Problems:

1-7. Write out the two-step mechanism by which the hydroxyl free radical catallytically destroys ozone by Mechanism I. By adding together the steps, deduce the overall reaction.

Step 1 O₃ + OH → O₂ + HO₂

Step 2 O + HO₂ → O₂ + OH

Net rxn: O₃ + O → 2O₂

(note: OH = hydroxyl radical – written with O first; HO₂ = hydroperoxy radical – written with H first)

1-8 By analogy with its reaction with methane, write a balance equation for the reaction by which O* produces OH from water vapor.

O* + H₂O → 2OH

2-3 Why is the mechanism involving dichloroperoxide of negligible importance in the descruction of ozone, compared to the one that proceeds by ClO + O, in the upper level of the stratosphere?

Because of the higher concentration of O in the upper stratosphere, it will react faster with ClO, keeping ClO concentrations low. For the dimer reaction to be important, the concentration of ClO must be high, as its formation rate depends on the square of the ClO concentration. Also, at higher temperatures, it can convert back to ClO.

Review Questions:

1-11 Explain why the phrase ozone layer is a misnomer.

The ozone layer is a misnomer because ozone is never a major constituent (ozone never gets much above the ppm level – even in the ozone layer).

1-15 Explain why, atom for atom, stratospheric bromine destroys more ozone than chlorine.

Both Cl and Br effectively react with ozone through mechanism I and II cycles. However, while ~99% of Cl typically is tied up in reservoir species such as HCl and ClONO₂, very little of Br is tied up in similar species.

2-1 What is a Dobson Unit? How is it used in relation to atmospheric ozone units?

A Dobson Unit is the amount of ozone in a column of air, if reduced to pure ozone under standard conditions, given by its height in hundredths of mm. It is a measure of the total column ozone, which is a useful measure of the UV absorbing capability of the ozone.

2-2 If the overhead ozone concentration at a point above the Earth’s surface is 250 DU, what is the equivalent thickness in mm of pure ozone at 1.0 atm pressure?

The thickness would be 2.5 mm.

2-3 Describe the process by which chlorine becomes activated in the Antarctic ozone-hole phenomenon.

Reservoir species of Cl, namely, HCl and ClONO₂, react on the surface of atmospheric particles or polar stratospheric cloud particles, leading to the production of Cl₂ and HClO, while nitrogen gets locked up as NO₃^- in the particles. Both Cl₂ and HClO are readily photolyzed to Cl when sunlight becomes available in the Antarctic spring.

2-4 What are the steps in Mechanism II by which atomic chlorine destroys ozone in the spring over Antarctica?

Once Cl is released from reservoir species (described in Review Question 2-3), it rapidly is converted to ClO. With high ClO concentrations, Mechanism II occurs effectively according to:

step1: 2ClO → ClOOCl

step2: ClOOCl + hn → ClO₂ + Cl

step 3: ClO₂ → Cl + O₂

2-5 Describe the reasons why the Antarctic ozone hole closes in late spring/early summer.

The Antarctic ozone hole closes in late spring/early summer through a combination of

decreased loss rate and less isolation of Antarctic stratospheric air (more mixing of relatively ozone plentiful air from mid-latitudes). The decrease loss rate occurs from the release of NOspecies, which can lock up Cl in reservoir species, as PSC particles disappear from atmospheric warming.

2-6 Explain why full-scale ozone holes have not yet been observed over the Arctic.

There are two reasons why large scale ozone holes in the Arctic have not been observed. One is that there tends to be more mixing from mid-latitudes in the winter compared to the Antarctic. This is due to a different geography where the Arctic is an ocean surrounded by continents vs. the Antarctic being a cold continent surrounded by warm (relatively) oceans. A much more consistently strong polar jet surrounding the Antarctic occurs, better isolating the Antarctic stratosphere. The second reason is due to relatively warm conditions in the Arctic from more frequent mid-latitude air intrusion.

2-7 What are two effects to human health that scientists believe will result from ozone depletion?

Skin cancer rates and cataract occurrence should increase from ozone depletion.

2-9 Explain what CFCs were and some of their uses. Did they have a tropospheric sink? Why did their emissions in air lead to an increase in stratospheric chlorine?

CFCs are chlorofluorocarbons and were used for air conditioning, as industrial solvents, and foam blowing agents. They have no significant tropospheric sinks. Due to transport to the stratosphere and photolysis, they have been a significant source of reactive stratospheric chlorine.

2-12 Chemically, what are halons? What was their main use?

Halons are chemicals containing bromine, carbon, fluorine and chlorine. They are mainly used as fire extinguishers.