Chapter 2: Protecting the Ozone Layer

1. Why do we need to do to protect the ozone layer?

2. Isn’t ozone hazardous to human health?

3. Why is the ozone layer getting smaller?

4. What can we do (if anything) to help stop the depletion of our ozone layer? Catatan: Diambil dari berbagai sumber

Ozone Formation Ozone Formation

Energy + 3 O

2 O

3 Energy must be absorbed

(endothermic) for this reaction to occur.

Ozone is an allotropic form of oxygen. The name ozone comes from a Greek word meaning “to smell”

An allotrope is two or more forms of the same element that differ in their chemical structure and therefore their properties.

Element Allotropes oxygen O , O

carbon graphite, diamond, buckminister fullerenes

The sum of the protons and neutrons.

16.00 Atomic number (Z) Mass number (A)

The number of protons.

(nuclear charge)

2.2 The electrons in the outermost energy levels are called valence electrons.

The group number (of the representative elements) on the periodic table tells you the number of valence electrons.

Group 1A: 1 valence electron

8A Group 3A: 3 valence electrons

3A 4A 5A 6A 7A

Isotopes are two or more forms of the same element (same number of protons) whose atoms differ in number of neutrons, and hence in mass.

Carbon 14 (

6 Isotopes of carbon: C-12, C-13, C-14 – Used in dating old materials

12 also written as: C

– Atomic number:

14 C C

– Atomic mass:

– # of neutrons = Atomic mass-

Atomic #

– 6

protons,

6 electrons, 8 neutrons

The average mass is very close to 12.000 b/c C is by far the most abundant

₆

Representing molecules with Lewis structures : H

Consider water, H O: H

2 O

1. Find sum of valence electrons:

1 O atom x 6 valence electrons per atom = 6

2 H atoms x 1 valence electron per atom = +2 8 valence electrons

2. Arrange the electrons in pairs; use whatever electron pairs needed to connect the atoms, then distribute the remaining electron pairs so that the octet rule is satisfied:

lone pair O H H bonding pair Representing molecules with Lewis structures : Typical valence for selected atoms = the # of bonds an atom typically forms

Element Typical Classification valence

H, 1 monovalent X (X= F, Cl, Br, I)

O 2 divalent N 3 trivalent

C 4 tetravalent Representing molecules with Lewis structures : Multiple bonds O O H C C H

Triple bond Double bond

Occasionally a single Lewis structure does not adequately represent the true structure of a molecule, so we use resonance forms:

N O O O N O O O N O O O Frequency () = number of waves passing a fixed point in one second (waves/s or 1/s or s

The Nature of Light Low E High E Wavelength () = distance traveled between successive peaks (nm).

1 or Hz).

The Electromagnetic Spectrum The various types of radiation seem different to our senses, yet they differ only in their respective  and 

Visible:  = 700 - 400 nm

M M UD A S

I R Dec rea sin g w ave len gth

Infrared (IR) : longest of the visible spectrum, heat ray absorptions cause molecules to bend and stretch.

Microwaves: cause molecules to rotate.

Short range: includes UV (ultraviolet), X-rays, and gamma rays. The energy of a photon of electromagnetic radiation is calculated by: E = h

 where h = 6.63 x 10

. s (Plank’s constant) The wavelength and frequency of electromagnetic radiation are related by: c =

  where c = 3 x 10

8 m/s (the speed of light)

Energy and frequency are directly related- higher frequency means higher energy. What is the energy associated with a photon of light with a wavelength of 240 nm? E = h  C = 

34 . 15 -1

E = (6.63 x 10 J s) (1.3 x 10 s ) = C

 

E = 8.6 x 10 J

8 m/s x 10 ₉ 15 -1

1.3 x 10 s

10 m = 240 nm x nm

UV radiation has sufficient energy to cause molecular bonds to break

The Chapman Cycle  ≤

 ≤ A steady state condition

Factors influencing steady-state Factors influencing steady-state concentration of O concentration of O

Intensity of UV radiation

– Expect O concentration will depend on season and

sun cycle

and other reacting species

Concentration of O

Temperature • Reaction rates (some of the steps are faster than others)
• *Many of these factors depend on altitude, so O

3 concentrations also depend on altitude. Biological Effects of Ultraviolet Radiation The consequences depend primarily on: 1. The energy associated with the radiation.

2. The length of time of the exposure.

3. The sensitivity of the organism to that radiation.

An Australian product uses “smart bottle” technology; bottle color changes from white to blue when exposed to UV light. The most deadly form of skin cancer, melanoma, is linked with the intensity of UV radiation and the latitude at which you live.

As  decreases, damage to DNA increases.

Note the logarithmic scale on the y-axis. O

3 concentrations vs. altitude O concentrations over time

3 http://earthobservatory.nasa.gov/cgi-bin/texis/webinator/ printall?/Library/Ozone/ozone_2.html ₉

1 Dobson unit (DU) = 1 O molecule per 1 billion (10 ) molecules of air

₃ 320 Du is average ozone level over the northern hemisphere 250 DU is typical at the equator “hole” is the area over Antarctica with < 220 DU

Natural causes of O destruction Natural causes of O destruction

1) Reactions with water vapor and its breakdown products:

H O + UV photon  H• + •OH

2 These free radical products can

participate in reactions that convert O to O :

2 Free radicals very

O H•  + O •OH

reactive b/c they

2 have unpaired e- Octet Rule is NOT

2O H• + O + •OH 

2 satisfied

Example of a chain reaction. Free radicals are very destructive

2) Reactions with stratospheric nitrogen 2) Reactions with stratospheric nitrogen

Formation of natural NO: N O + O 

2 monoxide (NO) monoxide (NO)

2NO

– N O produced by microorganisms

Formation of man-made NO: N + O

2 energy

energy  NO

– Energy is from lightning or high-

temp engines of supersonic transport airplanes (like the Concorde) that are designed to fly at altitudes of 15-20 km (the region of the ozone layer)

Lewis dot structure of NO?
Turns out that NO is also a free radical…

Source of VOC and NO in the Upper

However, these natural causes do not

Atmosphere leading to Tropospheric Air explain the amount of ozone destruction

Pollution Chemistry seen in recent years…

CFCs seem to be the culprit CFCs seem to be the culprit

Properties and uses of CFC

Originally developed as refrigerants to
Stable

replace the toxic ones that had previously been used (SO and ammonia—NH )

Nontoxic • Also used as:
Nonflammable • Low boiling point (makes it a – Solvent for computer chips good refrigerant)

– Propellants for aerosol cans

Cheap

– Styrofoam • Widely available
– Fire extinguishers

Before we knew about the ozone hole… Before we knew about the ozone hole…

Two scientists, Roland &
~ 5 years after release in Molina, set out to study fate of troposphere, CFCs make their way into the atmospheric CFC molecules stratosphere.
The reactions
A high-energy photon hypothesized would occur corresponding to suggested that CFCs could wavelengths  220 nm has destroy significant amounts of enough energy to break the O

3 chlorine-carbon bond:

CCl F + photon  •CClF

– The destruction of O by

Cl CFCs later shown by scientific experiments.

Revolutionized modern life! Throughout the 1960s and 70s, cheap air conditioning using CFCs helped spur the booming growth of Southern cities such as Atlanta, Dallas-Fort Worth and Houston.

Reactions with •Cl Removal of •Cl

Cl +O  O + •OCl

Carried to lower
OCl + O  •Cl + O

atmosphere by winds— O + O  2O

3 2 once in lower atmosphere,

there are many other compounds for •Cl to react

You can see that once •Cl is created that it begins a chain reaction—it can

with.

destroy many (~ 100,000!) O ₃

If it encounters another •Cl molecules.

the following reaction may

Cl is acting as a catalyst—it is not

occur: •Cl + •Cl  Cl

2 used up in the reaction

May react with other

The presence of •Cl will affect the

species to form stable

Chapman cycle balance!!!

compounds such as HCl and ClONO

Proof of relationship b/w ozone Proof of relationship b/w ozone depletion and stratospheric Cl depletion and stratospheric Cl

75-85% is from human activity

– CFCs
– Rocket fuel from space shuttles (<1%)

15-20% is from methyl chloride

– Most from natural sources and burning of biomass

~2% - Large, explosive volcanoes

– Increase amount of HCl which can be converted to •Cl
– Only a few volcanoes have enough explosive power to project material into the stratosphere.

Why is the hole over Antarctica? Why is the hole over Antarctica?

• A special mechanism appears to be in effect here, related to the fact that

the lower stratosphere over the South Pole is the coldest spot on Earth.

– During the Antarctic winter (June to September), a strong circumpolar wind develops in the middle to lower atmosphere

keeps warmer new air (w/ new O ) from entering

3 o

– Temps can get as low as -90 C!

– At temps < 80

C, clouds of ice crystals containing sulfates and nitric acid can develop in the stratosphere.

In spring, when the sun comes out, chemical reactions occurring on the surface of these cloud particles convert

otherwise safe (non-ozone depleting) molecules to more reactive species. http://earthobservatory.nasa.gov/Study/Tango/

– ClONO & HCl converted to HOCl &

2 Cl

2 2 •Cl

– HOCl •Cl + •OH ; Cl

Summer conditions replenish hole

In summer, sunlight warms the atmosphere

summer

– Ice crystals evaporate

No ice crystals means the conversion of ClONO & HCl

₂

– Air from lower latitudes flows into the polar regions, replenishing depleted ozone levels.
– Thus, “Ozone Hole” is

spring largely replenished by the end of summer.

Same process is developing in the Arctic

The Global Response The Global Response

CFCs in spray cans banned in North America in

1978, and use as foaming agents for plastics discontinued in 1990.

1987 Montreal Protocol —established a schedule to reduce production & consumption of CFCs.
The US and 140 other countries agreed to a complete ban on CFC production after Dec. 31, 1995.
Other ozone depleting compounds to be eliminated b/w 2002-2
CFBr compounds (fire-fighting)
CCl

₄

₃

Black-market CFCs Black-market CFCs

• While the protocols set dates for the halt
of production, sale of existing stockpiles will remain legal.

– US govt. is promoting conversion to less
harmful substitute refrigerants by imposing a tax of $5.35/lb on CFCs

Price of CFCs has risen $1/lb in 1989 to $15/lb

(as of 2003)

Creates a black-market for CFCs, which are 2 only to illegal drugs as the most lucrative illegal import.
“Freon busts” in 1997 led to the confiscation of 12 million lbs of illegal CFCs

What to use Ban on CFCs is making a difference--slowly instead?

Stratospheric concentration
DON’T want to go back to of chlorine peaked at 4.1 previous technologies ppb in the mid-90s and is

(NH3, SO2 as refrigerants)! now declining.

Need to balance 3
However, the stratospheric undesirable properties and chlorine is not expected to come up with the best reach 2 ppb* until 2050 or compromise 2075.

– Toxicity

Antarctic ozone hole first appeared when chlorine

– Flammability levels reached 2 ppb.
– Extreme Stability

Fluorocarbons (FCs) Fluorocarbons (FCs)

Made of only carbon and fluorine

– Non-toxic
– Nonflammable – Don’t decompose in the stratosphere

• The negative: No destruction pathways
— the molecules will accumulate in the

atmosphere and contribute to the greenhouse effect by absorbing IR radiation.

Hydrochlorofluorocarbons Hydrochlorofluorocarbons

(HCFCs) (HCFCs)

• Replacing one or more of the halogen atoms

with hydrogen makes the compound more
reactive

– Compounds decompose long before reaching stratosphere

However:

– Too many H atoms increases flammability
– Too many Cl atoms increases toxicity

production ban of HCFCs by 2030.

Suitable HCFCs

Decompose in the troposphere more readily than CFCs

HCFC-22 is the most widely-used HCFC

– Used in air-conditioners and in the production of foamed fast food containers
– 5% ozone depleting potential of CFC-12

HCFC-141b used to form foam insulation

– 250 million lbs produced worldwide in 1996 for this purpose

Montreal Protocol calls for a

CFC-12

Hydroflurocarbons (HFCs)

H C C

Example: HFC-134a
Has no Cl atoms to interact with ozone

US Emissions

The 2 H atoms facilitate decomposition in the lower atmosphere, without making it flammable under normal conditions.

http://www.eia.doe.gov/oiaf/1605/vr98rpt/chapter6.html

Timeline

1928 – CFCs invented
1950s-1970s – Consumption and use of CFCs rises rapidly
1971 – CFCs measured in the atmosphere
1974 – Rowland & Molina link CFCs with ozone depletion
1985 – First scientific assessment of stratospheric ozone levels
1987 – “Smoking gun” evidence linking decreasing ozone levels with increasing stratospheric chlorine levels

– Montreal Protocol established a schedule to reduce production & consumption of CFCs.

1991 – Multinational fund established to provide financial and technical assistance to developing countries to enable them to comply with control measures.

B/w 1991 & 2004 $1.6 billion given to more than 100 countries

1995 – Production ban of CFCs in US and 140 other countries goes into effect

– Rowland & Molina win Nobel Prize for their work

2030 – Production ban of HCFCs goes into effect

Chapter 2: Protecting the Ozone Layer