LECTURE 15 AOSC 434 AIR POLLUTION RUSSELL R. DICKERSON 2014.

LECTURE 15

AOSC 434

AIR POLLUTION

RUSSELL R. DICKERSON

2014

STRATOSPHERIC POLLUTION

Without ozone in the atmosphere there could be no life as we know it on the surface of the Earth. All of the atmospheric ozone, that is the “ozone column” is only about 0.3 atm cm. In other words, if all the air were squeezed out of the atmosphere, and the remaining ozone were brought to STP, it would be only 0.3 cm thick.

– Murphy’s Law is strictly obeyed by NOx pollution in the atmosphere.

– Chemistry of the stratosphere different from troposphere.

Table 15.1 Solar intensity at the Earth’s surface assuming 0.30 atm cm (300 D.U.) ozone. Note that the maximum flux is about 7x10¹⁵ (photons/(cm²s)/10 nm).

Copyright © 2013 R. R. Dickerson & Z.Q. Li

3

Layers in the atmosphere

Where O₃ stops absorbing, sunlight begins to reach the surface of the Earth. Hartley (1880) measured the ozone spectrum. Fabry and Buisson (1913) measured the solar spectrum at the Earth’s surface and concluded that the UV radiation reaching the surface of the Earth must be controlled by ozone in the upper atmosphere, they even made an accurate estimate of the amount of ozone!

Today we will examine the various catalytic cycles that control the level of ozone in the stratosphere. We will calculate the O₃ abundance for a highly simplified atmosphere containing only O₂ and N₂.

λ

(nm)

σ

(atm⁻¹cm⁻¹)

I/Io

250 305 1.0x10 ⁴⁻ ⁰

275 162 1.0x10⁻²¹

300 9.5 6.0x10⁻²

325 0.27 9.2x10⁻¹


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Why do we care about the UVB dosage? Cholesterol photolysis to Vitamin D

h

→


8

Folic acid (vitamin B-9)

If you have a weak stomachGo get a cup of coffee for the next 3 min.


10

Too little UV radiation means

rickets;UV converts cholesterol to

Vitamin D.

UVC - 100 to 290 nm

UVB - 290 to 320 nm

UVA - 320 to 400 nm


11

Too much UV radiation causes skin cancer and photodissociates folate, also called

Vitamin B9. Deficiency

causes anemia and birth defects.

VII. A) OZONE CATALYTIC CYCLES

1) Chapman Reactions (1931)

O₂ + h → 2O (1)

O + O₂ + M → O₃ + M† (2)

O₃ + h → O₂ + O (3)

O + O₃ → 2O₂ (4)

By way of qualitative analysis, Reactions (1) plus (2) produce ozone.

O₂ + h → 2O (1)

2 x ( O + O₂ + M → O₃ + M ) (2)

3 O₂ + h → 2 O₃ NET

While Reactions (3) plus (4) destroy ozone.

O₃ + h → O₂ + O (3)

O + O₃ → 2O₂ (4)

2O₃ + h → 3 O₂ NET

Reactions (3) plus (2) add up to a null cycle, but they are responsible for converting solar UV radiation into transnational kinetic energy and thus heat. This cycle causes the temperature in the stratosphere to increase with altitude. Thus is the stratosphere stratified.

O₃ + h → O₂ + O (3)

O + O₂ + M → O₃ + M* (2)

NULL NET

By way of quantitative analysis, we want [O₃]ss and [O]ss and [Ox]ss where “Ox” is defined as odd oxygen or O + O₃. The rate equations are as follows.

(a)

(b)

(a+b)

From the representation for O atom chemistry:

In the middle of the stratosphere, however, R₃ >>2 R₁ and R₂ >> R₄ thus:

(I)

This does not mean that R₄ is unimportant, but it can be ignored in an approximation of [O]ss at the altitude of the ozone layer.

The ratio of [O] to [O₃] can also be useful:

413

4321

4323

22/][/][

2/][

/][

RRdtOxddtOOd

RRRRdtOd

RRRdtOd

][]][[

])[(2])[(][

3422

2233

OkMOk

OOjOOjO SS

]][[

])[(][

22

33

MOk

OOjO SS

(II)

Reactions 2 and 3 set the ratio of O to O₃, while Reactions 1 and 4 set the absolute concentrations. Now we will derive the steady state ozone concentration fro the stratosphere. From the assumption that Ox is in ready state we know:

R₁ = R₄

Thus

j(O₂)[O₂] = k₄[O][O₃]

Substituting from (I), the steady state O atom concentration:

or

]][[

)(

][

][

22

3

3 MOk

Oj

O

O

SS

SS

]][[

])[(])[(

22

2334

22 MOk

OOjkOOj

SAMPLE CALCULATION

At 30 km

)(

][])[(][

34

22

223 Ojk

MkOOjO SS

ppmSS

O

scmk

scmk

sOj

sOj

30]3

[

101

105.4

101)(

106)(

13154

16342

133

1112

This is almost a factor of ten above the true concentration! What is wrong? There must be ozone sinks missing.

2) Bates and Nicolet (1950) “HOx”

Odd hydrogen “HOx” is the sum of OH and HO₂ (sometimes H and H₂O₂ are included as well).

HO₂ + O₃ → OH + 2O₂ (5)

OH + O₃ → HO₂ + O₂ (6)

2O₃ → 3O₂ NET

The following catalytic also destroys ozone.

OH + O₃ → HO₂ + O₂ (6)

HO₂ + O → OH + O₂ (7)

O + O₃ → 2O₂ NET

The second catalytic cycle speeds up Reaction 4, that is it effectively increases k₄. Note that any loss of odd oxygen is the same as loss of ozone. These catalytic losses are still insufficient to explain the observed ozone concentration.

3) Crutzen (1970); Johnston (1971) “NOx”

Odd nitrogen or “NOx” is the sum of NO and NO₂. Often “NOx” is used as “odd nitrogen” which includes NO₃, HNO₃, 2N₂O₅, HONO, PAN and other species. This total of “odd nitrogen” is better called “NOy” or “total reactive nitrogen.” N₂ and N₂O are unreactive.

NO + O₃ → NO₂ + O₂

O + NO₂ → NO + O₂

O + O₃ → 2O₂ NET

This is the major means of destruction of stratospheric ozone. The NOx cycle accounts for about 70% of the ozone loss at 30 km. We will calculate the implied steady ozone concentration later.

4) Stolarski & Cicerone (1974) “ClOx”

Cl + O₃ → ClO + O₂

ClO + O → Cl + O₂

O + O₃ → 2O₂ NET

This reaction scheme is very fast, but there is not much ClOx in the stratosphere … yet. Today ClOx accounts for about 8% of the ozone loss at 30 km. If all these catalytic destruction cycles are added together, they are still insufficient to explain the present stratosphere O₃ level.

The general for of a catalytic ozone destruction cycle is:

X + O₃ → XO + O₂

XO + O → X + O₂

O + O₃ → 2O₂ NET


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Molina and Molina (1987)

2(Cl + O₃ → O₂ + ClO)

ClO + ClO + M → (ClO)₂ + M

(ClO)₂ + hv → Cl + ClOO

ClOO + M → Cl + O₂ + M

2O₃ → 3O₂ NET

McElroy, Salawitch, et al. (1986)

Cl + O₃ → ClO + O₂

Br + O₃ → BrO + O₂

ClO + BrO → Cl + Br + O₂

2O₃ → 3O₂ NET

Table 15.2 Stratospheric ozone destruction cycles

Cycle Sources Sinks Reservoirs

HOx H₂O,CH₄,H₂ HNO₃ · nH₂O

H₂SO₄ · nH₂O

H₂O,H₂O₂

NOx N₂O + O(¹D) HNO₃ HO₂NO₂,ClONO₂

ClOx CH₃Cl,CFC HCl HCl, HOCl

The sinks involve downward transport to the troposphere and rainout or other local loss. Note that some sinks are also reservoirs:

HCl + OH → H₂O + Cl

Antarctic Ozone Hole

In the Antarctic winter there is no sunlight and even in the spring there is too little UV to generate enough O atoms to destroy ozone. The annual loss of ozone over Antarctica is driven by heterogeneous chemistry and visible radiation. A good current review Is provided by Solomon Nature, 1990, and “Scientific Assessment of Ozone Depeation :1991” (WMO). The destruction of ozone is usually moderated by the production of chlorine nitrate, an important reservoir species.

NO₂ + ClO + M → ClONO₂ + M

In the Antarctic winter, heterogeneous reactions “denitrify” the stratosphere (Molina et al., Science, 1987).

Molecular chlorine is only weakly bound, and can be dissociated by visible radiation.

2ClhνCl

*(aq.)HNO(gas)ClClONOHCl

2

32ice

2

Cl + O₃ → O₂ + ClO

ClO + ClO + M → (ClO)₂ + M

(ClO)₂ + hv → Cl + ClOO

ClOO + M → Cl + O₂ + M

2O₃ → 3O₂ NET

Two types of Polar Stratospheric Clouds (PSC’s) exist.

Type I = HNO₃ · 3H₂O Nitric acid trihydrate, formed at T ≤ 195K

Type II = H₂O Ice formed at T ≤ 190K

• They move NOy species from the vapor phase to the condensed phase as HNO₃.

• The are involved in catalytic cycles with chlorine and bromine compounds that speed the reaction of ozone with itself to form oxygen.

• They move chlorine from the reservoir species HCl and ClONO₂ to ClOx.


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October 24, 2009From NASA

http://ozonewatch.gsfc.nasa.

gov/index.html


25

Airborne Antarctic Ozone Expedition: Punta Arenas, Chile,1987

Anderson et al., Science, 1991


26Polar Stratospheric Clouds (PSCs)


27


28


29

World Production of CFCs


30


31

From Farman et al., Nature 1985.

32

TOMSOMI

Ground Based

Antarctic Ozone Loss: Hole cannot get wider or deeper.

After Farman et al., Nature, 315, 207, 1985

• Models now provide good overall simulation of Antarctic ozone loss.

• Scientific understanding of polar ozone depletion led to international ban of CFC production


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AL

TIT

UD

E (

km)

0

5

10

15

20

25

30

35

D. Hofmann, NOAA CMDL

0 5 10 15OZONE ABUNDANCE

(PARTIAL PRESSURE, mPa)

OCTOBERAVERAGE1967 - 1971

282 DU

SEP 29, 1999

90 DU

Ozone Hole Update, II

OZONE PROFILES, SOUTH POLE:

UPDATE


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Accommodation Coefficients

• Condensed phase has lower entropy than gas phase.

• Accommodation coefficients (reaction probabilities) should be greater at lower temperatures.


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Heterogeneous Chemistry: Faster at low temperatures

In all cases, must be measured in the laboratory (thanks, RJS 2010)

Reaction probabilities given for various surface types, with formulations of variousdegrees of complexity, in Section 5 of the JPL Data Evaluation.

Atmospheric Chemistry and Physics by Seinfeld and Pandis provides extensive treatmentof aqueous phase chemistry, properties of atmospheric aerosol, organic aerosols, etc.


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Summary of Ozone Hole Formation

• Threat to DNA-based life forms.• Not predicted by any models• First observed by Farman et al., (Nature 1985).• Ozone destruction nearly complete.• Halogen (Cl & Br) reactions are responsible.• Polar stratospheric Clouds play a central role.• Multiphase (heterogeneous) reactions denitrify

stratosphere.• Reaction rates depend on accommodation

coefficients, f(T).• Replacement of CFC’s should heal ozone hole.


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Summary of Ozone Hole Formation

• Threat to DNA-based life forms.• Not predicted by any models• First observed by Farman et al., (Nature 1985).• Ozone destruction nearly complete.• Halogen (Cl & Br) reactions are responsible.• Polar stratospheric Clouds play a central role.• Multiphase (heterogeneous) reactions denitrify

stratosphere.• Reaction rates depend on accommodation

coefficients, f(T).• Replacement of CFC’s should heal ozone hole.

LECTURE 15 AOSC 434 AIR POLLUTION RUSSELL R. DICKERSON 2014.

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