ABSORPTION OF
ULTRAVIOLET LIGHT
IN THE ATMOSPHERE
3 attributes of UV light from the sun
reaching the Earth:
UV-C
λ
UV-B
λ:
UV-A
wavelength
Shorter λ = higher
energy
< 280nm
λ
= 280-320 nm
λ
= 320-400 nm
Wavelength required to dissociate:
O₂
O₃
O₂ - λ<242 nm • UV-C part of the spectrum
O₃ - up to 330 nm • mostly UV-B
WHY?
→ 2O•
→ O• + O₂
O₂ has a double bond i.e. bond order 2, while O₃ has a delocalized structure
with bond order 1.5.
The O—O
.... bond in O₃ is weaker than the O=O bond in O₂, hence lower energy
is required to break it.
⁎⁎⁎
Catalysis of ozone depletion by CFCs and NOx
Chlorofluorocarbons (CFCs) are stable at
ground level but are broken down by absorbing
UV radiation in the upper atmosphere:
CCl₂F₂
→ •CClF₂ + Cl•
UV
The chlorine free radical can take part in a chain reaction that uses up ozone and
regenerates chlorine free radicals to react with more ozone:
→
/
/
•Cl + O₃ ClO• + O₂
ClO• + O• O₂ + Cl•
→/
→ /
O₃ + O• → 2O₂
•Cl + O₃ ClO• + O₂
ClO• + O• O₂ + Cl•
→
NOx are nitrogen oxides, NO and NO₂. Both have unpaired electrons therefore they can
have similar reactions to those of CFCs regarding ozone destruction. For example, the
destruction of ozone catalyzed by NO is represented as:
/
/
→ /
→
/
O₃ + O• → 2O₂
•NO + O₃ •NO₂ + O₂
•NO₂ + O• O₂ + •NO
The chlorine free radical
and NO are catalysts in the
reaction - they are used up
in the first step but not the
second - hence not used up
overall.
⁕
OZONE
OZONE IS
IS IMPORTANT...
IMPORTANT...
Ozone absorbs and protects us from
the UV-B radiation, which are very
harmful for our health and living
creatures on Earth. Increased UV
radiation can cause certain skin
cancers, eye cataracts and immune
deficiency disorders.