Protecting the Ozone Layer

Learning Objectives Describe the chemical nature of ozone, the ozone layer, and factors affecting its existence Describe the electromagnetic spectrum in terms of frequency, wavelength, and energy Use appropriate calculations to relate energy, wavelength, and frequency of light Understand how the ozone layer protects against harmful ultraviolet radiation Discuss the interaction of radiation with matter and changes caused by such interactions, such as biological sensitivity and the use of the UV index

But first, we need a little chemistry…

number of protons = atomic number How many electrons and protons in these elements? Iodine (I)

number of protons = atomic number How many electrons and protons in these elements? Iodine (I) Silver (Ag) = 53

number of protons = atomic number How many electrons and protons in these elements? Iodine (I) Silver (Ag) = 53 = 47

Valence Shells The first shell holds 2 valence electrons Most subsequent shells hold 8 valence electrons

For the “A” elements, Group number = number of outer electrons

Electron Arrangements *Number of outer electrons are critical. Atom is most stable when outer shell is filled.

What makes an atom happy? Ben and Jerry’s ice cream (Well, maybe not ice cream… :) Would you believe… –having full “valence shells” of electrons

If they don’t have their own, they can share… So H is happy when it can share its one electron with another hydrogen they both “feel” like they have 2 electrons, which is a full (first shell) –While they are still also electrically neutral Therefore, H often goes around as H 2

Single Bonds – Lewis Dot Structures H HF shared pair of electrons = covalent bond : ::..

Alternative notation H HF shared pair of electrons = covalent bond - -

Multiple Bonds ON double bond = 4 shared electrons triple bond = 6 shared electrons :: :::.. ::

Alternative Notation ON double bond = 4 shared electrons triple bond = 6 shared electrons = 

Occasionally a single Lewis structure does not adequately represent the structure of a molecule, so we use resonance forms. Lewis Structure for Ozone

What’s the structure of water?

The 3-D shape of a molecule is determined by electrons…. VS. Water Valence Shell Electron Pair Repulsion Theory The most stable molecular shape has the electron pairs surrounding a central atom as far away from one another as possible.

Water has two electron pairs that are bonded and two that are not bonded. The electron pairs are tetrahedrally arranged, but the shape is described only in terms of the atoms present: So, water is said to be bent in shape. Predicting molecule shapes

Now look at CO 2 Two groups of four electrons each are associated with the central atom. The two groups of electrons will be 180 o from each other: the CO 2 molecule is linear. Group

4 common molecular shapes Bent –water (H 2 O) Linear –carbon dioxide (CO 2 ) Pyramidal –ammonia (NH 3 ) Tetrahedral –methane (CH 4 )

Now we need a little physics…

Waves of Light Energy can be described as waves or particles When described as waves, we consider three important characteristics: wavelength, frequency, and energy The principal relationship is: = c Where –c = speed of light = 3.00 x 10 8 m/s – = frequency (cycles per second, or Hertz) – = wavelength (m)

= c (the speed of light, a constant)

Electromagnetic Spectrum

“Particles” of Energy Planck argued in 1900 that energy distributions are not continuous but consisted of individual ‘steps’; i.e., they are ‘quantized’ Einstein in 1905 argued that radiation should be viewed as bundles of energy called ‘photons’ Wave theory and particle theory are connected Energy per photon: E = h = hc/ where h is Planck’s constant = 6.63 x Joule second (and 1 J is approximately the energy required to lift 1 kg 10 cm).

Energy Calculations Using E = h = hc/, you should be able to calculate the energy of a photon, given the wavelength of that energy Example: What is the energy of a photon with a wavelength equal to 220 nanometers (nm)?

Solar Spectrum

Solar Energy on Surface of Earth (This figure shows only the UV portion of solar radiation)

Color scale (Dobson units) visit

What’s a Dobson Unit?

UV-A = nm UV-B = nm UV-C = nm (ref: ASTM) Biological Sensitivity to UV

Some Effects of Exposure to UV vitamin D production skin tanning or burning melanoma skin cancer eye damage (e.g. snow blindness, cataracts) effects on plants and animals  crop yields in Australia (e.g. wheat, peas)  amphibian populations (frogs?)

NWS UV Index (Table 2.3)

Ozone Concentration vs Altitude

Chapman Cycle O 2 + O O 3 O + O 3 2 O 2 O 2 2 O  < 240 nm O 3 O 2 + O  < 320 nm (1) (2) (3) (4) OZONE FORMATIONOZONE DECOMPOSITION

Steady-State Analogy

Ozone Loss through Catalysis. X + O 3 XO. + O 2 XO. + O. X + O 2 Catalysts are not destroyed and make the net reaction occur faster! Net reaction:O 3 + O2 O 2

Catalysts: Free Radicals in the Ozone! Natural:. H,. OH,. NO Anthropogenic:. NO from SSTs. Cl from CFCs

O3O3 ClO.

How do CFCs interact with Ozone?

Ozone Hole over Antarctica

Ozone Depletion at Mid-Latitudes Chemistry in Context: Applying Chemistry to Society, 3e. A Project of the American Chemical Society. Copyright © 2000 by the American Chemical Society. All Rights Reserved.

Largest extent in 2003 = 28 million km 2

September 08, _front.php?date=

Seasonal Explanation for the Antarctic Ozone Hole austral seasons:Summer: January - March Winter: July - September Vortex winds isolate the polar atmosphere In polar winter, no sunlight means no O 3 formation! Polar Stratospheric Clouds = ice crystal surfaces Chlorine is stored as inactive HCl and ClONO 2 Rising sun in Spring causes “puff” of Cl·

Responses to the Problem Montreal Protocol signed in 1987 Rowland and Molina win 1995 Nobel Prize Production of CFCs halted in US in 1996 Alternatives: HFCs, HCFCs

CFCs: Properties and Uses Primarily used as refrigerants, foaming agents, solvents, aerosol propellants

Usage History of CFCs Montreal Protocol of 1987 (amended in 1990 and 1992) banned CFC manufacturing by year 1996 See also Fig of the text for production through 1996

Current Status Source: NASA TOMS data,

Montreal Protocol of 1987 CFC production to ½ by – ban CFC production by – added bromine compounds to ban –BFCs –CFCs & HCFCs eliminated by 2010 Developing countries vs. Industrialized countries

Fig CFC production…

Substitutes Substitutes must be economic and provide the same technical benefits that CFCs provided (including toxicity, flammability, and stability) Most reasonable substitutes are HCFCs, which decompose faster, but still can cause problems (banned by 2030) HFCs Issues of equity for developing countries

Black Market Bootlegging –Russia, China, India, etc. 2 nd only to illegal drugs –1997: 2 million pounds of illegal CFCs confiscated Pssst… Wanna buy some CFCs?

The Future Stratospheric chlorine peaked in the late 90s Even with our bans in place –2ppb reached after 2050 Replacement products? –toxicity (increase chlorine atoms) –flammability (replace halogens with hydrogens) –stability & other harmful effects (flourocarbons) HFCs Pyrocool FEF