11 terms

Ozone Depletion

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1. What is the difference between stratospheric and tropospheric ozone? What does it mean to say that average column ozone is 300 Dobson Units?
Stratospheric ozone protects Earth from harmful UV radiation. Tropospheric ozone = smog (toxic). This is all measured throughout the atmosphere vertically in a "column" in Dobson Units (300 DU on average, which is equal to one layer about 3mm thick).
2. What causes stratospheric ozone loss? Name two ozone-depleting compounds.
Stratospheric ozone loss occurs due to the presence of ozone-depleting compounds in the atmosphere catalyzing reactions which deconstruct O3 molecules.
-Chlorine does this, as do H2O, NO and N2O
-Also, solar radiation naturally destroys ozone (O3 + hv => O + O2 => O+ O3 => 3O2)
-OR with catalyst: X(catalyst) + O3 => XO + O2 => O3 + hv => O + O2 => O+XO => X +2O2
-Chlorine destruction: O3 + Cl => ClO + O2 O3 + hv => O+O2 O + ClO => Cl + O2 (Cl is preserved and continues to destroy).
3. Of the three bands of UV light (A,B,C), UV-B has the most potential for harm. Why is this - what about UV-A and UV-C?
O3 absorption is low at UVb wavelengths but solar flux is high so most UVb reaches the surface.
4. Name one effect that increased UV-B has on humans.
Skin Cancer
5. What are the Chapman reactions, and how do they explain why most of the ozone is located in the stratosphere?
Ozone creation requires (and absorbs) UV radiation, which is less available in the troposphere due to ozone creation/destruction in the stratosphere, which as outlined by chapman in his Chapman Reactions, which describe how ozone is formed and destroyed with the help of solar radiation in the atmosphere.
Creation: O2 + hv (wavelengths under 242nm) => O+O O+O2+M(molecule to absorbe excess energy) => O3 + M.
Destruction: O3+hv => O+O2 O+O3 => 2O2 net: 2O3 + hv => 3O2
6. When does the Antarctic ozone hole occur, and what conditions are needed?
Antarctic ozone hole occurs in the southern hemisphere spring due to intense cold which causes nitric acid cloud formation (polar stratospheric clouds). Clouds take up and freeze NO/N2O which would normally trap ClO in the atmosphere (ClONO2), releasing ClO to catalyze ozone loss. Must be below -78C and ClONO2 must be present for these clouds to form.
7. Why is atmospheric lifetime of a compound relevant to its Ozone Depletion Potential?
The higher the lifetime of the molecule/the more stable the molecule the higher the Ozone Depletion Potential.
8. HCFCs are substitute compounds for CFCs. What are the issues with these compounds?
HCFCs are still ozone-depleting but they have a shorter lifespan.
9. HFCs are also substitute compounds for CFCs. What are the issues with these?
HFCs do not deplete the ozone layer but they do contribute significantly to global warming.
10. In the polar stratosphere, does the presence of both ClO and NO2 mean much greater ozone destruction? Why or why n
ot?
Not together. They combine to form unreactive reservoir (ClONO2) but separately they're both very damaging to the ozone layer (Alien vs. Predator)
11. What is the Montreal Protocol? What was the importance of the amendments to the Montreal Protocol?
Montreal Protocol (1987) agreement to phase out Ozone-Depleting substances by strategic dates (key: 1998 for CFCs). The amendments added on many other ozone depleting substances (originally the protocol only included CFCs, not HCFCs and other ODSs like bromine-containing substances).
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