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Stratospheric Ozone Depletion

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					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                    O
  composed of three oxygen atoms.          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                 formation
                 b) O+O2+M -> O3+M
          c) O3 + hv (<320nm) O +O2
                                                     Destruction
                  d) O + O32O2
 Where M is a random air molecule (O2 or N2)


 Steady-state O3               [O3]     k a kb [M ] 1/ 2
                                     (            )
 concentration                 [O 2]       kc kd

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
Prediction by Chapman theory vs. Observation




                                  Using Chapman theory
  There must be other O3 destruction pathways
  Catalytic ozone destruction

                     X + O3 = XO + O2
                     XO + O = X + O2
    Net reaction       O + O3 = 2 O2

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 + O2   HOx cycle
                       Net: O + O3 = 2 O2
• Chlorine and bromine (Cl and Br)
                      Cl. + O3 = ClO. + O2
                                             ClOx cycle
                      ClO. + O = Cl. + O2
                       Net: O + O3 = 2 O2
• Nitric oxide (NO)
                      NO + O3 = NO2 + O2
                                             NOx cycle
                      NO2 + O = NO + O2
                       Net: O + O3 = 2 O2
Hydroxy radical
• Accounts for nearly one-half of the total ozone
  destruction in the lower stratosphere (16-20 km).
• Sources
  O3 + 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, CH3Cl

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: 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. + HCl
                        Stable in the stratosphere
                        Removed from air by precipitation
                        when it migrates to the troposphere
    ClO. + NO2 + M ClONO2 + M
             Reservoir species
             Relatively unreactive but can regenerate
             reactive species upon suitable conditions
             ClONO2 + hvClO + NO2
             ClONO2 + hvCl + 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          Inhibit the HOx
   ClO. + NO2  ClONO2       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)

				
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posted:10/7/2011
language:English
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