HSC CHEMISTRY 2015​.

Problems Caused by CFC's

 

 

Ozone hole: The ‘ozone hole’ refers to the annual drop in ozone levels over Antarctica through August to October (the southern spring). This was first discovered by the British Antarctic Survey at Halley Bay in 1976, which revealed a 10% drop in ozone levels over spring.

Although CFC’s have caused a 50 to 90% ozone depletion over Antarctica during spring, they are also responsible for a world-wide 3 to 8% decrease all year-round.

Ozone depletion is a general decrease in world-wide ozone concentrations in the stratosphere whereas the ozone hole is a unique feature to the Antarctic, where the area experiences a dramatic drop in ozone levels throughout spring.

 

 

 

 

 

 

 

 

 

 

 

 

UV-B absorption: UV-B is short wave UV light, Ozone absorbs shortwave UV light when it decomposes to O2 and O∙. Thus a lack of Ozone in the stratosphere means less UV-B is absorbed and more infiltrates the troposphere.

 

UV-B effects:

• Increases sunburn and cancer caused by UV-B.

• Higher rates of eye damage and cataracts

• Lowered immune response, leading to higher susceptibility to disease

• UV-B interference with photosynthesis leading to plant/crop damage

• Damage to synthetic materials (brittleness and surface powder)

• Phytoplankton DNA damage, effecting the food chain of aquatic organisms.

 

Ozone Destruction:

Once diffused into the stratosphere CFC’s encounter short wave UV radiation (which wasn’t present in the troposphere)

This radiation decomposes the CFC’s, breaking of a Cl free radical.

 

 

CCl3F + uv.light à Cl∙ + CCl2F

 

 

This Cl∙ free radical takes an oxygen atom from O3 forming O2 and a very reactive ClO∙ free radical.

 

 

 

Cl∙ + O3 à ClO∙ + O2

 

 

This ClO∙ reacts with one of the O∙ atoms in the atmosphere producing O2 and Cl∙, thus beginning the process again.

 

 

 

ClO∙ + O∙ à O2 + Cl∙

 

 

As the reactive species (Cl∙) is reproduced in this reaction allowing the reaction to repeat thousands of times it is called a chain reaction and Cl∙ is the catalyst for this O3 destruction.

 

 Halting the Chain Reaction:

For this chain reaction to cease the chain carriers (Cl∙ and ClO∙) must be removed from the stratosphere, there are two main reaction which can do this:

In the stratosphere small quantities of methane (CH4) are present.

These molecules can react with Cl∙ atom forming hydrogen chloride (HCl)

and CH3∙, removing the Cl∙ atom, halting the chain reaction.

The CH3∙ has no effect on O3.

 

The other method of naturally halting O3 destruction involves removing

ClO∙ atoms from the stratosphere. Nitrogen dioxide (NO2) is also present

in the stratosphere and will react with ClO∙ to for chlorine nitrate from

which Cl∙ atoms cannot be regenerated thus halting the chain reaction.

 

 

 

 

Despite these methods on average every Cl∙ atom destroys ≈10 000 O3 molecules before the chain reaction is halted, causing a significant amount of destruction.

 

Antarctic Ozone Depletion/The ozone hole

Winter: The Ozone hole is most prominent during spring, however much less so during the winter months. This is due to:

• A period of continuous darkness

  • No UV.light to decompose CFC’s or Cl2

• Polar Vortex

  • Stops Antarctic air mixing with warm, low-latitude air, meaning the stratosphere is extremely cold.

  • Soild crystal particals form and catalyse a reaction between hydrogen chloride and chlorine nitrate. Which has no effect on O3 during winter

 

 

 

 

Spring: However, in spring when UV light returns the Cl2 is broken into 2Cl∙ atoms which destroy ozone in the same way as Cl∙ from CFC’s. This extra source of Cl∙ atoms accentuates the depletion of ozone resulting in the ozone hole.

 

Summer: By summer the additional Cl∙ radicals are exhausted as only a finite amount of Cl2 is produced during the winter. This means the rate of ozone destruction returns the usual CFC rate. Additionally, in contrast to winter, the polar vortex breaks up allowing Antarctic air to mix with lower-altitude air, which assists in bring the ozone concentration back to normal.

 

This can however lead to low ozone concentrations over the southern hemisphere, as while air from the southern-hemisphere mixes with Antarctic air the overall concentration decreases for the southern-hemisphere leading to issues associated with ozone depletion and high UV-B radiation.