1. Stratospheric Ozone
From UNEP/WMO, 2006

2. OZONE IN OUR ATMOSPHERE
Atmospheric ozone. Ozone is present
throughout the lower atmosphere (troposphere and stratosphere). Most ozone resides in the stratospheric “ozone layer” above Earth’s surface. Increases in ozone occur near the surface as a result of pollution from human activities.

3. OZONE IN OUR ATMOSPHERE : how ozone is formed in the atmosphere ?
λ < 240 nm
Stratospheric ozone production.
Ozone is naturally produced in the stratosphere in a two-step process. In the first step, ultraviolet sunlight breaks apart an oxygen molecule to form two separate oxygen atoms. In the second step, each atom then undergoes a binding collision with another oxygen molecule to form an ozone molecule. In the overall process, three oxygen molecules plus sunlight react to form two ozone molecules.
Chapman cycle (1930)

4. OZONE IN OUR ATMOSPHERE : Is the total ozone uniform over the globe ?
A total ozone value is obtained by measuring all the ozone that resides in the atmosphere over a given location on Earth’s surface. Total ozone values shown here are reported in “Dobson units” as measured by a satellite instrument from space. Total ozone varies with latitude, longitude, and season, with the largest values at high latitudes and the lowest values in tropical regions. Total ozone at most locations varies with time on a daily to seasonal basis as ozone-rich air is moved about the globe
by stratospheric winds.
Dobson units : mesure of the atmospheric trace gas density of
an atmospheric columns.
1 DU = 2.69×1016 ozone molecules . cm-2

5. OZONE IN OUR ATMOSPHERE : Why do we care about ozone ?
UV-B protection by the ozone layer.
The ozone layer resides in the stratosphere and surrounds the entire Earth. UV-B radiation (280- to 315- nanometer (nm) wavelength) from the Sun is partially absorbed in this layer. As a result, the amount of UV-B reaching Earth’s surface is greatly reduced. UV-A (315- to 400-nm wavelength) and other solar radiation are not strongly absorbed by the ozone layer. Human exposure to UV-B increases the risk of skin cancer, cataracts, and a suppressed immune system. UV-B exposure can also damage terrestrial plant life, singlecell organisms, and aquatic ecosystems.

6. OZONE IN OUR ATMOSPHERE : How ozone is measured in the atmosphere?
Ozone measurements. Ozone is measured throughout the atmosphere with instruments on the ground and on board aircraft, high-altitude balloons, and satellites. Some instruments measure ozone locally in sampled air and others measure ozone remotely some distance away from the instrument. Instruments use optical techniques, with the Sun and lasers as light sources, or use chemical reactions that are unique to ozone. Measurements at many locations over the globe are made regularly to monitor total ozone amounts.

7. OZONE IN OUR ATMOSPHERE : How ozone is measured in the atmosphere?
λ : 305 / 325 nm

8. OZONE IN OUR ATMOSPHERE : How ozone is measured in the atmosphere?
λ : 305 / 325 nm

10. Discovery of the ozone hole at Antarctica
Halley Bay, Antarctica, 1985
Anderson et al., 1987
Sources: British Antarctic Survey, NASA-WMO

11. OZONE IN OUR ATMOSPHERE : Depletion of the ozone layer.
Time-series (1996 to 2012) of total polar ozone mean values over the months of September, October and November as measured by GOME, SCIAMACHY and GOME-2 flown on ERS-2, Envisat and MetOp-A, respectively. Smaller ozone holes are evident during 2002 and The maps were generated using total ozone columns derived with the GODFIT algorithm (BIRA/IASB, RT Solutions Inc.), which has been consistently applied to the three different satellite instruments.
Credit:BIRA/IASB Read more at:

12. OZONE IN OUR ATMOSPHERE : Depletion of the ozone layer.

13. OZONE IN OUR ATMOSPHERE : Depletion of the ozone layer.

14. OZONE IN OUR ATMOSPHERE : Halogens in the atmosphere

15. OZONE IN OUR ATMOSPHERE : Halogens in the atmosphere
Reactive chlorine gas observations.
The abundances of chlorine source gases and reactive chlorine gases as measured from space are displayed with altitude for a midlatitude location. In the troposphere (below about 10 kilometers), all chlorine is contained in the source gases. In the stratosphere, reactive chlorine gases increase with altitude as chlorine source gases decrease. This is a consequence of chemical reactions involving ultraviolet sunlight. The principal reactive gases formed are HCl, ClONO2, and ClO. Summing the source gases with the reactive gases gives total available chlorine, which is nearly constant with altitude up to 47 km. In the ozone layer, HCl and ClONO2 are the most abundant reactive chlorine gases.

16. OZONE IN OUR ATMOSPHERE : Halogens/ozone chemistry
Ozone destruction Cycle 1.
The destruction of ozone inCycle 1 involves two separate chemical reactions. The net or overall reaction is that of atomic oxygen with ozone, forming two oxygen molecules. The cycle can be considered to begin with either ClO or Cl. When starting with ClO, the first reaction is ClO with O to form Cl. Cl then reacts with (and thereby destroys) ozone and reforms ClO. The cycle then begins again with another reaction of ClO with O. Because Cl or ClO is reformed each time an ozone molecule is destroyed, chlorine is considered a catalyst for ozone destruction. Atomic oxygen (O) is formed when ultraviolet sunlight reacts with ozone and oxygen molecules. Cycle 1 is most important in the stratosphere at tropical and middle latitudes, where ultraviolet sunlight is most intense.

17. OZONE IN OUR ATMOSPHERE : Halogens/ozone chemistry
Polar ozone destruction Cycles 2 and 3.
Significant destruction of ozone occurs in polar regions because ClO abundances reach large values. In this case, the cycles initiated by the reaction of ClO with another ClO (Cycle 2) or the reaction of ClO with BrO (Cycle 3) efficiently destroy ozone. The net reaction in both cases is two ozone molecules forming three oxygen molecules. The reaction of ClO with BrO has two pathways to form the Cl and Br product gases. Ozone destruction Cycles 2 and 3 are catalytic, as illustrated for Cycle 1, because chlorine and bromine gases react and are reformed in each cycle. Sunlight is required to complete each cycle and to help form and maintain ClO abundances.

18. OZONE IN OUR ATMOSPHERE : Halogens in the atmosphere

19. OZONE IN OUR ATMOSPHERE : Why does it occur over antarctic ?
Arctic and Antarctic temperatures.
Stratospheric air temperatures in both polar regions reach minimum values in the lower stratosphere in the winter season. Average minimum values over Antarctica are as low as –90°C in July and August in a typical year. Over the Arctic, average minimum values are near –80°C in January and February. Polar stratospheric clouds (PSCs) are formed when winter minimum
temperatures fall below the formation temperature (about –78°C). This occurs on average for 1 to 2 months over the Arctic and 5 to 6 months over Antarctica (see heavy red and blue lines). Reactions on PSCs cause the highly reactive chlorine gas ClO to be formed, which increases the destruction of ozone (see Q9). The range of winter minimum temperatures found in the Arctic is much greater than in the Antarctic. In some years, PSC formation temperatures are not reached in the Arctic, and significant ozone depletion does not occur. In the Antarctic, PSCs are present for many months, and severe ozone depletion now occurs in each winter season.

20. OZONE IN OUR ATMOSPHERE : Why does it occur over antarctic ?
Polar stratospheric clouds.
This photograph of an Arctic polar stratospheric cloud (PSC) was taken from the ground at Kiruna, Sweden (67°N), on 27 January PSCs form during winters in the Arctic and Antarctic stratospheres. The particles grow from the condensation of water and nitric acid (HNO3). The clouds often can be seen with the human eye when the Sun is near the horizon. Reactions on PSCs cause the highly reactive chlorine gas ClO to be formed, which is very effective in the chemical destruction of ozone.

21. Summary of the stratospheric ozone destruction

22. How severe is antarctic depletion ?

23. Are there regulations on the production of ozone-depleting gases?

24. Sumary of the stratospheric ozone destruction

26. Look at « true » image

27. Measuring Ozone in the Earth’s Atmosphere
Scientists have been measuring ozone since the 1920’s using ground-based instruments that look skyward. Data from these instruments, although useful in learning about ozone, only tell us about the ozone above their sites, and do not provide a picture of global ozone concentrations. To get a global view of ozone concentrations and its distribution, scientists use data from satellites.
The principle of measuring ozone is simple. We know from measurements how much incoming UV-B sunlight arrives at the top of the Earth’s atmosphere every second. We also know how much light (including UV-B) is scattered by air molecules in the atmosphere--this is called “Rayleigh scattering,” and this is the same phenomenon that makes the sky appear blue. So we can calculate the amount of UV-B sunlight that would make it to the Earth’s surface if there were no ozone to absorb it. When we measure the amount of UV-B sunlight from the ground, we find it is much less than what we calculated. The difference is the amount of UV-B absorbed by ozone, and from that we can calculate the amount of ozone.
Measuring ozone from space is similar, but you have to know the amount of the solar UV-B light that is backscattered, or bouncing, off molecules in the atmosphere (Rayleigh scattering, again) in the direction of the satellite. This measurement technique is depicted in the figure on left. We can calculate how much UV-B light the space-based instrument would observe if there were no ozone. However, the amount of UV-B measured is much less because UV-B is passing through the atmosphere a second time. Again, from the amount of UV-B that’s “missing,” we can calculate the amount of ozone.
The longest satellite record of ozone data has been from instruments using this backscatter method. The first measurements were taken by the BUV instrument on Nimbus-4 satellite in 1970 followed by the Total Ozone Mapping Spectrometer (TOMS) instruments on the Nimbus-7, Earth Probe, and Meteor-3 satellites, several SBUVs on NOAA satellites, the Ozone Monitoring Instrument (OMI) onboard the EOS Aura satellite, and the Ozone Mapping and Profiler Suite (OMPS) on the Suomi NPP satellite. The Europeans also flew BUV-type instruments on their environmental satellites.
Another type of instrument uses occultation to measure ozone, such as the Stratospheric Aerosol and Gas Experiment (SAGE) instruments including the upcoming SAGE-III-ISS onboard the International Space Station, the Atmospheric Chemistry Experiment (ACE), and Halogen Occultation Experiment (HALOE) on the UARS satellite. Other techniques include measuring the radiation emitted from the atmosphere in the infrared and microwave wavelengths where ozone also absorbs and allows instruments to detect the radiation change.
In addition to the OMI instrument, the Aura satellite carried three other ozone-measuring instruments, the High Resolution Dynamics Limb Sounder (HIRDLS), the Tropospheric Emission Spectrometer (TES) using infrared emission, and the Microwave Limb Sounder (MLS). (Note: All these instruments measure several other atmospheric constituents in addition to ozone.)