Ozone Layer-2:
Physics and Chemistry of the
Ozone Layer
CHEM/ENVS 380
1. Become familiar with major regions of the electromagnetic spectrum.
2. Understand the relationship between energy and wavelength of light.
3. What wavelengths of light do O2 and O3 absorb? What are the consequences?
4. What portion of the incoming solar radiation reaches the Earth’s surface, and why?
Some relevant topics from Gen Chem:
electromagnetic radiation, photons, wavelength
enthalpy, thermochemical equations, enthalpy change of a reaction, exothermic
reactions, endothermic reactions, bond energies
absorption spectra, photodissociation
This is bad,
but it would
be a lot
worse
without the
ozone layer.
https://theconversation.com/how-does-the-ozone-layer-protect-earth-from-radiation-9206
accessed Feb 10, 2014
The electromagnetic spectrum
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
Major regions of the electromagnetic spectrum
122
Baird and Cann (2008), Figure 1-6.
far UV
Wave-particle duality of light
•  Light has particle-like and wave-like properties
•  Photon = finite packet of light that is absorbed or
emitted by a molecule or an atom
•  Energy E of a photon is inversely related to its
wavelength λ:
1
E is directly proportional to
λ
or,
1
E∝
λ
Relationship between E and λ
hc
E of a single photon =
λ
h = Planck’s constant (6.63 x 10-34 J s)
c = speed of light in vacuum (3.00 x 108 m s-1)
λ = wavelength in m (or typically in nm = 10-9 m)
E is also commonly expressed as:
E = hν
where ν = frequency of light (s-1), and:
c
ν=
λ
Energy E of a mole of photons
hc
E of a mole of photons =
× Avogadro's number
λ
Energy E of a mole of photons
hc
E of a mole of photons =
× Avogadro's number
λ
What is the E of a mole of photons with
wavelength λ = 240 nm (in UV-C region)?
h = Planck’s constant (6.63 x 10-34 J s)
c = speed of light in vacuum (3.00 x 108 m s-1)
λ = wavelength in nm (nanometers = 10-9 m)
PROBLEM 1: What is the energy, in kilojoules per mole (kJ mol-1), associated with photons having
the following wavelengths? What is the significance of each of these wavelengths? (a) 280 nm, (b) 400 nm, (c) 750 nm, (d) 4000 nm
(from Baird and Cann 2008)
Absorption of light by molecules
•  A given molecule has its unique, finite number of discrete
energy levels (electronic, vibrational, rotational).
•  For absorption of radiation to occur, the energy of the
exciting photon must exactly match the energy difference
between the ground state and one of the excited states.
Excited States
E
ΔE3
ΔE2
ΔE1
Ground State
Question
The ozone layer reduces the amount of
harmful UV radiation reaching the surface
of the Earth.
Would you predict that ozone
absorbs photons in the UV region? YES
NO
O2
Sola
r flu
x
O3
Van Loon and Duffy (2003) Environmental Chemistry, Oxford Press.
Spiro et al. (2012)
Penetration of solar radiation through the
atmosphere
A large portion of
harmful shortwavelength radiation is
absorbed prior to
hitting the troposphere
by O2 and O3 (and N2).
Baird and Cann (2008) Fig 1-5.
Absorption of light by molecules
•  Excited molecules rapidly lose excess E through
various avenues:
–  Release of heat
–  Emission of a photon of lesser E (longer λ)
E
ΔE3
ΔE2
ΔE1
Photon emission
Ground State
-  Or it can undergo photolysis, or photochemical dissociation
Question
Given:
Energy of a mole of photons with
wavelength (λ) 240 nm = 498 kJ mol-1
and
O3 à O2 + O
ΔH°rxn = 107 kJ mol rxn-1
(ΔH°rxn = standard enthalpy of reaction)
What will happen to O3 when it
absorbs UV-C photons?
PROBLEM 2:
What is the longest wavelength of light that could dissociate ozone into
O2 and O? Does ozone absorb in this region?
O3 à O2 + O
ΔH°rxn = 105 kJ mol-1
(from Baird and Cann 2008)
UV-C is sufficiently energetic to photodissociate
O2 into atomic O
O2 à O + O
ΔH°rxn = 498.4 kJ mol rxn-1
PROBLEM 3: Calculate the longest wavelength
of light needed to photodissociate O2.
Solutions to problems
1.  (a) 427 kJ mol-1, at the boundary between UV-B and UV-C
(b) 299 kJ mol-1, at the boundary between UV-A and VIS
(c) 160 kJ mol-1, at the boundary between VIS and IR
(d) 29.9 kJ mol-1, beginning of the thermal IR region
2.  1140 nm (IR region). Ozone does not absorb in the IR.
3.  240 nm