1. Ozone depletion today – introduction and ozone chemistry
tomorrow – Antarctic ozone hole

2. Ozone molecule that contains 3 oxygen atoms – O3
bluish color and unique odor
powerful oxidizing agent and relatively unstable – explosive in high concentrations
arose naturally as a byproduct of photosynthesis and the rise of atmospheric oxygen
shields against harmful solar UV radiation
important for almost all of life on Earth – we should protect it

3. Ultraviolet radiation
UVA: 320 – 400 nm relatively harmless, causes tanning but no burns, not absorbed by O3
UVB: 290 – 320 nm harmful – has biological effect: causes sunburn, skin cancer, damage eyes, partly absorbed by O3
UVC: 200 – 290 nm very dangerous but mostly absorbed by O3

4. Ultraviolet radiation
UVB has strong biological effect on humans, other animals (under water too) and plants
biological effect is determined by the dose rate – number of UV photons per unit time that lead to biological response
Example:
erythemal action spectrum – appearance of sunburns in humans

5. Ultraviolet radiation
Inverse relation between O3 and UVB flux determined theoretically and experimentally – less O3 - more UVB radiation
solar flux of UVB is high and absorption by O3 relatively low – sensitive balance
small change in O3 concentrations can cause a big change in UVB dose rates

6. Ozone distribution Vertical distribution:
most of O3 is in the stratosphere
this layer of ozone is also called ozonosphere
peak at 25 – 30 km above ground
O3 concentrations measured indirectly by satellite or Dobson spectrophotometer
direct measurements by ozonesondes

7. Ozone distribution Vertical distribution:
amount of O3 present is measured in column depth – total amount of O3 per unit area over a certain location (number of molecules per square centimeter)
column depth can be expressed in atmosphere-centimeter – thickness that a layer of pure O3 would have at surface pressure (1 mbar)
also used Dobson unit: 1 DU = 1000 atm-cm
average value about 0.3 atm-cm = 300 DU
layer of O3 0.3 cm thick (< 1/8 in)

8. Ozone distribution Spatial distribution:
highest concentrations of O3 are in mid to high latitudes in spring (both hemispheres)
lowest at equator (prior to ozone holes)
concentrations vary in time and space
O3 produced mostly over tropics – stratospheric circulation redistributes it to higher latitudes

9. Ozone distribution

10. Ozone distribution Spatial distribution:
UV radiation flux on the surface depends on solar zenith angle (and latitude) and column depth of O3
easier to get sunburn in Florida than in New York (on a sunny day)
current value in New York: 312 DU Miami: 248 DU (4 Dec 2004)

11. Ozone distribution Tropospheric (ground-level) ozone:
10 % of all O3 is in troposphere (first 10 km of atmosphere)
O3 occurs naturally in troposphere in small conc. (from hydrocarbons and intrusions of O3 from stratosphere)
helps to keep the air clean
it is very reactive – cleans pollutants like CO, SO2
too much of O3 in troposphere is bad (becomes pollutant itself)
part of photochemical smog (UV radiation reacts with hydrocarbons and NOx emitted by cars, fossil fuel burning...)
toxic to plants, irritates eyes and lungs
ground level O3 concentrations have doubled since 1900

12. Production of O3 – Chapman mechanism
production of O3 (photochemistry) first explained by Sydney Chapman, English scientist, in 1930
assumes pure oxygen – nitrogen atmosphere
production:
UV photon with wavelength λ < 240 nm – UVC
M – some molecule which accelerates the reaction but remains unchanged itself - catalyst

13. Production of O3 – Chapman mechanism
destruction:
photon can be visible light – very fast reaction that can happen anywhere in the atmosphere
ozone destroyed permanently – slow but important
O from (3) can also recombine with O2 to form O3
O3 is constantly being produced and destroyed
solar radiation needed – maximum production in tropics
Chapman mechanism overestimates O3 production by 30%
there are other processes in real atmosphere that destroy O3

14. Catalytic cycles of N, Cl, Br – destruction of O3
Chapman mechanism ignores the effect of trace atmospheric constituents: nitrous oxide (N2O), water vapor, freons (contain chlorine), ...
they can be photolyzed and produce highly reactive radicals which destroy ozone
Chlorine catalytic cycle:
similar reactions happen for other radicals NO, Br, OH
most radicals are anthropogenic but some are natural, e.g. hydroxyl (OH)
complex chemistry

15. Catalytic cycles of N, Cl, Br – destruction of O3
Chlorine cycle:
chlorine is the most important for stratospheric O3 chemistry
current stratospheric chlorine concentration is ~ 3.3 ppb
natural sources of chlorine:
CH3Cl (methyl chloride)
HCl (hydrogen chloride)
produced by marine plankton
most of it destroyed in troposphere
in stratosphere only 0.6 ppb Cl (18% of total stratospheric chlorine)
main sources are volcanic eruptions and sea spray
most of it dissolves in water droplets and falls out – doesn’t reach stratosphere

16. Catalytic cycles of N, Cl, Br – destruction of O3
Chlorine cycle:
anthropogenic sources of chlorine:
chlorofluorocarbons (CFCs) or freons
freon-11 (CCl3F): used as propellant, blowing agent for foams, cleaning electronics
freon-12 (CCl3F2): refrigerant in air conditioners
banned in US and replaced – concentrations decreasing since 1990s
freons are very inert (and harmless) in troposphere, but also mix out to stratosphere where they photolyze with UV light and release Cl atom
other chlorocarbons
CCl4 (carbon tetrachloride), CH3CCl3 (methyl chloroform)
concentrations strongly decreasing since early 1990s (usage decreased and short life time)
sink: chlorine reacts with methane, produces HCl which precipitates to the surface

17. movie

18. Catalytic cycles of N, Cl, Br – destruction of O3
Nitrogen cycle:
nitrogen radicals destroy O3 in catalytic reaction very similar to chlorine catalytic reaction
radicals with odd nitrogen (NO, NO2) – very reactive
sources: nitrous oxide (N2O) from troposphere (microbial activity in soils and ocean, fertilizers, supersonic transport airplanes which fly in stratosphere)
converts to reactive radical NO:
sink: NO2 eventually reacts with OH, converts to inert nitric acid (HNO3) and precipitates back to surface

19. Catalytic cycles of N, Cl, Br – destruction of O3
Bromine cycle:
bromine destroys O3 in similar catalytic reaction
sources of bromine:
natural
anthropogenic
methyl bromide (CH3Br)
produced in oceans
halons (CF3Br, CF2ClBr)
used in fire extinguishers
halons defuse to stratosphere and photolyze with UV light releasing Br
sink: precipitates to troposphere as hydrogen bromide HBr

20. Ozone hole – Antarctic ozone hole