Environmental Geology Spring 2010

 

Stratospheric Ozone

 

The stratosphere from 8 to 30 miles above Earth’s surface contains most of the naturally occurring ozone (O3)

 

Ozone absorbs harmful UV radiation from the sun which in turn warms the stratosphere.

 

Temperature in the stratosphere rises from about -58 degrees C at the tropopause to about 0 degrees C at the top of the stratosphere.

 

There is not much O3 below the stratosphere because O3 is formed by energetic UV radiation.  The O3 that is present in the troposphere is primarily related to industrial pollutants, and automobile emissions, as well as that produced by lightning.

 

The Antarctic Ozone Hole was first reported in 1985 (the satellite observations of stratospheric O3 began in 1978)

 

From 1979 to 1996 there was a decrease in O3 over Antarctica from about 400 dobson units to about 200 dobson units (one Dobson unit = 1 ppm O3).

 

Over this same period over the Arctic the decrease has been from about 450 dobson units to about 300 dobson units.

 

There was a progressive decline in O3 observed that continues today.

 

Globally, loss rate is about 0.3%/yr with the worst loss occurring in the Austral Spring (Sept. – Oct).

 

During this period there is as great as a 50% reduction in the region of the hole.

 

Who Cares?

  • Increased skin cancer – 1% decline in O3 >>> 2% increase in cancer
  • Increased incidence of cataracts
  • Damage to the immune system
  • Lower crop yields
  • Lower phytoplankton production

 

What Drives the Loss of O3?

 

Chlorofluorocarbons (CFC’s) in the stratosphere.  The CFC’s used as propellants and refrigerants reach the stratosphere because they are lighter (less dense) than air allowing them to float through the tropopause.

 

UV radiation splits O2 (stable state of oxygen) into O  +  O

 

O2  +  O  >>>> O3  Ozone – short lived, unstable

 

There is a dynamic equilibrium between the amount of incoming UV radiation and the production of O3 in the stratosphere.

 

Introduction of CFC’s, and other gases such as NOx disrupt this dynamic equilibrium.

 

CFC’s interact with stratospheric ice particles formed during the polar night and split to form chlorine gas (Cl2)

 

UV radiation in early-spring spits Cl2 into Cl  +  Cl

 

Cl interacts with O3 to form O2  +  ClO

 

Cl  +  O3 >>>>> ClO  +  O2

 

This process becomes a chain reaction where one Cl atom can destroy as much as 100,000 atoms of O3 before it combines with anther element to become inert.

 

Natural stratospheric Ozone Production and Modulation

 

Polar Night

  • No solar radiation  >>>>>>> no UV radiation  >>>>> no O3 production

 

Extreme Cold Forms Polar Stratospheric Clouds (PSC’s)

  • When temps reach about -80 degrees C ice crystals form that make up the PSC’s

 

Circumpolar Vortex forms

  • Encircles Antarctica during the Austral Winter
  • This reduces the potential for mixing of stratospheric O3 with outside regions.

 

Reactive Nitrogen Compounds Act Similarly to CFC’s.

  • These gases are often produced in volcanic eruptions

 

Solar Sunspot Cycle is not Constant

  • Modulated UV radiation impacts the amount of O3 produced on an annual basis.

 

Montreal Protocal 1987

  • Activated by 141 major industrial nations to eliminate CFC production and use.  Since established there has been a noticeable decrease in CFC concentrations in the stratosphere but not real healing of Ozone depletion has yet been observed.