U2 T4 Flashcards
Since 1970s a reduction in atmospheric ozone has been observed in 2 different ways, which ones?
- A general reduction –> 4% per decade over de hole planet
2. “Ozone holes” in polar regions where there is an annual drop stratospheric ozone concentrations
Ozone concentration in Antarctica and Arctic have dropped up to…
Antarctica: dropped up to 2/3
Arctic : 1/3
Why is there a concern about ozone depletion?
If UBV is not absorbed in the atmosphere then it h the Earth surface and may be absorbed by living cells.
UV A (characteristics and wavelength)
- not absorbed by ozone
- 320-400nm
UV B (characteristics and wavelength )
- almost fully absorbed by ozone
- 280-320 nm
UV C
<280 nm Completely absorbed by ozone and normal oxygen
The UV light considered in studying ozone depletion is :
UVB
Which molecules absorbed UVB
UVB is absorbed by both diatomic and triatomic oxygen in photosynthesis reactions where the molecules are split.
Why chlorine is very reactive ?
A single chlorine atom is a chlorine radical because it has an unpaired electron. This makes it very reactive
O3
Ozone
O2
Dioxigen (the one we breath)
O
Monoatomic oxygen
Cl
Chlorine
ClO
Chlorine monoxide
ClO2
Chlorine dioxide
3 successional reaction between chlorine and ozone
- a chlorine radical can react with an ozone molecule, breaking it down leave Ana oxygen molecule and chlorine monoxide.
- The chlorine monoxide can then react with a monatomic oxygen atom to make chlorine dioxide
- The ClO2 can then break down to release the chlorine again which may repeat these reactions thousands of times
The important overall summary reaction
Cl + O3 + O—> 2O2 + Cl
How does chlorine get to the stratosphere ?
Chlorine is very reactive so if it is released in the troposphere it will probably react with something else and never reach the stratosphere.
What do CFCs contain?
Only chlorine, fluorine and carbon
When were developed CFCs?
In the 1920s as the fluids of refrigerators and were later used as aerosol propellants, solvents and as the gases for expanded plastics such as expanded polystyrene
What are the properties of CFCs that made them suitable for many use?
- they are chemically very stable
- Can be liquefied with pressure
- good solvents of grease and oils
- are not flammable
- most are not toxic
HCFC that have replace CFCs in refrigerator also contain chlorine. Are they as dangerous as CFCs? Why?
HCFCs contain chlorine but they are less chemically stable so they break down before they reach the stratosphere and thus cannot damage the ozone layer
Why is ozone depletion greatest in Polar Regions?
The chemical reactions involving chlorine and ozone require UVB but occur most easily at very los temperatures when ice crystals form that provide catalytic surfaces. During the Polar winter the temperatures drop and the allow the ice crystals to form, but there is no sunlight. When spring arrives, the crystals still exist and the sunlight causes the ozone- depleting reactions to take place.
NOx
Oxides of nitrogen
Do other gases cause ozone depletion ?
Oxides of nitrogen have been identified as possible causes of ozone depletion but releases in the troposphere do not reach the stratosphere as they dissolve in rain or react with other substances. High flying aircraft do release NOx into the stratosphere, but they are relatively very few so no obvious damage is caused
