Stratospheric Ozone: Production, Destruction, & Trends Antarctic Ozone Hole: Sept. 12, 2012 http://ozonewatch.gsfc.nasa.gov/

Take away concepts Why is the Ozone layer important? Atmospheric thermal structure UV radiation and stratospheric O2 and O3. Natural ozone production and destruction The life cycle of CFCs, and their role in ozone destruction. Why the Antarctic ozone hole is larger than the arctic, and why there is no "hole" over the tropics. How large is the ozone hole now? Future trends

Ozone (O3) Triatomic oxygen molecule Max concentration is ~10 ppm, occurring in Stratosphere (15-30 km). Stratosphere contains 90% of all ozone. Tropospheric ozone from auto emissions (bad)

What does the ozone layer do for us? Ozone absorbs UV radiation (between 210 and 290 nm). Radiation at these wavelengths harm biomolecules (DNA) The ozone layer is a consequence of oxygen-only chemistry. Significant oxygen appeared on earth about 2 billion years ago due to rise of cyanobacteria. cyanobacteria

Why it matters … UV damages DNA

Structure of the Ozone Layer note difference in profile due to absolute and relative concentrations. Absolute conc is more important in terms of ability to absorb UV light and in terms of reaction rates. absolute conc profile peaks around 25-30 km (lower-to-mid stratosphere) typical VMRs for ozone in the stratosphere: 1 – 10 ppmv. Typical number densities: 1011 – 1012 /cc questions: why does the ozone layer exist? Is it connected with the fact that there is a thermal inversion throughout the stratosphere? (Answer: dioxygen photodissociation essentially provides the heating mechanism for the stratosphere. Observations: (i) O3 is not the most concentrated gas in the ozone layer (not even close!) (ii) maximum concentration is in the middle stratosphere. Why does the ozone layer exist in the stratosphere? What processes are responsible for its formation and maintenance?

Radiation and Matter Also dependent upon the frequency of radiation!

Emission Spectra: Sun and Earth

INCOMING Solar Radiation budget Incoming Solar energy… Percentage Reflected 35% Absorbed by atm (ozone) 18% Scattered to Earth from blue sky 10% Scattered to Earth from clouds 15% Radiation going directly to Earth surface 22% 100%

Composition of the atmosphere Nitrogen: 78% Oxygen: 21% Argon: 1% CO2: 380 ppm Plus: CH4, H2O, He,

Atmospheric layers 60 km 15 km

Why is the Sky blue? Ozone scatters short-wavelength (blue) incoming radiation from the sun. Rayleigh scattering of incoming, short wavelength radiation (photons with specific energy) Radiation scattered by O3, O2 in stratosphere (10-50 km)

Why are sunsets red? Blue wavelengths are scattered/absorbed Red and orange pass through to surface

Blackbody emission curves and absorption bands

Photon Absorption: Vibration, Dissociation, and Ionization

Absorption of incoming solar radiation 1.The shortest wavelengths of the solar spectrum, the ultraviolet < .12 microns are absorbed by oxygen and nitrogen in the upper atmosphere above 100 kilometers. 2.UV radiation between between .12 and .18 microns are absorbed by oxygen above 50 kilometers. 3.UV radiation between .18 and .34 microns are absorbed between 50 kilometers and 10 kilometers by ozone. 4.The atmosphere is nearly transparent to visible radiation between .34 and .7 microns. 5.The infrared part of the solar spectrum is partially absorbed by water vapor in the troposphere below 10 kilometers.

Thermal Structure of the Atmosphere

Why are the Thermosphere and Stratosphere so “hot”? Photodissociation of O2 O2 + hn  O + O ionization l 120-210 nm Stratosphere: Ozone production and destruction is exothermic. O2 + O  O3 (production) O3 + O  2O2 (destruction) produces heat l < 310 nm

Photodissociation of Oxygen O2 + UV light (120-210 nm) --> O + O Makes “free O” for making ozone (O3) Occurs above 50 km in atmosphere (Upper Stratosphere)

“Good” and “Bad” Ozone Now

Natural Ozone Production The Chapman Profile: balancing density and photon flux

Factors controlling the rate of photodissociation 1.The wavelength of light.
The wavelength must be short enough so the wave has sufficient energy to break the bond between the two atoms in the oxygen molecule. The most efficient wave lengths for photodissociation occur in the ultraviolet (0.15µm). 2.Variation of oxygen density.
As altitude increases oxygen density decreases (Chapman Profile). The higher the oxygen density the greater the likelihood of having an interaction between an oxygen molecule and a photon. 3.Variation of photon flux Photon flux decreases with decreasing altitude because of photon absorption by the atmosphere. Rate of photodissociation of oxygen is greatest at an altitude of about 100 km:

The Chapman Cycle 1930s, Sydney Chapman proposed a series of reactions to account for the ozone layer: the Chapman Cycle The Chapman Cycle explains how the ozone layer is formed and maintained. Describe this process in some detail. Four chemical reactions: Initiation O2 + light  2O (120 – 210 nm) Propagation (cycling) O + O2 + M  O3 + M* (exothermic) O3 + light  O2 + O (220 – 320 nm) Termination O3 + O  2O2 (exothermic)

The Chapman Cycle Oxygen-only Chemistry Give the four Chapman reactions: O2 + hv  2O (< 242nm) O + O2 + M  O3 + M* (generates heat) O3 + hv  O2 + O(1D) (usually followed by rapid deactivation to 3P) O + O3  2O2 Need to talk about the time frames of the reactions. In one cc, typical rates for the four reactions are: 106 molecules of O2 photodissociate per second 105 molecules of O3 react with O per second 108-109 “cycles” of Ox conversions occur per second Thus, the typical “chain length” of the cycles is about 1000. In other words, the rate of interconversion among Ox species is (very roughly) one thousand times faster than the rate of Ox production or loss. “odd-oxygen” species (Ox) are rapidly interconverted Ox = O + O3

Ozone Production (>50km) Ozone Destruction (50-15 km)

O3 production & destruction

DU: Ozone measurement unit UV A not blocked by Ozone (0.3-0.4 µm), UV B partially (0.28-0.31), UV C (0.1-0.28) totally blocked by Ozone. Dobson Sp[ectrometer measures UV C / UV A ratio In 1985, Joe Farman of British AA survey measured dramatic loss in ozone.. Dobson Unit (100 DU = 1 mm O3 at STP)

Enter CFCs What are they? Chlorofluorocarbons, organic compounds developed (by Dupont) in 1930s as safe, odorless substitutes for refrigerants, aerosol propellants, spray foam and insulation. Example: CFC-12  CCl2F2

Rowland & Molina & Crutzen (1974) Nobel Prize (1995) Discovered that CFCs can last 10-100s of years in atmosphere. CFCs susceptible to break down by UV Predicted that CFCs will reduce ozone inventories. Proof that this was occurring came in 1985. Montreal Protocol 1987

Key ingredients to make an Ozone Hole: Chlorine: supplied by manmade CFCs Cold: Antarctic Polar Vortex Seasons: Dark and Light seasons Clouds: Polar Stratospheric Clouds UV radiation: Springtime sunlight

CFCs: Chlorofluorocarbons CFCs introduced 1950s “Miracle compounds”: inert, cheap, many applications. Uses: Foam & Insulation Propellants Air conditioning Electronics

CFC Compounds

Antarctic Polar Vortex Large polar land mass Ice covered, large temperature gradients Circular airflow around Antarctica APV effectively creates an atmospheric “fence” impeding air exchange with other regions (this is arctic polar vortex)

Polar Stratospheric Clouds (PSCs) Ice clouds during Austral winter (no light, very cold) PSCs concentrate, activate Cl (as Cl- and CLO) Ice crystals act as reaction sites for O3 destruction CFCs accumulate in stratosphere

O3 loss by UV photolysis Austral spring: sunlight appears, UV UV radiation splits off Cl atom from CFC molecule Ozone destroying reactions: Cl + O3 --> ClO + O2 ClO + O --> Cl + O2 (Cl is free to react with another O3 again) Net: O3 + O --> O2 + O2

UV radiation and CFCs

CFC and O3

The Ozone Hole – Explained! review again the sequence of events leading the the formation of the ozone hole

Discovery of the Ozone Hole British Antarctic Survey (Farman et al 1985) Molina et al. (1974) Farman et al. (1985)

Antarctic Ozone Hole in Dobson Units

Area of the Ozone Hole http://jwocky.gsfc.nasa.gov/eptoms/dataqual/oz_hole_avg_area_v8.jpg

The Ozone Hole right now Antarctic Ozone Hole: Sept. 12, 2012 http://ozonewatch.gsfc.nasa.gov/

The Ozone Hole in 2007 Blue = low ozone levels

Current Status (2012) Ozone Hole Area SH Dobson units http://jwocky.gsfc.nasa.gov/eptoms/dataqual/ozone_v8.html

Future projections

Full recovery takes a long time (50 years)

The world avoided… April, 70-90˚N

By 2020, an Arctic “ozone hole” is apparent

As over Antarctica, the ozone losses are extreme, and spread to mid-latitudes

A Brief History June 28, 1974, Drs. Sherry Rowland and Mario Molina published the first study warning that CFCs could harm the ozone layer (Molina and Rowland, 1974). They calculated that if CFC production continued to increase it would cause a global ozone loss of 30-50% loss by 2050. (current number is 70%!). They warned that the loss of ozone would significantly increase the amount of UV-B light reaching the surface, increasing incidences of skin cancer. Although no stratospheric ozone loss had been observed yet, CFCs should be banned. At the time, the CFC industry was worth about $8 billion in the U.S., employed over 600,000 people directly, and 1.4 million people indirectly (Roan, 1989).

Tactics of the Ozone Hole Skeptics (1970s - 90s) Launch a public relations campaign. Predict dire economic consequences. Find and pay a respected scientist to argue your point. Elevate discredited scientific studies. Emphasize scientific uncertainty. “Cherry-pick” data to support your view. Disparage and impugn specific scientists. Compliance puts the nation at an economic disadvantage. More research is needed before action should be taken. Argue that it is less expensive to live with the effects. In October of 1994, Rep. Tom Delay introduced a bill that would have essentially repealed the entire section of the Clean Air Act that deals with ozone depleting substances. Some time later, Rep. John Doolittle (Republican from California) introduced a somewhat less extreme measure that would have pushed back the phase-out date from CFCs from 1996 to the year 2000. (A friend of mine remarked that "Doolittle and Delay" sounded like a pair of characters from a Gilbert and Sullivan production.) Either measure would have amounted to a US withdrawal from the Montreal Protocol and its successors. After the Republicans took control of Congress in the 1994 elections, the House Subcommittee on Energy and Environment, now led by Rep. Dana Rohrabacher, decided to hold a hearing related to these proposals. At the first panel, dealinf with the scientific issues, Singer and Baliunas represented the skeptics, while Robert T. Watson (then at the Office of Science and Technology Policy) and Dan Albritton, director of the NOAA Aeronomy Lab, represented the conventional scientific wisdom. (A fifth scientist, Richard Setlow from Brookhaven, also spoke, but he dealt only with the biological effects of UV radiation, not with whether ozone depletion was actually taking place or not. I'm not sure why the Republicans brought him in. A subsequent panel dealt with economic and policy issues.) The hearings were held on September 20, 1995. A great link to Jeffrey Masters’ article on this