Ozone: Earth s shield from UV radiation

Transcription

1 Outline Ozone: Earth s shield from UV radiation Review electromagnetic radiation absorptivity by selective gases temperature vs. height in atmosphere Ozone production and destruction natural balance anthropogenic influence Trends in stratospheric ozone ozone hole Hazards of exposure to UV radiation electromagnetic radiation

2 Ozone (O 3 ) Ozone is a form of oxygen. The molecule contains three oxygen atoms (O 3 ) Ozone is unstable and will readily combine with other atoms. Ozone is found in the stratosphere, where it blocks the sun's ultraviolet (UV) waves and prevents them from reaching the earth's surface. Ozone is also found in the troposphere, where it can damage living tissue and human-produced objects. It is generated both from certain types of pollution and natural sources.

3 Major issues related to ozone? 1. Stratospheric ozone is a UV-shield: Its depletion will increase surface UV, with consequences to health and composition of troposphere. This is not a theory to be tested - it is reality! O 3 hole, trends in O 3 abundance, etc.: need predictions. 2. Stratospheric ozone is an important greenhouse gas: Its changes influence earth s climate (circulation, temperatures and composition.) 3. Ozone-depleting substances are also greenhouse gases: What is the net influence of changing stratospheric O 3 and CFCs simultaneously? What are the impacts of substitutes, CFCs, etc.? 2

4 Electromagnetic spectrum To understand how ozone is generated and the functions it serves in Earth's atmosphere, it is important to know something about the electromagnetic spectrum the energy emitted from the sun. Electromagnetic energy is sometimes described as traveling in waves and sometimes as traveling in packets of energy referred to as photons. Progressing from short wavelengths to long wavelengths, scientists have identified gamma rays, x-rays, ultraviolet radiation, visible light (between 400 and 700 nanometers), infrared radiation (heat), microwaves, and radio waves. Short wavelengths have more energy per photon than long wavelengths

5 Ultraviolet radiation Energy from the sun reaches the earth as visible, infrared, and ultraviolet rays UV account for 2% of incoming solar radiation

6 Ultraviolet Radiation Energy from the sun reaches the earth as visible, infrared, and ultraviolet rays Ultraviolet A (UVA) is made up of wavelengths 320 to 400 nanometers (nm) in length Ultraviolet B (UVB) wavelengths are 280 to 320 nm Ultraviolet C (UVC) wavelengths are 100 to 280 nm Only UVA and UVB ultraviolet rays reach the earth's surface. Earth's atmosphere absorbs UVC wavelengths. UVB rays cause a much greater risk of skin cancer than UVA. However, UVA rays cause aging, wrinkling, and loss of elasticity. UVA also increases the damaging effects of UVB, including skin cancer and cataracts.

7 Blackbody curves

8 Structure of Earth s Atmosphere in the stratosphere temperature increases with height in the stratosphere ozone absorbs incoming ultraviolet radiation in the troposphere temperature decreases with height the average lapse rate is about 5-9 degrees C per kilometer depending on the humidity 80% of the mass of Earth s atmosphere is in the troposphere, we all live in the troposphere temperature vs. height

9 Ozone In the stratosphere, we find the "good ozone that protects life on earth from the harmful effects of the sun's ultraviolet rays. In the troposphere, the ground-level or "bad" ozone is an air pollutant that damages human health, vegetation, and many common materials. It is a key ingredient of urban smog.

10 Stratospheric ozone layer maximum at km

11 Ozone production O 2 + hν ---> O + O (1) O + O 2 ---> O 3 (2) (1/ν = wavelength < ~ 240 nm)

12 Ozone production and destruction Although the UV radiation splits the ozone molecule, ozone can reform through the following reactions resulting in no net loss of ozone: O 3 + hν --> O 2 + O (3) hν + O 2 --> O 3 (2) as above Ozone is also destroyed by the following reaction: O + O 3 --> O 2 + O 2 (4) Chapman reactions

13 Measurement of atmospheric ozone Ground based measurement Absorption spectroscopy using light from sun, moon, or star Satellite measurement Backscatter ultraviolet spectrometers

14 Dobson units

15 Chlorofluorocarbons (CFCs) Because of their chemical stability, low toxicity, and valuable physical properties, these chemicals, versatile and stable in the lower atmosphere, at least, have been extensively used since the 1960s as refrigerants, industrial cleaning solvents, propellants in aerosol spray cans, and to make Styrofoam.

16 CFCs Wonder Chemicals CFCs 11 and 12 developed in 1930s as refrigerants Chemically stable and non-toxic By 1970s, used as Aerosol propellant Refrigerant (refrigeration, building and mobile air conditioning) Blowing agent for flexible and rigid plastic foams CFC-113 used as solvent, especially for microelectronic production

17 CFC-11

18 CFC-12 The more recent trend for CFC11 and CFC12: mixing ratios in ppt Source: NOAA/CMDL

19 SST 1960s and early 1970s US and UK/France developing supersonic transports (SST) Concerns that HOx then NOx in exhaust would deplete ozone US cancelled SST, largely for economic reasons UK/France built Concorde Later disputes over US landing rights for Concorde Ozone discussed, but mostly about noise, technology

20 Discovery of Threat to Ozone June 1974 Molina and Rowland paper identifying threat from CFCs published Attracted little attention September 1974 Molina and Rowland held press conference and discussed work at American Chemical Society Widely publicized (e.g., two New York Times articles)

21 The Nobel Prize in Chemistry 1995 "for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone Paul J. Crutzen Mario J. Molina F. Sherwood Rowland Discovery of Threat to Ozone

22 CFCs destroy ozone The following animation shows the destruction of an ozone molecule by a chlorine atom.

23 CFCs destroy ozone 1. UV radiation breaks off a chlorine atom from a CFC molecule. 2. The chlorine atom attacks an ozone molecule (O 3 ), breaking it apart and destroying the ozone. 3. The result is an ordinary oxygen molecule (O 2 ) and a chlorine monoxide molecule (ClO). 4. The chlorine monoxide molecule (ClO) is attacked by a free oxygen atom releasing the chlorine atom and forming an ordinary oxygen molecule (O 2 ). 5. The chlorine atom is now free to attack and destroy another ozone molecule (O 3 ). One chlorine atom can repeat this destructive cycle thousands of times.

24 Ozone is also a greenhouse gas

25 Tropospheric ozone 1. Ozone is a naturally occurring gas found in the troposphere and other parts of the atmosphere. 2. Tropospheric ozone is often called "bad" ozone because it can damage living tissue and break down certain materials. 3. Concentrations of ozone are not uniform in the troposphere. 4. Longer exposure to ozone will increase the negative effects. 5. The amount of ozone present in the troposphere varies from day to day and from place to place. place to place

26 Ozone can damage cells

27 Ground-Level Ozone Primary constituent of urban smog Secondary pollutant formed through photochemical reactions involving NOx and VOCs in the presence of bright sunshine with high temperatures Exposure to elevated concentrations associated with Increased hospital admissions for pneumonia, chronic obstructive pulmonary disease, asthma, allergic rhinitis, other respiratory diseases Increased mortality Outdoor ozone concentrations, activity patterns, and housing characteristics are the primary determinants of ozone exposure

28 Trends in tropospheric ozone concentrations Background concentrations have risen since pre-industrial times, and this trend is expected to continue over the next 50 years Future concentrations depend on future emissions and weather patterns Emissions depend on assumptions of population growth, economic development, and energy use Fraction attributable to climate change is the portion that is the consequence of climate change on local temperature & UV Assuming no change in the concentration of precursor emissions, the frequency of future ozone episodes will depend on the occurrence of the requisite meteorological conditions

29 Ozone trends

30 Ozone trends

31 Ozone hole

32 Ozone over Antarctica

33 Ozone holes

34 Ozone holes

35 TOMS

36 The UV Index provides a daily forecast of the expected risk of overexposure to the sun. The Index predicts UV intensity levels on a scale of 0 to 10+, where 0 indicates a minimal risk of overexposure and 10+ means a very high risk uv index

37 UV index

38 UV index

39 UV index

40 UV index