lecture of env. chemistry

7/30/2019 lecture of env. chemistry

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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)


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Distribution of ozone layer and temperature variation


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Ozone selectively absorbs UV radiation


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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).


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Absorption of UV light by Oxygen molecule


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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:


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Fate of O2 and O3 in the presence of UV light

Wavelength (longest) of the light required for dissociation of oxygen molecule


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Reaction which occurs in the presence of lightare called ?Photochemical


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What is the longest wavelength of light that could dissociate ozone in this manner?


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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.


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Destruction of Ozone Layer

Chapman Mechanism:


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2 O2


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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.svg


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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.


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Adding relations A and B

Subtracting relation B from A


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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."


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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


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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


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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

lecture of env. chemistry

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