Chapter 25

Ozone Depletion

Erin KreegerAndy KangJason LooneyMatt ProcopioSteven Tepper

Ozone

http://www.youtube.com/watch?v=xOR82MZxEiA

25.1

Ozone

• Ozone is made up of 3 oxygen atoms(O₃)–Diatomic oxygen has 2 oxygen

atoms(O₂)• Oxygen is a strong oxidant

– It chemically reacts with many materials and gasses in the atmosphere.

Ozone in the Atmosphere• Ozone in the stratosphere blocks

harmful UV rays from the sun– About 90% of the ozone is found in the

stratosphere.• In the lower atmosphere however,

ozone acts as a pollutant– It is produced by photochemical

reactions involving sunlight, nitrogen oxides, hydrocarbons, and diatomic oxygen.

UV Radiation & Ozone

• The ozone layer in the stratosphere is called the ozone shield.–Absorbs most of the harmful UV

rays that enter the Earth’s atmosphere.

UV Radiation

• Ultraviolet C (UVC)– Shortest wavelength and most energetic– Has sufficient energy to break down

diatomic oxygen (O₂)– UVC is strongly absorbed and negligible

amounts reach the surface of the Earth

UV Radiation

• Ultraviolet A (UVA)– Has the longest wavelength– Weakest of the three types of UV rays– Not affected by stratospheric ozone and

is transmitted to the surface of Earth– Can cause some damage to cells

UV Radiation

• Ultraviolet B (UVB)– Strongly absorbed by the ozone shield– Ozone is the only known gas that absorbs

UVB• Depletion in ozone allows UVB to reach the Earth’s

surface

– UVB is hazardous to living cells.

Measurement of Statospheric Ozone

• Dobson ultraviolet spectrometer– An instrument used to measure the

concentration of atmospheric ozone• Measurements through the years have

shown an overall loss of ozone in the atmosphere, with certain areas having a significant reduction of ozone levels.– Antartica (ozone hole)

“Ozone hole”

Ozone Depletion &CFCs (25.2)

CFCs

• The hypothesis that ozone was being depleted in part by CFCs (chlorofluorocarbons) was first suggested in 1974 by Mario Molina and F. Sherwood Rowland.

• The public became concerned because CFCs could be found in everyday propellant products such as shaving cream, hair spray, deodorants, paints and insecticides.

• Many people made the decision to buy fewer of these propellants.

CFCs

• Major features of the Molina and Roland hypothesis:

1) The CFCs are emitted in the lower atmosphere by human activity are extremely stable. They are un-reactive in the lower atmosphere and therefore have a very long lifetime (around 100 years). No significant tropospheric CFC filters have been discovered. Soil does however remove an unknown amount of CFCs at the surface-level.

CFCs

2) Because CFCs have a long lifetime in the lower atmosphere and because the lower atmosphere is very “fluid,” the CFCs eventually wander upward and enter the stratosphere. Once in the stratosphere, they may be destroyed by UV radiation, releasing chlorine.

CFCs

3) The chlorine released may then enter into reactions that deplete ozone in the stratosphere.

4) The result of the depletion of ozone is an increase in the amount of UVB radiation (causing skin cancers and immune system deficiencies).

CFCs

• Equations for this hypothesis:

Cl + O3 ClO + O3

ClO + O Cl + O2

Emissions and Uses of Ozone-Depleting Chemicals

• Emissions of chemicals thought to destroy ozone amounted to about 1.5 million metric tons per year, with CFCs accounting for around 60% of these emissions.

• CFCs are found in aerosol propellants, as well as refrigerators, air-conditioning units, and the foam-blowing process that creates styrofoam.

Emissions and Uses of Ozone-Depleting Chemicals

• A variety of cleaning solvents (such as tetrachloride and methyl chloroform) contain chlorine and thus destroy ozone.

• Fire extinguishers use bromine which is contained in halon, and can also deplete ozone.

• Since many have bought fewer propellants, ozone depletion from these products has decreased, but increased regarding refrigeration and blowing-agents.

1986 CFC Use

Propellants

Refrigerants

BlowingAgents

CleaningAgents

Other

1991 CFC Use

Propellants

Refrigerants

BlowingAgentsCleaningAgentsOther

1974 CFC Use

Propellants

Refrigerants

BlowingAgentsCleaningAgentsOther

Chemical Cycle

• Chlorine can enter into a reaction in the atmosphere over and over again; called a catalytic chain reaction.

• Because chlorine is not removed from the equation depicted previously, the process may be repeated over and over again.

• It has been estimated that chlorine may destroy 100,000 molecules of ozone over a period of 1-2 years before chlorine is removed from the atmosphere through other chemical reactions and rain-out.

Chemical Cycle

• The catalytic chain reaction can be interrupted through storage of chlorine in other compounds in the stratosphere. Two possibilities are as follows:

1) UV light breaks down CFCs to release chlorine, which combines with ozone to form chlorine monoxide (ClO). The ClO may then react with nitrogen dioxide (NO2) to form a chlorine nitrate (ClONO2). If this occurs, ozone depletion is minimal.

Chemical Cycle

2) Chlorine released from CFCs may combine with methane (CH4) to form hydrochloric acid (HCl). The acid may then diffuse downward. If it enters the troposphere, rain may remove it; removing the chlorine from the ozone-depleting chain reaction. This is the ultimate end for most chlorine atoms in the stratosphere.

Chemical Cycle

• It has been estimated that the chlorine chain reaction that destroys ozone may be interrupted by the processes described as many as 200 times while a chlorine atom is in the atmosphere.

• Because of this, concentrations of ozone have been depleted in northern and southern temperate latitudes.

Ozone Concentration

• Under natural conditions in the Southern Hemisphere, the highest concentration of ozone is found in the polar regions, and lowest near the equator.

• This may seem like a paradox because ozone is produced by solar energy, and more solar energy is found near the equator. However, the ozone in the stratosphere moves from the equator toward the poles with global air circulation patterns.

25.3

The Antarctic Ozone Hole

• The thickness of the ozone layer above Antarctica has been decreasing since the 70s.

• It’s total size now exceeds the area of North America.

What caused the ozone depletion over

Antarctica?• During the winter, specific clouds called polar stratospheric clouds form.

• At the same time, a mass of cool air called the polar vortex forms. This is caused by a lack of sunlight and poor airflow.

• These supercooled clouds allow small sulfuric acid particles to freeze, allowing sulfur oxide and nitrogen oxides to form. Other chemicals like chlorine and fluoride mix in.

• When these clouds are exposed to sunlight, it creates a process that causes a chlorine molecule to break off.

• This process facilitates the depletion of ozone.

Possible complications in the Arctic circle

• The same process is being observed in the arctic circle.

• There, a polar vortex also forms and during some months, ozone losses can be as high as 40%.

• If the situation worsens, effected air masses could travel south and effect populated areas.

http://www.youtube.com/watch?v=qUfVMogIdr8&feature=related

25.4 Tropical and Midlatitude Ozone Depletion

• Ice particles can also form in the stratosphere over the tropics causing Ozone depletion similar to the Arctic Ozone.

• Also, Sulfuric acids aerosols are abundant in the stratosphere because of injection of sulfur by volcano eruptions.

• These are only assumptions on Ozone depletion in the tropics. There is no solid evidence to whether these particles cause Ozone depletion.

• A reason why Environmentalists believe that Volcanic eruptions cause Ozone Depletion is because in 1982 a Volcano called El Chichon in Mexico caused a 10% decrease in Ozone in the Northern Hemisphere.

• Ozone in midlatitude areas has decreased with the years similar to the Arctic areas.

• While midlatitude ozone depletion is not as severe as Arctic ozone depletion, it shows that global warming has a negative effect on ozone in regions all around the world.

25.5 Future of Ozone Depletion

• Even if the manufacture, use and emission of all ozone-depleting chemicals were to completely stop, the problem would still not be solved.

• Millions of metric tons of ozone-depleting chemicals in lower atmosphere, moving up to stratosphere.

• Several CFCs have atmospheric lifetimes of 75 to 140 years.

• As of 1992 CFC-11 has peaked and is now decreasing and CFC-12 has leveled off with very little increase. (CFC-12 accounts for 50% of ozone depletion.

Environmental Effects

• Several serious potential environmental effects.• Damage to Earth’s food chains both on land and

in the ocean.• Damage to human health including increases in

all types of skin cancers, cataracts, and suppression of immune systems.

• A 1% decrease in ozone can cause a 1-2% increase in UVB radiation and a 2% increase in the incidence of cancer.

Environmental Effects (continued)

• May lead to a reduction of primary productivity in the world’s oceans of phytoplankton

• Phytoplankton being the base of the food chain, this would have a negative impact on a variety of other marine organisms.

• In recent years, ozone over the Antarctic has been depleted as much as 80%.

• A study done on the waters beneath the mass of ozone depleted air in the Antarctic suggests that there is a 6-12% reduction of primary productivity due to ozone depletion.

Big Picture

• If ozone depletion is to become more widespread it will affect major crops such as beans, wheat, rice, and corn.

• A small decrease in food production would have large social and economic consequences.

Montreal Protocol• Outlined a plan for the eventual reduction of global

emissions of CFCs to 50% of the 1986 emissions.• 27 nations signed the agreement originally with 119

additional nation later.• US and most industrialized countries stopped CFC

emissions by the end of 1995.• The goal of the Montreal Protocol is the eventual phase-

out of all CFC consumption.• Two substitutes for CFCs being experimented with today

are hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons(HCFCs).

• Problem is that most chemical replacements for CFCs are more expensive than CFCs.