CHEM 2505: The Ozone Layer and Ozone Holes

O2 and gases like N2 filter UV wavelengths of...

50 to 120 nm

Describe the chemical makeup of the atmosphere (when unpolluted).

78% N2, 21% O2, 1% Ar, and 0.04% CO2

What is an ozone "sink"?

A natural removal process such as dissolution in rain, oxidation by atmospheric gases or photolysis.

Describe UV-C

Absorbance spectrum: 200-280 nm High energy Highly damaging O3 filters most, with a little help from O2

Describe UV-B

Absorbance spectrum: 280-320 nm Medium energy Medially damaging Partly absorbed by O3

Describe UV-A

Absorbance spectrum: 320-400 nm Low energy Low damage Not absorbed by O3 or O2

How does inactive chlorine get converted to active chlorine?

Activation caused by special polar winter weather. Occurs on the surface of liquid particles formed by water, sulphuric acid, and nitric acid. No atomic oxygen is present, so that normal rxn of the lower stratosphere can't occur. Temp drops to -80*C, causing air pressure drop, then vortex created, then PSCs result, ice crystals, activation of Cl

What is the Montreal Protocol?

An international agreement to phase out CFC production, 1st international environmental agreement to use the Precautionary Principle. Signed by 24 countries in 1987. Now, all ODSs are being phased out in all nations

What is an ODS?

An ozone depleting substance

What is the Precautionary Principle?

Assume a chemical is dangerous until its proven safe. Action taken before scientific proof of a problem because the effects are so severe.

Where does most ozone lie?

Between 15-35 km, called the ozone layer. Middle and lower stratosphere.

What does full spectrum sunscreen block?

Blocks UV-A and UV-B light. Sunscreens that only block UV-B may lead to an increase in skin cancer as people can withstand more UV without burning.

Describe nitric oxide's role as a catalyst in the destruction of ozone.

Can exist in unpolluted air. Nitrous oxide (N2O) molecules rise from the troposphere and collide with excited oxygen atoms (produced by photochemical destruction of ozone). N2O + O* --> 2NO NO + O3 --> NO2 + O2

Name an ODS.

Carbon tetrachloride (CCl4). Decomposed similar to CFCs. Used as a solvent, some lost in atmosphere when used as an intermediate in manufacture of CFCs. Used as dry-cleaning solvent. Lifetime 26 years

True or false: ozone is transported downward after production in the stratosphere.

True

What is temperature inversion?

Occurs when the air is warmer than the air below it. Heat generated by ozone production causes the temperature of the stratosphere to be higher than the air above or below it.

Describe the Arctic ozone hole.

Ozone destruction is less severe since stratospheric temperatures do not fall as low and air circulation is not as limited. The northern hemisphere has a flow of tropospheric air over mid-latitude mountains, warming the Arctic stratosphere; warmer temperatures decrease the frequency and duration of PSCs, and small crystals are not large enough to fall out & denitrify air

What is the equation for measuring partial pressure of gas in ATM, kPa or bar?

PV = nRT

What are PSCs

Polar stratospheric clouds; result from condensation of gases within the vortex

Describe the green chemistry behind the replacement of CFCs.

Polystyrene a common polymer. Foam is made by combining the melted polymer with a gas under pressure and allowing expansion into a lower pressure environment, gas used was a CFC. CO2 was introduced as a blowing gas; not as detrimental as CFCs. Used from other processes.

What are the 2 types of sunscreen?

Reflect or scatter sunlight (i.e. particles of ZnO or TiO) Absorb UV light (i.e. soluble organic compounds)

What is TFA?

Trifluoroacetic acid. An intermediate in one possible degradation mechanism. Removed from the atmosphere by rainfall, may accumulate in plants and inhibit growth.

True or false: all of the energy for dissociation must come from a single photon.

True

True or false: cool air is denser so it does not spontaneously rise

True

When does Arctic ozone destruction increase?

- When the vortex is cool and prolonged, causing denitrification by PSC formation and ozone depletion by Mechanism II - Irregular in shape, sometimes a portion passes over sunlit area causing temporary ozone depletion - Increases in CO2 (cools stratosphere, extending life of PSCs)

Describe the annual ozone hole that occurs above Antarctica.

-Occurs in Antarctic spring (Sept-Nov) and closes until next spring -Ozone levels reduced by ~50% -Approx. size of North America -Will continue until middle of century -Sometimes appears over the Arctic

What is the absorption spectrum of ozone?

200-280 nm (really can't absorb past 280 nm)

What are the 3 requirements for photolysis reactions?

1. Light of sufficient energy must come from a single photon. Energy required is delta H of the rxn. 2. The wavelength of light must be absorbed by the molecule. 3. The light must be used by the molecule for reaction rather than other processes.

The O2 above the stratosphere filters UV wavelengths of...

120 to 200 nm

What are halons?

Chemicals containing bromine but no hydrogen. Used in fire extinguishers. Nontoxic, leave no residue upon evaporation. Lack of hydrogen means they have no tropospheric sinks and eventually rise to stratosphere. Photolysis releases atomic Br (or Cl) which act as catalysts for ozone destruction.

What are CFCs?

Chlorofluorocarbons

What chemicals may cause ozone destruction?

Chlorofluorocarbons (CFCs) --> increased levels of stratospheric Cl and Br due to release of anthropogenic compounds

What do Dobson Units (DU) measure?

DU measures the total atmospheric ozone over a given point on Earth (1 DU = 0.01 mm thickness of O3 at the density it would have at 1 atm and 0*C)

Describe the vortex created during chlorine activation.

Decreased temperature causes decreased air pressure, causing a vortex in which is isolated and very cold.

How is ozone depleted in the Arctic?

Denitrification is decreased, because the vortex breaks up before sunlight returns. NO2 containing air mixes with the vortex air. Stratospheric temperatures rise more quickly than in Antarctica converting nitric acid particles back to NO2. Both deactivated chlorine. Decreased destruction compared to Antarctica.

What is the equation for energy?

E = hv = hc/wavelength where h = Planck's constant v = frequency c = speed of light

What determines how much energy of light a compound will absorb?

Energy levels of electrons & its structure

What would occur to energy if wavelength decreases?

Energy would increase

True or false: UV wavelengths of 50-200 nm reach the Earth

False

True or false: a large hole in the ozone layer appeared over Antartica in the mid-90's.

False: the mid-80's.

True or false: CFCs have a sink in the troposphere.

False; they gradually diffuse from the troposphere into the stratosphere and are decomposed into Cl and Br atoms by UV-C

Describe how hydroxyl free radical acts as a catalyst in the destruction of ozone.

Formed by: O* + CH4 --> CH3 + OH Destruction: OH + O3 --> HOO + O2

Describe the species of oxygen in the stratosphere.

High concentration of O2 and a little atomic O. Most UV-C has been filtered by O2 above, air is denser.

Describe the species of oxygen at the bottom of the stratosphere.

High concentration of O2 and very little atomic O. Almost all high energy has been filtered, air is denser.

What are the consequences of ozone depletion?

However levels of ozone allow for more UV-B to penetrate. UV-B causes sunburn, suntan, and skin cancer. UV-B is absorbed by DNA molecules. Its most damaging effects are at 300 nm.

What are HCFCs?

Hydrochlorofluorocarbons. Removed from the troposphere by hydrogen abstraction by OH. Long term ozone depletion is small, short term ozone depletion is greater since HCFC-22 will decompose more readily to release atomic Cl.

What are HFCs?

Hydrofluorocarbons. Main long term replacement for CFCs. Longer atmospheric lifetime than HCFCs, undergo OH attack.

How does stratospheric chlorine exist?

In two catalytically inactive (reservoir) molecules: HCl gas and ClONO2 gas -reconversion to the active Cl form also occurs

What is denitrification of the lower stratosphere?

Inactivation of ClO normally occurs by reacting with NO2. All NO2 is bound in PSC crystals. Larger crystals move down into the upper troposphere by gravity and increase life of ozone hole. When vortex breaks, clouds disappear and nitrogen is released. Cl is deactivated, ozone hole closes but not before ozone depleted air is transported.

Describe how atomic choline plays a role as an X catalyst.

Interaction of chloride ions in oceans with decaying vegetation produces methyl chloride gas or chloromethane, CH3Cl. Some CH3Cl reaches the stratosphere where CH3Cl + UV-C --> Cl + CH3 or CH3Cl + OH --> Cl + other products -chlorine is also produced during manufacture or usage of CFCs

Which wavelength of UV light is most harmful?

Low wavelength.

Describe the skin cancers from UV exposure.

Malignant melanoma (25% mortality)- directly related to short periods of very high UV exposure, usually in early life. Lag time of 15-25 years from first exposure to development of cancer. Non-malignant- more common, lag time of 30-40 years. Effects of ozone depletion not yet apparent due to long lag time involved

Describe how atomic bromine is produced as an X catalyst.

Methyl bromide (CH3Br) is produced similarly to methyl chloride. It catalytically destroys ozone in a similar way using mechanism 1. Almost all stratospheric Br remains in the active free radical forms of Br or BrO. HBr and BrONO2 are photochemically decomposed. Formation of HBr is slower since the reaction is more endothermic.

What do relative concentration scales of gases in air measure?

Mole fractions or molecule fraction.

What are the units for absolute concentration sales of gases in air?

Molecules per cubic centimetre of air.

Describe the species of oxygen located above the stratosphere.

Most oxygen is in atomic form. Here the air is thin, UV-C causes dissociation of molecules.

Describe bromine vs. chlorine as catalysts.

Much more bromine is in the active form than chlorine, therefore it is more efficient at destroying ozone. There is much less bromine in the stratosphere, it is less important than chlorine.

Describe the destruction of stratospheric ozone.

Much of the ozone destruction is caused by O3 + UV photon (wavelength < 320 nm) --> O2* + O*. Most of the produced O2 will react with O to reform O3.

Do fluorine atoms act as catalysts?

No: while fluorine is in halons, CFCs, HCFCs, & HFCs, fluorine is liberated upon decomposition in stratosphere. Reaction with F of any H-containing molecule produces HF, a stable strong-bonded molecule. H-F bond is stronger than OH bond, precluding OH activation.

Describe general properties of CFCs.

Nontoxic, nonflammable, nonreactive Useful condensation properties Leak from devices, ascend to lower stratosphere via molecular collision Vertical motion in stratosphere is slow, CFCs must rise to mid-stratosphere where UV-C decomposes them and releases Cl atoms (so atmospheric lifetimes are long)

Describe how ozone is created in the stratosphere.

O + O2 --> O3 + heat -requires high concentration of O2 and low concentration of O

What is the Chapman mechanism?

O2 --> (UV-C) --> O --> (O2) --> O3

Describe the troposphere.

The atmosphere layer closest to Earth, where almost all weather occurs. Stretches from ground level to ~15 km altitude.

Describe the steady state of ozone.

The concentration of ozone does not change much with time. The rate & formation and the rate of elimination are about equal, even though many reactions are constantly occurring. Ozone has achieved a steady state.

What does the rate of formation of ozone depend on?

The concentrations of reactant species (dependent on the amount of UV light) at a specific altitude.

Describe how UV effects the eye.

The cornea and lens filter out almost all of UV before it enters retina. UV-B absorbed by the cornea produces free radicals that attack the cornea, producing cataracts. Macular degeneration is also linked to UV exposure.

Describe the second mechanism of the catalytic destruction of ozone without atomic oxygen.

The lower stratosphere has little oxygen. X + O3 --> XO + O2 Overall.... 2 O3 --> 3 O2

Define ozone layer.

The ozone layer is a region of the atmosphere that filters out harmful UV radiation from sunlight before it reaches earth's surface.

Describe the stratosphere.

The stratosphere is above the troposphere, from ~15-50 km altitude. The region where temperature trends undergo reversal.

True or false: energy and wavelength are inversely proportional

True

True or false: most ozone is produced at the equator

True, but stratospheric winds transport it toward the poles (tropics ~250 DU, sub polar regions ~450 DU)

True or false? A third molecule (any kind) is required to carry away the generated heat.

True. O + O2 + M --> O3 + M + heat

Describe the vertical mixing of air.

Vertical mixing of air in stratosphere is slow, the air is stratified. The troposphere undergoes vertical mixing as sun heats the ground and adjacent air more than higher air. As altitude increases, air temperature falls, warm less dense air rises, causing vertical air exchange.

Describe how sunlight and vitamin D are related.

Vitamin D is synthesized from precursor chemicals by UV light absorbed by the skin. Vitamin D is an anti-cancer agent, needed for calcium utilization and bone regeneration. Moderate UV exposure (especially in winter) in high latitudes may be beneficial.

When does maximum ozone formation occur?

When the product of UV intensity and concentration of O2 is maximum.

Describe the first mechanism of the catalytic destruction of ozone.

X + O3 --> XO + O2 (abstraction of O from O3) XO + O --> X + O2 (abstraction of O from XO) overall rxn: X + XO + O3 + O --> XO + O2 + X + O2 O3 + O --> 2 O2

What is bond enthalpy?

energy required to break a bond

What are most atmospheric concentrations expressed as?

ppb (10^9 molecules of air) *or ppm? *clarify

What does SPF stand for?

sun protection factor. SPF 30 = person can stay in sun 30 times longer than without sunscreen before they burn.