Post on 04-Apr-2018
7/30/2019 lecture of env. chemistry
1/21
The Ozone Layer Lecture Questions
At what altitudes is the ozone layer located?
What is the maximum concentration of ozone in the ozone layer?
Maximum of absolute conc about 23 km (up to 1013 molecules/mL)
Maximum of relative conc about 35 km (up to 10 ppm)
16 35 km (above bkgd level)
Stratosphere contains about 90%
of all atmospheric ozone
Total column ozone: ~300 DU (1
DU = 0.3 cm thick layer at 1 atm)
7/30/2019 lecture of env. chemistry
2/21
Distribution of ozone layer and temperature variation
7/30/2019 lecture of env. chemistry
3/21
Ozone selectively absorbs UV radiation
7/30/2019 lecture of env. chemistry
4/21
Absorption Spectrum of Ozone
Ozone filters out all of the Sun's ultraviolet light in the 220-290-nm range, which
overlaps the 200-280-nm (UV-C).
7/30/2019 lecture of env. chemistry
5/21
Absorption of UV light by Oxygen molecule
7/30/2019 lecture of env. chemistry
6/21
Variation in Light Energy with
wavelength
hE
E
hc
f
c
hcE
h= 6.63 x 10 - 34 Js and c= 3 x 108
For convenience, the product hc in the equation above can be evaluatedon a
molar basis to yield a simple formula relating the energy absorbed by 1 mole of
matter when each molecule in it absorbs one photon of a particular wavelength of
light. If the wavelength is expressed in nanometers, the value of hc is 119,627 kJ
mol-1 nm, so the equation becomes:
7/30/2019 lecture of env. chemistry
7/21
Fate of O2 and O3 in the presence of UV light
Wavelength (longest) of the light required for dissociation of oxygen molecule
7/30/2019 lecture of env. chemistry
8/21
Reaction which occurs in the presence of lightare called ?Photochemical
7/30/2019 lecture of env. chemistry
9/21
What is the longest wavelength of light that could dissociate ozone in this manner?
7/30/2019 lecture of env. chemistry
10/21
Creation of Ozone Layer
Reactions responsible for ozone layer formation in stratosphere
O2 absorbs the energy in UV-C region and dissociates in to oxygen atoms.
Oxygen atoms combine with O2 molecules in stratosphere to form ozone which is
an exothermic reaction. This process results the temperature inversion in
stratosphere.
Vertical mixing in stratosphere is absent unlike troposphere due to temperature
inversion.
Stratosphere is perfect zone for the production of ozone: Above stratosphere there
is very less molecular oxygen (less density) available for combination with oxygen
atom. Below stratosphere there are very less oxygen atoms available due to the
absence of UV-B and UV-C.
7/30/2019 lecture of env. chemistry
11/21
Destruction of Ozone Layer
Chapman Mechanism:
7/30/2019 lecture of env. chemistry
12/21
2 O2
7/30/2019 lecture of env. chemistry
13/21
Chapman Mechanism
Dr. Harminder Singh
http://upload.wikimedia.org/wikipedia/commons/2/28/Ozone_cycle.svghttp://upload.wikimedia.org/wikipedia/commons/2/28/Ozone_cycle.svg7/30/2019 lecture of env. chemistry
14/21
The steady state analysis (kinetics) of atmospheric reactions
Above stratosphere, dissociation of O2 molecule:
Applying steady state, rate of change of [O] should be zero
As we go higher and higher above the stratosphere: the air pressure drops, and therefore so
does [M], so the O2 re-formation rate decreases.
7/30/2019 lecture of env. chemistry
15/21
7/30/2019 lecture of env. chemistry
16/21
Adding relations A and B
Subtracting relation B from A
7/30/2019 lecture of env. chemistry
17/21
Multiplying relations C and D
The steady-state ratio of ozone to diatomic oxygen depends on the square root of the air
density through [M]. The ratio is also proportional to the square root of the product of
the rate constants for the reactions, 1 and 2, in which atomic oxygen and then ozone are
produced, and inversely proportional to the square root of the product of the ozonedestruction reaction rate constants.
The magnitude for the ozone/diatomic oxygen ratio is about 1O-4 in the mid-
stratosphere. Ozone is never the main oxygen containing species in the atmosphere, not
even in "the ozone layer."
7/30/2019 lecture of env. chemistry
18/21
7/30/2019 lecture of env. chemistry
19/21
The Chapman Cycle 1930
Sydney Chapman proposed a series of reactions to account for the ozone
layer: the Chapman Cycle
Lecture Question The Chapman Cycle explains how the ozone layer is formed and maintained.
Describe this process in some detail.
Four chemical reactions
Initiation O2 + light 2O (120 210 nm)
Propagation (cycling)
O + O2 + M O3 + M* (generates heat)
O3 + light O2 + O (220 320 nm)
Termination O3 + O 2O2
7/30/2019 lecture of env. chemistry
20/21
The Chapman CycleOxygen-only Chemistry
odd-oxygen species (Ox) are rapidly interconverted
Ox = O + O3
Oh
O2O
10-4 - 10 s
O3
60 - 3 min
O2
h
O2
7/30/2019 lecture of env. chemistry
21/21
Evaluation of Chapmans Model
How to evaluate Chapmans Theory?
Qualitative agreement: Predicts stratosphere as a source of ozone
Predicts thermal inversion in the stratosphere
Quantitative agreement? Check by comparing measured ozone levels with those predicted by
Chapmans model