1

Ozone depletion

Ultra-Violet Radiation (U.V.): wavelength less than 0.4μm. 1.2% of the Sun's radiant energy occurs at <0.3μm.

U.V. with <0.3 μm (photon energy>4eV) is strongly ionising. Can lead to skin cancers, cataracts, etc. It can harm corps and disrupts aquatic ecosystems.

The Ozone Layer is the peak in O3 concentration that occurs in the stratosphere.

O3 absorbs almost all U.V. with <0.290μm.

h + O2 O + O ; O + O2 O3 ; hn + O3 O2 + O O3

The above occurs many times until some interference reaction removes either or both the O3 and O species.

2

Chloroflurocarbons (CFCs)

Used as coolants, propellants, solvents, etc

Very unreactive. Half-live 70- 200 years in the troposphere.

Diffuse into the stratosphere where they are dissociated by U.V. releasing free Cl atoms.

Cl atoms remove O3 and O in the catalytic chlorine cycle

Cl + O3 O2 + ClO ClO + O Cl + O2

RESULT Cl + O3 + O Cl + 2O2

Each chlorine atom can destroy about 105 O3 molecules.

3

Interference Reactions Interference reactions e.g.

Cl O + NO2 ClONO2

and H + Cl HCl

 rapidly remove the Cl.

Experiment and calculation show that such reactions should keep the Cl in check.

So CFCs should have little impact on the ozone layer.

4

British Antarctic Survey

Have carefully monitored ozone levels above Antarctica for last 50 year

5

Antarctic Ozone Hole

First reported in 1985.

In the Antarctic spring up to 80% of the ozone is lost. In December (Mid Summer) the Antarctic U.V. level is about 3 times normal.

In late spring the low O3 air moves out from Antarctica leading to record low levels of O3 over southern Australia and New Zealand.

6

Polar VortexPolar Stratospheric Clouds

In winter air gets trapped over the Antarctic in the Polar Vortex.

In the extreme cold Polar Stratospheric ice clouds form which trap the NO2.

The reaction HCl + ClONO2 Cl2 + HNO3 can occur efficiently

on the crystals surfaces.

Thus the interference reactions are stopped and the Cl is released as soon as the temperature rises slightly in spring.

7

Extent of the Ozone hole

8

The Global implicationsSulphuric acid crystals, which are found throughout the stratosphere, can act in a similar way to PSCs.

The 1990 Pinatubo volcanic eruption put very large quantities of aerosol material including H2SO4 crystals into the stratosphere.

This led to strong reductions in ozone levels at more northerly latitudes including northern Europe.