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					                                                                          Chem 302 Stratospheric chem 3 - 1



Polar Ozone Holes

- the British Antarctic Survey (BAS) had been measuring ozone concentrations regularly at Halley Bay
station in Antarctica for many years
 they first detected a decline in springtime ozone concentrations in 1982
 in 1984 measured a 30% total ozone loss, with an increased loss each year thereafter

- we now know that this had already been going on for more than a decade at that time
 more sophisticated ozone instruments such as the Total Ozone Mapping Spectrometer (TOMS) and
the Solar Backscattered UltraViolet (SBUV) on the Nimbus 7 satellite had apparently not detected the
ozone depletion
 but when they checked the data collected, they found that they had detected the same low
concentrations as the BAS  but they were being rejected from the satellite data by software on the
grounds that they lay outside what was thought to be a ”reasonable” range

 the hole has been defined as an ozone column of 220 DU or less
 keeping in mind that the pre-hole column amounts were typically 300 – 350 DU at Halley Bay during
October

- the Antarctic ozone hole is very clearly a seasonal phenomenon, with maximum depletions in late
September or early October
 the monthly total ozone in recent years has been 40 - 55% below the pre-hole values with up to 70%
deficiencies in short periods of a week or so
 although there has been some decline in summer-season (Jan – Mar) ozone levels, the losses are
much more modest at ~ 10%

Is it really an ozone “hole"?
 up to 80% of the ozone in the lower stratosphere (12 – 20 km) has been lost in recent years in
October, and at specific altitudes virtually all the ozone is lost

Fig. 4.28  compares a vertical profile of ozone measurement over the Antarctic station of Syowa in
1997 Oct 6 to a long-term average for Oct in 1968-80
 the profile for the earlier years, when ozone depletion was only just becoming noticeable shows the
normal ozone layer centred on an altitude of ~ 17 km
 in Oct 1997, the ozone in much of this critical region had entirely vanished

 this had not been predicted by any of the models  even for 50 to 100 year in the future

Special Features of Polar Meteorology

 remember  winter in the southern hemisphere is Aug - Sept  spring is Oct - Nov
 and at the poles, in winter, there is no sun (ie 24 hour nights)

- there are two features of polar stratospheric meteorology and dynamics that appear to have a close
bearing on the interpretation of the polar ozone loss:

1. the very low temperature (< - 80C in the Antarctic) leads to the formation of polar stratospheric clouds
(PSC's)
 these are much common in the Antarctic than in the Arctic
 there is clear evidence that the PSC's are involved in polar ozone destruction and years when
stratospheric temperatures are particular low ( and  frequency of PSC's is high) are also those in which
ozone depletions are greatest

2. a related feature of polar meteorology is formation of a vortex as air cools and descends during the
winter resulting in a westerly circulation
                                                                          Chem 302 Stratospheric chem 3 - 2



 this vortex has very high wind speeds (~100 m/s or more) in the spring
 the vortex develops a core of very cold air  it is these low temperatures that allow the polar
stratospheric clouds to form in the lower stratosphere
 when the sun returns in September, temperatures rise, the winds weaken, and the vortex breaks down
in November
 but in the winter and early spring, the stability of the vortex is so great that air at polar latitudes is
essentially sealed off from that at lower latitudes

 can look at the air over the pole as more or less confined to what is effectively a giant reaction vessel
 ie as a giant beaker
 there is a slow downward circulation which drives the polar air through the cold core of the vortex that
contains the polar stratospheric clouds
 with the downward circulation allowing the core region to act as a “chemical processor”
 the concept is thus of the vortex as a containment vessel
 however, in reality material is slowly lost from the vortex, perhaps at ~ 1% a day during the period of
early Aug to late Oct
 in that case the vortex would be best characterized as a “flow reactor” with a continual slow supply into
it of reactants and a flow out of products

- so the low temperatures, the presence of PSC's and the unusual dynamics and meteorology of the
vortex most assuredly provides the backdrop to some unexpected and surprising chemistry

Anomalous Chemical Composition

- in 1987 a large international collaborative effort was made to measure ozone, and other important
species etc at the Antarctic
 these measurements confirmed that there is anomalous chemistry going on within the vortex region

Fig 4.30  shows the ozone concentrations and catalytically active ClO radical concentrations in late Aug
at the Antarctic
 ozone concentrations are normal but the chlorine oxide concentrations show a sharp rise at a latitude
of ~ 65S i.e. it goes up a factor of 10 over a few hundred km
 in the perturbed region, the concentrations are in more than 100 times greater than they are at lower
latitudes

Fig 4-31  only a few weeks later, in mid-Sept, there is a dramatic change in the behaviour of ozone
 this is a time by which the hole has developed fully
 ozone concentrations decrease over exactly the latitude range where the ClO concentration increases
on entering the chemically perturbed region within the polar vortex
 the strong anti-correlation between the ozone and chlorine oxide concentrations is a strong indication
that Cl chemistry is somehow responsible for the ozone depletion

Fig 4-32  shows that the stratosphere within the disturbed region is abnormally dry and highly deficient
in nitrogen oxides
 the drop from normal stratospheric concentrations occurs at just those latitudes where the ClO
concentrations increase and the ozone concentrations decrease

 the dehydration and denitrification are explained by the condensation of water and the conversion of
the nitrogen oxides to nitric acid in the polar stratospheric clouds
 the PSC particles may become large enough to undergo sedimentation over appreciable distances
 if the PSC's contain HNO3, NOy may be removed irreversibly

NOy = NO + NO2 + N2O5 + HNO3 + HNO4 + ClNO2 + PAN
NOx = NO + NO2
                                                                            Chem 302 Stratospheric chem 3 - 3




 up to 30% of the available reactive nitrogen has been observed to be lost in this way in the Antarctic
(and the Arctic) vortex
 it is this removal of the nitrogen oxides which leads to the anomalous Cl chemistry
 only the core of the vortex is cold enough for the larger particles to form, so we see again that it is the
combination of low temperature and special dynamics in the atmosphere that set up the conditions
needed for perturbed chemistry

- dehydration may be brought about by similar sedimentation of PSC particles containing large fractions
of water-ice
 this requires very low temperature,  dehydration is observed in the Antarctic, but not in the Arctic
where temperatures are higher

Polar Stratospheric Clouds (PSC's)

two main classes of PSC's:

Type I  small (< 1 m in diameter) HNO3-rich particles: HNO3/H2O = 1/3
 mass mixing ratio of ~ 10 ppb

Type II  larger (from 10 m to > 1 mm diameter) particles, composed primarily of H2O-ice together with
minor amounts of HNO3 as hydrates
 these can constitute up to 1000 ppb of the stratosphere when they are present

 Type I’s are formed at substantially lower temperatures (~ 195 K) than Type II’s (~ 187 K)
 both the physical size and the chemical composition are thus temperature-dependent
 the extent of denitrification and dehydration will be affected by the rate of sedimentation which is
obviously more rapid for larger particles

 of the various heterogeneous reactions that I mentioned earlier  some occur with greater efficiency
on Type I’s than on Type II’s

Perturbed Chemistry

 along with the PSC's, also present within the vortex are the accumulated gases  ie reservoir
chlorine- and nitrogen-containing species
 the central feature of the perturbed chemistry of the polar stratosphere is the conversion of reservoir
compounds to catalytically active species (or their precursors) on the surface of the PSC's

 most of the Cl in the stratosphere is usually bound up in the reservoir molecules HCl and ClONO 2, as a
result of the reactions:

                 Cl + CH4  CH3 + HCl                                       63
                 ClO + NO2 + M  ClONO2 + M                                 49

 liberation of the active Cl from the reservoirs is normally rather slow
 but the two reservoir molecules can react together on PSC particles:

                 HCl + ClONO2          
                                  on Cl2 + HNO3
                                      PSCs
                                                                            64

 the outcome is that molecular chlorine is released as a gas, and the nitric acid remains in the ice
particles (as hydrate) which can ultimately transport water and nitric acid out of the vortex, and perhaps
even to the troposphere if the temperature is low enough
                                                                           Chem 302 Stratospheric chem 3 - 4



 the molecular chlorine is photodissociated to atoms:

                Cl2 + h  Cl + Cl                                          65

 very low energy (long wavelength) radiation is required to effect this reaction,  if any sunlight is
present, even at very low intensities, it will occur
 in the polar spring (late October), when the sun returns to the poles, this process occurs

 the PSC's disturb the balance between active and reservoir Cl in two related ways
 they provide surface's on which unusual chemical change can occur and they also transport active
nitrogen out of the stratosphere, in the form of HNO 3, reducing the amount of ClONO2 reservoir that can
be formed in the first place
 denitrification (ie the permanent loss of NOy) requires lower temperatures than denoxification (loss of
NOx which is possibly temporary  ie NOx  HNO3  NAT  NOx)
 low temperatures thus particularly favour reduction of ClONO2 concentrations

 the surface reaction given above (# 64) can obviously play an important role in atmospheric chemistry
whenever particles are present
 but their involvement in atmospheric chemical transformations was frequently neglected before 1985
 this is why the ozone hole was not predicted

 there are other surface reactions that may be involved in polar chemistry:

                ClONO2 + H2O  HOCl + HNO3                                  66
                N2O5 + H2O  2HNO3                                          43

 these also produce HNO3   removing N   decreasing reservoir of ClONO2 and  increasing
reactive Cl
 these reactions occur on ice particles (with dissolved HCl in the second case)
 HNO3 remains in the ice particles after reaction
 HOCl (hydrogen hypochlorite), the gas phase product above is readily photolyzed by near-UV and
visible light to yield ClOx radicals

                HOCl + h  Cl + OH                                         19

 because the conversion can occur on the surface of the polar stratospheric clouds, release of
molecular Cl from the major reservoir molecules can continue in the chemically perturbed region
throughout the polar winter and early spring
 the vortex largely isolates the air within it, so that this part of the stratosphere can become chemically
altered  or pre-conditioned over the long polar night

- so let’s consider the molecular chlorine released via:

                HCl + ClONO2          
                                 on Cl2 + HNO3
                                     PSCs
                                                                            64

 when the sun finally returns again in the spring (late October), the Cl 2 generated from the reservoir
gases as a result of the pre-conditioning is rapidly split into chlorine atoms which can destroy ozone, and
at the same time liberate chlorine monoxide:

                Cl2 + h  Cl + Cl                                 65
                Cl + O3  ClO + O2                                 16

 the chlorine radical is then available to deplete ozone by the standard catalytic cycle
                                                                           Chem 302 Stratospheric chem 3 - 5



 the beginnings of an explanation for enhanced concentrations of ClO accompanying ozone depletion
are already apparent
 however, on their own, these two reactions cannot lead to much ozone loss due to a chain process in
the mid-stratosphere:

                Cl + O3  ClO + O2                                 16
                ClO + O  Cl + O2                                  17

 this process destroys large numbers of ozone molecules for each Cl atom made available
 something of the sort must be going on in the perturbed Antarctic stratosphere, but it cannot be this
particular chain, because the concentration of O atoms in the lower stratosphere is far too small

 alternative catalytic cycles are required to explain substantial ozone depletion in the polar stratosphere
in early spring
 the most important of these cycles involves the ClO dimer (ClO) 2 formed in the self-reaction of ClO
 these dimers are readily photolyzed to yield by an indirect route , two free Cl atoms:

                ClO + ClO + M  (ClO)2 + M                         67
                (ClO)2 + h  Cl + ClOO                            68
                ClOO + M  Cl + O2 + M                             69
                2(Cl + O3  ClO + O2)                              16
net rn          2O3 + h  3O2                                     70

 similar to the cycles discussed earlier involving chlorine, the presence of atomic oxygen is unnecessary
for depletion to occur

 note that these dimers are only formed at low temperatures, so that, once again, the low Antarctic
polar temperatures are an essential component of another part of the perturbed chemistry
 high concentrations of ClO also favour the formation of the dimer, and such high concentrations are a
feature of the chemically perturbed region of the Antarctic vortex

 all the Cl-activation reactions proceed more rapidly on Type II PSC's than they do on Type I PSC's
 stratospheric temperatures are important

 in situ and remote-sensing measurements have confirmed most of the theory
 chlorine is converted from inactive to active forms during winter, and the inactive reservoirs are
reformed again in the spring
 the surface conversion of the reservoirs ClONO2 and HCl to active Cl occurs rapidly, and virtually to
completion in early winter
 destruction of ozone occurs rapidly so that in a matter of days, ozone levels fall dramatically to half or
less of their winter value

 this situation persists until the air temperature raises, causing the vortex to break up and the polar
stratospheric clouds to dissipate
 when that occurs in mid to late spring, there is no further chemical processing on PSC surfaces
 the Cl radicals again become tied up by the formation of HCl and chlorine nitrate (ie the reservoir
species), and the ozone level begins to recover to “prehole” levels

 there is concern that the air mass, while low in ozone concentration, will extend out over the most
southern land masses, exposing people in the southern parts of South America, Australia and NZ to
unusually high levels of UV-B radiation
                                                                           Chem 302 Stratospheric chem 3 - 6



Summary

- due to very cold temperatures, get the build-up of a vortex over the south pole  acts as a big beaker in
which PSC's form
 HCl and ClONO2 combine on PSC's to form Cl2 and HNO3
 when the sun returns in the summer, Cl2  2Cl which destroys O3 via the usual catalytic cycles

The Arctic Ozone Hole

- the winter stratosphere is general warmer over the Arctic (temperatures are roughly 10C higher) than in
the Antarctic
 PSC's are far less abundant and persistent in the North than in the South polar regions
 the Arctic vortex is general smaller, less stable, and shorter lived than the Antarctic vortex
 also, the interannual variability’s are much greater in the Arctic

 but have detected large ozone losses
eg in the spring of 1995, 1996, 1997 and 1998  saw ozone losses for the first time in the Arctic that
rivalled in extent those seen in the Southern hemisphere roughly a decade earlier
ie lows of 219 DU
 these particular years were characterized by particularly low polar stratospheric temperatures
 the winter polar vortex of 1996/1997 was unusually strong and persisted into March

- recently in the Arctic atmosphere, as a result of the eruption of Mt Pinatubo in the Philippines during
1991, sulphate was found to be present at higher concentrations than usual
 stratospheric sulphate is usually in the form of an aerosol and acts as a catalyst for the removal of
N2O5 gas by forming nitric acid:

                                  sulphate
                                 
                                   
                                  2HNO3
                                  aerosol
                N2O5 + H2O                                                  43a

 the ultimate consequence of this is that it becomes a means by which NOx species are removed from
the polar stratosphere  thus eliminating one of the species that is associated with holding cycles that tie
up ClO radicals
 higher concentrations of reactive Cl species are present, thus leading to more rapid ozone depletion
 there is also concern that this reaction could occur to a large extent throughout the stratosphere and
not just at the poles, resulting in a lowering of the world-wide stratospheric ozone concentrations

- the temperature sensitivity of ozone depletion adds to the concern about further increases in
greenhouse gases
 although the build–up of such species in the troposphere leads to global warming near the earth’s
surface, where radiation is trapped, at the same time it causes cooling in the stratosphere , where the IR
absorption is optically thin and energy is radiated to space
 quite small changes in concentrations of radiative gases might tilt the balance between the production
of PSC's and their absence
 this could result in the Arctic vortex being more stable and itself producing significantly lower
stratospheric temperatures  adding yet further to the greenhouse cooling of the stratosphere
 according to one model, the combination of cooling effects might be so great as to double ozone loss
in the arctic by the year 2020

- one very important lesson learned from the surprise discovery of the Antarctic ozone hole was that the
stratospheric chemistry thought to be essentially complete in the mid-1980’s was in reality, missing a vital
part  the surface chemistry (ie heterogeneous chemistry)

				
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