1. Ozone Depletion O3

2. Ozone Factoids Good in the stratosphere, bad in the troposphere
Produced at a height of ~50 km, settles to a height of 23 km
Filters out 95 – 99% of incoming UV-A and UV-B (0.2m - 0.3m; m= 10-6m)
Formation may be the mechanism that explains the warming trend in the stratosphere
O3 depletion is a global problem

3. O3 Chemistry – Formation of Ozone
O2  O. + O. (O. = free radical, very reactive)
O. + O2 + M + UV  O3 + M + IR
O O M  O M
(M = a non reactive body that absorbs the energy)

4. Reasons for Depletion
Freons (CFCs) – 1 freon molecule can destroy 100,000 O3 molecules
Methyl bromide (CH3Br) - fumigant
Halons – used in fire extinguishers
High flying jets (NOx)

5. OZONE DEPLETION IN THE STRATOSPHERE
One CFC molecule can destroy 100,000 ozone molecules

6. What is a Dobson Unit? How is Ozone Measured?
A dobson unit is the most basic measure used in ozone research. The unit is named after G.M.B. Dobson, one of the first scientists to investigate atmospheric ozone (~ ). He designed the 'Dobson Spectrometer' - the standard instrument used to measure ozone from the ground. The Dobson spectrometer measures the intensity of solar UV radiation at four wavelengths, two of which are absorbed by ozone and two of which are not.

7. Dobson Unit
The illustration below shows a column of air, 10 deg x 5 deg, over Labrador, Canada. The amount of ozone in this column (i.e. covering the 10 x 5 deg area) is conveniently measured in Dobson Units.
If all the ozone in this column were to be compressed to STP (0oC and 1 atmosphere pressure) and spread out evenly over the area, it would form a slab approximately 3mm thick.
1 Dobson Unit (DU) is defined to be 0.01 mm thickness at STP; the ozone layer over Labrador then is ~300 DU.

9. Ozone Depletion
Size of ozone hole

10. Impacts
Increases in rates of skin cancer (1% decline in O3 layer  4 – 5% increase in skin cancer)
Cataracts
Impaired immune systems
Decline in phytoplankton population

11. Solutions Ban CFCs Find replacements for HFCs Ban halons
Ban methyl bromide

12. Global Agreements 1987  Montreal Protocol
1992  Copenhagen Agreements