REVIEW QUESTIONS

Life Cycle of an Ozone Molecule: Basic Photochemistry

1. What are the two reactions that make up the Chapman Cycle for the production of ozone? Explain what the different terms are and provide numbers for wavelengths. You may refer to Figure 5.03.

 

2. The ozone created in the above reaction is quickly lost by additional reactions with ultraviolet photons. What is the reaction? Explain the terms.

 

3. The amount of ozone predicted by Chapman Cycle ozone production is higher than that actually observed. What is the reaction for ozone loss that balances the production reaction?

 

4. What is odd oxygen? What is the source of odd oxygen? How is it lost? What is the role of odd oxygen in the Chapman life cycle of ozone? You may refer to Figure 5.05.

 

5. It turns out that the Chapman life cycle for ozone results in ozone concentrations higher than actually observed in the tropics by a factor of 2 and concentrations lower than actually observed in the middle and higher latitudes. What other factors need to be considered?

 

Ozone UV Radiation Shielding and Photolysis

1a. What is meant by absorption cross-section of a molecule?

1b. Explain absorption cross-section of oxygen or ozone molecules as shown in Figures 5.01 and 5.06.

 

2a. How does ozone photolysis depend on altitude? Refer to Figure 5.07.

2b. How does ozone photolysis depend on latitude? Refer to Figure 5.08.

2c. How does ozone photolysis depend on season?

 

3a. What is the average lifetime for a free oxygen atom and for an ozone molecule in the daytime, middle latitude, lower stratosphere?

3b. Given the lifetime of free oxygen and ozone, why isn't the atmosphere completely depleted of ozone?

3c. What is odd oxygen partitioning? What does partitioning depend on? What sort of values are encountered in the stratosphere?

 

Catalytic Loss and Life Cycle of a Pollutant

1. What is a catalyst?

 

2a. What are the observed distributions of water vapor and methane with altitude in the stratosphere? Refer to Figures 5.12 and 5.13.

2b. Explain these observed distributions of water vapor and methane with altitude in the stratosphere given that both gases have source regions in the troposphere.

 

3. Why are water vapor and methane important in ozone chemistry?

 

4a. List the two species that make up reactive hydrogen, HOx.

4b. What are the two reactions involving HOx shown in Figure 5.15a? What is the net effect on odd oxygen? What happens to the reactive hydrogen species?

 

5a. What are reservoir species?

5b. What kind of reservoir species interrupt the HOx catalytic cycle in 4b?

 

6a. List the two species that make up reactive nitrogen, NOx.

6b. What are the two reactions involving NOx shown in Figure 5.18? What is the net effect on odd oxygen? What happens to the reactive nitrogen species?

 

Refer to Figure 5.21.

7. What are interference cycles? What are the reactions involved in the interference cycle shown in Figure 5.21? What is the net change?

 

8a. List the reservoir species for reactive nitrogen. Which one has the longest lifetime (i.e., the slowest photolysis rate)?

8b. What are the consequences of the slower photolysis rate of HNO3 in terms of acting as a reservoir species of NOx?

 

9a. List the two species that make up reactive chlorine, Clx.

9b. What are the two reactions involving Clx shown in Figure 5.23? What is the net effect on odd oxygen? What happens to the reactive chlorine species?

 

10a. What are three chlorine reservoir species?

10b. List three reactions that transform reactive chlorine, Clx, into the reservoir species listed above.

 

11. Give an interference cycle between reactive nitrogen and reactive chlorine that can actually reduce ozone loss.

 

12a. List the two species that make up reactive bromine, Brx. List two nonreactive bromine species.

12b. Why aren't nonreactive bromine species referred to as reservoir species?

 

13. What are the reactions and the net effect of the Brx-Ox catalytic reaction cycle?

 

14. What are the reactions and the net effect of the Brx-NOx-Ox catalytic reaction cycle? Discuss what happens to the reservoir species BrONO2 in this set of reactions and what does this imply about bromine in the stratosphere?

 

Heterogeneous Chemistry

1. What is heterogeneous chemical reaction? Why are such reactions important in the stratosphere?

 

2. What are sulfate aerosols? Where do they come from?

 

3a. What are polar stratospheric clouds (PSCs)? Where do they form? How are they classified?

3b. Why are PSCs important in heterogeneous stratospheric chemistry?

 

4. What are the five basic heterogeneous reactions involved in stratospheric chemistry?

 

5. What happens to the Cl2 and HOCl species created in the reactions in question 4?

 

6. What role does HNO3 (nitric acid) play in ther eactions in question 4? Why is it siginificant in ozone chemistry?

 

7. Explain the significance of Figure 5.29 in terms of the activation of reactive chlorine. How does PSC heterogeneous chemistry play a role?

 

8. Explain the significance of Figure 5.30 in terms of chlorine activation and nitrogen deactivation in the top panel and chlorine deactivation and nitrogen activation in the bottom panel.

ANSWERS

Life Cycle of an Ozone Molecule: Basic Photochemistry

1. Reaction 1:

O2 + hc/ --> O + O

h : Planck's constant

c : speed of light, c=3 x 10⁸ meters per second

: wavelength of incident photon given in nanometers, where 1 nm =1 x 10^-9 meters; for photolysis of an oxgyen molecule, must be shorter than 242 nm.

O2 diatomic molecule is split (photolyzed) apart by an ultraviolet photon into two free oxygen molecules.

Reaction 2:

O2 + O + M --> O3 + M

The free oxgyen atom then reacts with another molecular oxygen molecule to form ozone. M represents any other molecule, such as N2 or O2, involved in the reaction.

2. Ozone is lost through photodissociation by an ultraviolet photon via the reaction O3 + hc/ --> O2 + O. Here an ultraviolet photon of energy hc/ breaks apart an ozone molecule into a diatomic oxygen molecule and a free oxygen atom.

3. The basic Chapman Cycle for ozone loss is O3 + O --> O2 + O2. Here an ozone molecule reacts with a free oxygen atom to create two diatomic oxygen molecules.

4. Because free oxygen and ozone molecules are rapidly interconverted, we may regard the sum of the two as a "family" of odd oxygen, denoted Ox. The source of odd oxygen is the photolysis of oxygen molecules and the loss of odd oxygen is the reaction of ozone with oxygen atoms. The lifetime of odd oxygen in a parcel of air is much longer than the lifetime of an individual O atom or O3 molecule participating in the Chapman production and loss cycle. This is why ozone concentrations in the middle stratosphere vary so slowly.

5. We must consider two additional chemical and transport processes. First, there are other ozone loss reactions with gases containing chlorine, bromine, nitrogen, and hydrogen that contribute to overall ozone loss. Second, there exists an Equator-to-pole stratospheric circulation known as the Brewer-Dobson circulation that transports ozone from the photochemical source region in the tropics to the middle and high latitudes. This circulation is discussed in detail in Chapter 6.

Ozone UV Radiation Shielding and Photolysis

1a. Absorption cross-section refers to the ability of a particular molecule to absorb a photon of a particular wavelength. It does not refer to an actual size area, even though it has units of area.

1b. Absorption cross-section conveys the probability of interaction between light photons of a given energy (wavelength) and the oxygen or ozone molecules. In Figure 5.01, we have the absorption cross-section for molecular oxygen and ozone as a function of photon wavelength. Figure 5.06 illustrates UV flux in terms of a steady rain of photons from the Sun, denoted by red lines. The size of the target is represented by the cross-section, which represents the probability of interaction between the photon and the molecule. If the cross-section is small, the probability that a UV photon will reach the surface is high. If the cross-section is large, the probability that a UV photon will reach the surface is low.

2a. The rate of photolysis depends upon both the number of UV photons and the number of ozone molecules available to interact with the photons. In Figure 5.07, we see that there is a steady rain of UV photons at the top of the atmosphere, but few ozone molecules to absorb them, hence little absorption occurs. Further down, the number (density) of ozone molecules increases, and absorption becomes very strong. As shown schematically in Figure 5.07, absorption reaches a maximum in the middle atmosphere. At even lower altitudes, even though air density is greater, there are few UV photons remaining, hence absorption again becomes small. In this way, the absorption process is density limited in the upper atmosphere and photon limited in the lower atmosphere.

2b. The position of the Sun in the sky is high in the tropics during the day and lower as you travel toward the poles. The intensity of sunlight reaching the surface at a given latitudes depends on how much atmosphere the light must go through. This is because the longer the path light must travel through the atmosphere, the more molecules the light will encounter, hence the more absorption that will occur. As a result, we expect the photolysis rate to decrease from tropics to poles simply because fewer UV photons can penetrateto higher latitudes.

2c The answer is an extension of the latitude dependence. Since the Sun is higher in the sky in summer at a given latitude, the path that UV photons must travel is shorter in summer than in winter. The shorter the path, the fewer the molecules encountered and the more UV light available. As a result, photolysis rates should be higher in summer than in winter, when most UV light has already been absorbed by the time it reaches a given location.

3a. The lifetime for an O atom is about 0.002 second and for an O3 molecule it is about 1000 seconds (less than 20 minutes).

3b. Despite the rapid photolysis of ozone (less than 20 minutes), photolysis of molecular oxygen, O2, also occurs in the upper atmosphere, which frees two O atoms. An O atom quickly reacts with another O2 molecule to create an O3 molecule. Thus, the creation of ozone is also rapid. The result is that, on average, the local amount of stratospheric ozone does not vary much.

3c. Odd oxygen consists of free O atoms and ozone molecules. The amount of Ox that is made up by O3 relative to the amount of Ox made up of O atoms is known as partitioning. The amount of partitioning depends upon the photolysis rate of ozone, the O+O2 reaction rate, and air density. The ratio of [O]/[O3] varies from 1 in 1 million to 1 in 100 in the stratosphere, depending on time of year and altitude.

Catalytic Loss and Life Cycle of a Pollutant

1. A catalyst is a substance, usually present in small amounts, that causes chemical reactions without itself being consumed by those reactions.

2a. The lower stratosphere contains small amounts of water vapor and elevated amounts of methane. The amount of water vapor increases through the stratosphere, reaching a maximum in the mesosphere while the amount of methane decreases.

2b. Moist air that is lifted up from the troposphere in the tropics passes through the very cold boundary of the tropopause where water vapor is "frozen out." This is why air entering the lower stratosphere is very dry. Methane, on the other hand, is unaffected by such cold temperatures as it enters the stratosphere through the tropical tropopause. However, methane undergoes oxidation reactions with the hydroxyl radical OH, leading to the production of water vapor. As a result, the upper stratosphere is depleted in methane while water vapor increases.

3. These two species are important in ozone chemistry because they transport and release hydrogen into the stratosphere. Reactive hydrogen species can then participate in catalytic cycles that destroy ozone. These reactions exist in addition to those explained by simple Chapman chemistry cycles.

4a. Reactive hydrogen, abbreviated as HOx, consists of OH (the hydroxyl radical) and HO2.

The net effect is simply to convert two odd oxygen molecules into two molecules of molecular oxygen while conserving the sum of OH and HO2. (This is why the reaction is a catalytic one.)

5a. Reservoir species are chemical compounds that store (like a reservoir) a particular species in a nonreactive form.

5b. H2O, HNO3, and HNO4 are reservoir species that act as stores of hydrogen, locking up (or sequestering) HOx and halting the reaction cycle in 4b.

6a. Reactive nitrogen, abbreviated as NOx, consists of NO (nitric oxide) and NO2 (nitrogen dioxide)

The net effect is to convert the odd oxygen molecules into two molecules of molecular oxgyen while conserving the sum of NO and NO2. (Again, this is another example of a catalytic reaction.)

7. Interference cycles refer to those reactions (illustrated by the pathways highlighted in yellow in Figure 5.21) in which an odd oxygen molecule is not lost. In Figure 5.21, the reactions involved are

8a. Reservoir species for reactive nitrogen include HNO3, NHO4, and H2O5. NHO3 (nitric acid) has the longest lifetime, about 20 hours.

8b. The relatively long lifetime (or slow photolysis rate) of HNO3 suggests that they are effective reservoirs of NOx, requiring more hours of sunlight to be transformed back into an active species. A typical HNO3 molecule will persist for most of the day, meaning that reactive nitrogen is locked up and can't destroy ozone over the course of a whole day.

9a. Reactive chlorine compounds include Cl (free chlorine atom) and ClO (chlorine monoxide).

In this set of reactions, the net effect is to convert two molecules of odd oxygen into two molecules of molecular oxygen while conserving the sum of reactive chlorine. (As in the case of HOx and NOx, the sum of Clx compounds are conserved in this catalytic reaction.)

10a. HCl (hydrochloric acid), HOCl (hypochlorous acid), and ClONO2 (chlorine nitrate).

10b. The three reactions are

12a. Reactive bromine species includes Br (bromine) and BrO (bromine monoxide). Nonreactive bromine species includes HOBr (hypobromous acid) and BrONO2 (bromine nitrate).

12b. The nonreactive bromine species, such as HOBr and BrONO2, are not referred to as reservoir species because they are easily photolyzed, even by visible light, and hence have very short lifetimes. This means that these species do not lock up reactive bromine in the same way that other reservoir species do.

13. The reactions are

The net effect is to convert two odd oxygen molecules into two molecules of molecular nitrogen while preserving the amount of reactive bromine (another catalytic reaction).

14. The reactions are

The net effect is to convert two odd oxygen molecules into three O2 molecules. The bromine nitrate (BrONO2) is photolyzed by a photon of near-UV or even visible light. Since such light penetrates into the lower stratosphere, bromine exists mostly in its reactive form in the lower stratosphere.

Heterogeneous Chemistry

1. A heterogeneous chemical reaction is a chemical process that involves solid, liquid, and gaseous phases (it is a multiphase process). Such a reaction occurs on or in a condensed particle that are in contact with gaseous molecules. Such reactions are the physical mechanism behind the Antarctic ozone hole phenomenon.

2. Sulfate aerosols are submicron sized particles that are typically composed of sulfuric acid and water. The sulfuric acid comes from carbonyl sulfide (COS) and sulfur dioxide (SO2) carried into the stratosphere by the Brewer-Dobson circulation or direct injection of SO2 into the stratosphere by major volcanic eruptions.

3a. Polar stratospheric clouds are another class of particles in the stratosphere. They consist of Type I and Type II and form only under extremely cold conditions such as are found in the winter polar vortex region. Type II PSCs are composed of water ice particles that form when temperatures drop below 188K (-85C) and they have a diameter of at least 10 microns. Type I PSCs are composed of a mixture of nitric acid, sulfuric acid, and water ice, as well as frozen nitric acid trihydrates (NAT). They form when the temperature drops below 195K (-78C) and have a diameter on the order of 1 micron.

3b. PSCs are important in heterogeneous stratospheric chemistry in that they provide the surfaces on which ozone destroying reactions occur

4. The five reactions are

5. The chlorine species Cl2 and HOCl created in the five heterogeneous reactions above are short lived. They are photolyzed even by visible sunlight into reactive chlorine, Clx, species such as Cl and ClO. The Cl2 is photolyzed into two Cl atoms, which then participate in the Clx catalytic cycles that destroy ozone. The HOCl photolysis liberates ClO (chlorine monoxide), which also participates in catalytic cycles that destroy ozone.

6. The HNO3 locks up reactive nitrogen, such as NO2, which is then unavailable to interfer with the Clx reactions. Hence, chlorine is free to destroy ozone.

7. Figure 5.29 shows an added reaction pathway for activating chlorine into a reactive form. The reservoir species ClONO2 is converted to reactive Cl by the heterogeneous reaction with HCl on a PSC surface.

8. Figure 5.30 (top panel) shows how reactive nitrogen is locked away (deactivated) as nitric acid (HNO3) and it therefore unable to deactivate ClO. Chlorine nitrate is converted to nitric acid via a heterogeneous reaction with HCl on a PSC particle. Such a reaction occurs only as long as it is cold enough to maintain PSC particles. Figure 5.30 (bottom panel) shows how reactive chlorine is deactivated when the Sun rises. HNO3 is photolyzed, producing reactive nitrogen (NO2), which then reacts with ClO to reform ClONO2, locking up (deactivating) reactive chlorine.