF2Cl2 + hv (185-210nm) CF2Cl. + Cl.

Subsequent reactions of CFCl2 and CF2Cl more Cl atoms

The principal Cl-containing species are:CF2Cl2, CFCl3, CFCl2, CF2Cl, CCl4, CH3CCl3,

CF2HCl, CH3ClSources 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: CFCl3, CF2Cl2, and CF2ClCFCl2.

• 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. + CH4 CH3. + HClStable in the stratosphere

Removed from air by precipitation when it migrates to the troposphere

ClO. + NO2 + M ClONO2 + MReservoir species

Relatively unreactive but can regenerate reactive species upon suitable conditions

ClONO2 + hvClO + NO2

ClONO2 + hvCl + NO3

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 (N2O), which is much less reactive than NO.N2O + hv N2 + O (90%)N2O + 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 O3 destruction.

• NOx inhibit the HOx and ClOx cycles for O3 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 O3.– (NOx accounts for >50% of total ozone

destruction in the middle and upper troposphere.)

– In the lower stratosphere, the net effect is to protect O3 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 O3 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)