5.6 depletion of stratospheric ozone

5.6 Depletion of Stratospheric Ozone

Composition of the atmosphere

• N = 78.1%• O = 20.9%• Ar = 0.9%• CO2= 0.03%

OO

O O

O O

O O

UV

UltraViolet is an energy coming from the sun. The Oxygen “Diatom” (literally two atoms joined) is joined together by sharing electrons. The energy in UV is enough to split the pair of atoms. Splitting the atoms uses up the energy in the UV ray.

The 2 separate Oxygen atoms are ‘high energy’ and have enough energy to combine with other oxygen ‘Diatoms’ to create Ozone.

It can also collide with other lone oxygen molecules to make diatoms again,Or it can collide with diatoms to make other ozone molecules.

So…….

•Ultra violet energy is absorbed when ozone is made and when it is destroyed.

A little about UV• UV is split into 3 main types depending on its

wavelength:

UVA – Long wavelength, low energy, less harmful, able to penetrate the atmosphere easily.UVB – Mid wavelength, mid energy, somewhat harmful, about 10 to 30% able to penetrate the atmosphere.

UVC – Short wavelength, Hi energy, very harmful, not able to penetrate the atmosphere easily.

Ozone creation and depletion video:

http://www.youtube.com/watch?v=k2kpz_8ntJY&feature=related

Ozone depletion by halogens• Chlorine, flourine, and bromine (halogens) have a strong

affinity(“liking”) for oxygen. • They can strip the ‘extra’ oxygen from an Ozone O3 molecule

because the bond is weaker than the normal bond of O2.

• A CFC molecule has 3 Cl and 1 Fl atoms joined to a carbon.

oo

oCl

c

Fl

ClCl

The Cl atoms can dissociate from the CFC.

If a Cl comes into contact with an ozone molecule it can strip an oxygen atom off it creating ClO and O2

• ClouroFlouroCarbons deplete ozone faster than it can get made by UV. So with more CFCs the ozone levels get less.

• CFCs can have long ‘Persistence’. (Up to 400 years!)

• One chlorine atom can destroy over 100,000 ozone molecules.

• CFCs and other widely-used chemicals produce roughly 84% of the chlorine in the stratosphere. The other 16% comes from natural sources like volcanoes.

http://www.epa.gov/ozone/science/sc_fact.html click here for more great stuff!

Environmental effects of ozone depletionIf more UV gets through to the lower atmosphere:

1. More ‘photochemical smog’ is formed as the UV creates more ozone at ground level and more Nitrous oxides, Sulphur oxides.

If more UV gets through to the lower atmosphere:

2. More acid deposition (rain) leading to damage to buildings, increased erosion, changes in soil (Al availability) and damage to plants.

Health effects of Ozone depletion

• Cataracts,• Sun burn, potentially leading

to skin cancer,• Respiratory disease,• Lowered immunity.

…and that’s just humans.• Have you ever seen a dolphin put on sun

screen?• If 1 hour in the sun damages the DNA in skin

cells what do you think the damage will be to leaves that spend 12hours? How do you think this might affect crop yields?

• Food chains may be damaged due to algae and plankton being damaged by too much UV. What effect will this have on the food chains?

Find out about Slip, slap, slop, seek, slide:

http://scarletwords.com/wp/images/2009/11/SlipSlopSlapSeekSlidewords.jpg

http://www.epa.gov/ozone/downloads/spd_poster_11x17-1.jpg click here for the bigger picture!

The World's Reaction• The initial concern about the ozone layer in

the 1970s led to a ban on the use of CFCs as aerosol propellants in several countries, including the U.S. However, production of CFCs and other ozone-depleting substances grew rapidly afterward as new uses were discovered.

Protocols and conventions.Through the 1980s, other uses expanded and the world's nations became increasingly concerned that these chemicals would further harm the ozone layer. In 1985, the Vienna Convention was adopted to formalize international cooperation on this issue. Additional efforts resulted in the signing of the Montreal Protocol in 1987. The original protocol would have reduced the production of CFCs by half by 1998.

After the original Protocol was signed, new measurements showed worse damage to the ozone layer than was originally expected. In 1992, reacting to the latest scientific assessment of the ozone layer, the Parties to the Protocol decided to completely end production of halons by the beginning of 1994 and of CFCs by the beginning of 1996 in developed countries. More information on the phase out of ozone depleting substances is found here.

Now the good news……Because of measures taken under the Montreal Protocol, emissions of ozone-depleting substances are already falling. Levels of total inorganic chlorine in the stratosphere peaked in 1997 and 1998. The good news is that the natural ozone production process will heal the ozone layer in about 50 years.

http://earthobservatory.nasa.gov/Features/Ozone/

• A little tutorial from NASA. Worth looking at.

Highly depleting substancesTable 1: Phase out Schedule for Class I Substances (in percent reduction from baseline)

Date (Jan. 1) CFCs Halons

Carbon Tetrachloride

Methyl Chloroform

Methyl Bromide HBFCs

1994 75% 100% 50% 50% 0% 0%

1995 75% 85% 70% 0% 0%

1996 100% 100% 100% 0% 100%

1997 0%

1998 0%

1999 25%

2000 25%

2001 50%

2002 50%

2003 70%

2004 70%

2005 100%

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Lower depleting substances will be phased out completely from 2014