The Ozone Layer: Formation and Depletion

This presentation explains the Earth's atmosphere and its composition and variation of temperature and pressure in different layers of the atmosphere. It contains atmospheric circulation in troposphere and stratosphere. It explains the process of ozone formation and how its stability affects by the other chemical components which lead to the ozone depletion and ozone hole. It also contains the cosmic ray theory of ozone hole.

1. THE OZONE LAYER: FORMATION AND DEPLETION Presented by: Kamran Ansari Center for Basic Sciences, PtRSU , Raipur Date: 06 - 03 - 2019

2. Contents Earth’s atmosphere and its composition. Variation of different atmospheric parameters (e.g. Temperature, Pressure and Density). Formation of stratospheric ozone. Ozone hole and ozone depletion. Cause of o zone depletion.

3. Earth’s atmosphere and its composition Atmosphere is a set of layers of different gases that surrounds a planet (e.g. Earth) and retained by planet’s gravity. Earth is a only planet in the solar system with an atmosphere having all the necessary component that can sustain life. - Protecting from high energetic and harmful radiation from Sun. - Create different weather system. - Appropriate pressure for forming important gases. - Maintains the surface temperature.

4. Fig. 1 : Different gas composition in Earth’s atmosphere. Courtesy : Wikipedia

5. Vertical thermal structure of Earth’s Atmosphere Troposphere – Temperature decreases with altitude due to less radiation comes from Earth’s surface with altitude. Stratosphere – Temperature increase with altitude due to ozone absorb UV radiation during photolysis. Mesosphere – Temperature decreases with altitude due to abundance of radiation absorbing molecules. Thermosphere – Temperature increases with altitude due to ionized O 2 and N 2 molecules present. Fig. 2 : Vertical structure of temperature in different layer. Courtesy : C . Donald Ahrens-Essentials of Meteorology - An Invitation to the Atmosphere

6. In the case of Hydrostatic equilibrium in atmosphere, the hydrostatic condition is, For isothermal condition, = standard atmospheric pressure at sea level = 1.01 x 10 5 Pa H = scale height = = density of air at sea level is approx. 1.225 kg/m 3 at temperature 15 ◦ C.   Vertical variation in Atmospheric pressure

7. Fig. 3 : (a) Variation of air density and air molecules with altitude, and (b) Variation of atmospheric pressure. Courtesy : C . Donald Ahrens-Essentials of Meteorology - An Invitation to the Atmosphere (b) (a)

8. Depends on the temperature, extent of water vapor, wind pattern in that latitude makes several pressure belts are: Equatorial Low Pressure Belt, Sub - Tropical High Pressure Belt, Sub - Polar Low Pressure Belt, and Polar High Pressure Belt. Horizontal variation in Atmospheric pressure

9. Fig. 4 : Atmospheric circulation on Earth’s troposphere. Courtesy : Wikipedia

10. Atmospheric circulation in stratosphere : - Temperature inversion in stratosphere makes warmer layers above and cooler layers below, makes the stratosphere dynamically stable in vertical. No regular convection and associated turbulence. The horizontal mixing of gaseous components proceeds much more rapidly than does vertical mixing.

11. Fig. 5 : Atmospheric circulation on Earth’s stratosphere (Brewer-Dobson circulation). Courtesy : Wikipedia

12. Formation of Stratospheric Ozone (O 3 ) Fig. 5 : The vertical distribution of atmospheric ozone. Courtesy : Wikipedia Ozone (O 3 ) is most abundant gas component in stratosphere absorb most of the Sun’s ultraviolet radiation, this region is also ozone layer . The photochemical mechanisms for the formation of ozone were discovered by the British physicist Sydney Chapman in 1930.

13. Fig. 6 : The process of ozone photochemical production. Courtesy : www.theozonehole.com/ozonecreation.htm

14. Fig. 7 : Global distribution of ozone by NASA AURA Satellite (OMI) on 27 th Feb 2019. Courtesy : NASA - Ozone Monitoring Instrument (OMI)

15. Normal ozone concentration is about 300 to 350 DU. The term “Ozone hole” is applied over region w hen stratospheric ozone depletion is so severe that levels fall below 200 DU. Ozone loss now occurs every late winter and spring time above Antarctica (South pole), and to a lesser extent the Arctic (North pole). Special meteorological conditions and very low air temperatures accelerate and enhance the destruction of ozone loss by man-made ozone depleting chemicals (ODCs). Ozone Depletion and Ozone Hole

16. NO x Catalytic Cycles In 1970, Dr. Paul Crutzen showed that the nitrogen oxides NO and NO 2 react catalytically (without themselves being consumed) with ozone, thus accelerating the rate of reduction of the ozone content . NO + O 3 NO 2 + O 2 NO 2 + O NO + O 2 Net Reaction: O 3 + O 2O 2 CAUSES OF OZONE DEPLETION Fig. 8 : Paul Crutzen . Courtesy : Photo from the Nobel Foundation archive.

17. 2. Chlorine Catalytic Reactions Dr. M. Molina and Dr. S. Rowland in 1974 suggested that a man-made group of compounds known as the chlorofluorocarbons (CFCs) were likely to be the main source of ozone depletion. CFCs (also known as Freon) are non-toxic , non-flammable and non-carcinogenic. CFCs are very stable gas and can remain 20 to 200 years. CFCs are broken down by ultraviolet (UV) rays from the Sun, releasing free chlorine which becomes actively involved in the process of destruction of ozone. Fig. 9 : Mario J. Molina. Courtesy : Photo from the Nobel Foundation archive. Fig. 10 : F. Sherwood Rowland .

18. Cl + O 3 ClO + O 2 ClO + O Cl + O 2 Net Reaction: O 3 + O 2O 2 Leaving the chlorine atom free to repeat the process up to 1,00,000 times, resulting in a reduced level of ozone. Highly reactive trace-gas molecules known as radicals dominate stratospheric ozone destruction, and a single radical in the stratosphere can destroy up to 10,000 ozone molecules before being deactivated and removed from the stratosphere

19. Fig. 10 : Destruction of ozone by Chlorofluorocarbons (CFCs). Courtesy : Sivasakthivel.T and K.K.Siva Kumar Reddy, Ozone Layer Depletion and Its Effects: A Review, 2011, International Journal of Environmental Science and Development.

20. Why has an “ozone hole” appeared over Antarctica when ozone- depleting substances are present throughout the stratosphere? Fig. 11 : Minimum Air Temperature in the Polar Stratosphere. Courtesy : www.esrl.noaa.gov/csd/assessments/ozone/2010/twentyquestions/Q10.pdf Polar stratospheric clouds (PSCs) are important components of the ozone depletion process in the polar regions of the Antarctic and the Arctic. PSCs provide the surfaces upon which chemical reactions involved in ozone destruction take place.

21. Fig. 11 : A comparison of chlorine monoxide (left) and ozone concentration (right) derived by MLS at approximately 18 km altitude on August 30, 1996. Courtesy : NASA Earth Observatory.

22. Ozone Depletion (DU/km) Years Fig. 12 : A comparison of Ozone depletion (up) and monthly average daily Sunspot number (down) during 1950 -2000. Courtesy : Hathaway, D.H. Living Rev. Sol. Phys. (2015) 12: 4. https ://doi.org/10.1007/lrsp-2015-4 Courtesy : Sivasakthivel.T and K.K.Siva Kumar Reddy, Ozone Layer Depletion and Its Effects: A Review, 2011, International Journal of Environmental Science and Development. Cosmic ray theory for ozone hole

23. They suggested that dissociation of chlorofluorocarbons by capture of electrons produced by cosmic rays and localized in polar stratospheric cloud ice may play a significant role in causing the ozone hole.

24. Fig. 13 : (a) Cosmic-ray ionization-rate variation as a function of altitude, (b) Dependence of ozone loss on altitude. Fig. 14 : (a) Cosmic-ray intensity as a function of latitude, (b) Monthly average ozone concentrations in pre ozone hole and ozone hole period. Courtesy :WMO Global Ozone Research and Monitoring Project–Report No. 44, p. 27 Courtesy :NASA TOMS satellite database

25. Earth’s atmosphere and its composition, Variation of different atmospheric parameters, Formation of stratospheric ozone, Ozone Depletion and Ozone Hole, Cause of ozone Depletion. Summary

26. References K. Mohanakumar - Stratosphere troposphere interactions: An introduction (2008, Springer). C. Donald Ahrens-Essentials of Meteorology: An Invitation to the Atmosphere-Brooks Cole (2011). Sivasakthivel . T and K . K. Siva Kumar Reddy, Ozone Layer Depletion and Its Effects: A Review, 2011, International Journal of Environmental Science and Development. Q.-B. Lu and L. Sanche Phys. Rev. Lett. 87. https://doi.org/10.1103/PhysRevLett.87.078501 Hathaway, D.H. Living Rev. Sol. Phys. (2015) 12: 4. https://doi.org/10.1007/lrsp-2015-4

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