Stratospheric Chemistry Problems:

1. Ozone in the stratosphere is catalytically destroyed by Mechanism I and Mechanism II with X being Cl, Br, NO or OH.

If one is only looking at one X species (e.g. only Cl), the Mechanism I and Mechanism II reactions only lead to O₃ loss. However, if we are looking at two different species (e.g. X = Cl and X’ = OH), reactions could lead to increased or decreased O₃ loss.

a. Give an example of a reaction between X or XO with X’ or X’O (where X ≠ X’) that will decrease ozone loss. Explain how this reaction decreases ozone loss (compared to having just independent single catalyst cycles).

Example reactions leading to a decrease in ozone loss:

1. ClO + NO₂ → ClONO₂

2. OH + NO₂ → HNO₃

Explanation:

1. This binds both NO and Cl in reservoir species making them unavailable for catalytic cycles until the backwards reaction occurs.

2. This binds both NO and OH in reservoir species making them unavailable for catalytic cycles until the backwards reaction occurs.

b. An example of a reaction between X/XO and X’/X’O (where X ≠ X’) that increases ozone loss over independent cycles is where X = Cl and X’ = Br in a Mechanism II reaction. Explain i) why one would see additional ozone loss over independent cycles? and ii) what species could possibly be measured to indicate if this reaction occurs to a significant extent?

Reactions:

1. Cl + O₃ → ClO + O₂

2. Br + O₃ → BrO + O₂

3. ClO + BrO → ClOOBr

4. ClOOBr → ClBr + O₂ or Cl + Br + O₂

5. ClBr + hn → Cl + Br

i) Because we end up with Cl and Br and the net process is to convert ozone to oxygen, we get more ozone destruction than by the independent cycles only. This is especially true for Br because concentrations of BrO are low making 2 BrOs colliding unlikely.

ii) One could measure ClBr which would indicate that this process is occurring

2. Two key steps in ozone hole reactions are: 1) activation of Cl from reservoir species and 2) removal of compounds that are capable of inactivating Cl.

a) Give one reservoir species that will activate Cl in ozone hole chemistry and explain (words or reactions) how Cl is reactivated.

1. ClONO₂ – it activates Cl through the following reactions:

ClONO₂ + HCl(s) → Cl₂ + HNO₃(s)

Cl₂ is readily photolyzed to catalytic Cl

2. HCl – it activates Cl through the same reaction as above (with HCl(g) → HCl(s) preceeding the first step above)

b) Give the species removed in ozone holes that could otherwise inactivate Cl.

NO_x is removed through the production of HNO₃

3. Mechanism II is not very important in the upper tropical stratosphere despite there being sunlight for photolysis. Give one reason why mechanism II is not important under those conditions.

1. At high altitudes, O is much more abundant so that an O₃ only loss is not required

2. In the tropical stratosphere, temperatures are higher. This leads to quick thermal decomposition of XOOX’ to XO and X’O.

3. At higher altitudes, halides spend a greater fraction of their time in the X (vs. XO) form, making the chance of two XO molecules colliding to form a dimer lower.