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ATMOSPHERIC CHEMISTRY
The atmosphere is a thin layer of gases which surrounds the
earth. 78% N2
21% O2
0.9% Ar
0.03% CO2
plus trace gases
- 90% of atmosphere lies
10 miles from surface –
thermosphere
troposphere: weather
mesosphere
53 mi area.
- 9% of the atmosphere
stratosphere
31 mi lies 10-50 miles above the
surface- stratosphere and
troposphere
6 mi mesosphere
earth
- O3 located 12-21 miles
above the earth’s surface
- 90% of earth’s atmosphere lies in the troposphere
- The atmosphere is classified by altitude and
temperature
- The temperature generally decreases with
increasing altitude except in the stratosphere where
absorption of hv increases the temperature
TROPOSPHERE CHEMISTRY
Photochemical Smog
Photochemical smog results mainly from the
burning of hydrocarbon fuels. The major
ingredients in photochemical smog are NOx, HC,
and hv. Minor components include SOx and O3.
Under normal conditions, low lying ground air (lower
troposphere) circulates faster and is warmer that the above air
(upper troposphere). The net effect allows pollutants to be
swept up and out of the low ground area.
Thermal Inversion
Sunlight helps create an inversion pattern while mountains
prevents both the polluted air from escaping and fresh air
from circulating. During thermal inversion, a layer of warm air
resides above cooler ground air, thus trapping pollutants in the
lower region. It is in this region that dangerous levels of NOx,
SOx, and O3 build up.
PHOTOCHEMICAL SMOG
NOx
Initial Steps:
N2 + O2  2•NO
2•NO + O2  2•NO2
•NO2 + hv  •NO + O*
Termination Steps:
O* + O2  O3
O* + HC  IRRITANTS
and
O3 + HC  IRRITANTS
O3 + •NO  •NO2 + O2
SOx
S + O2  SO2 (or)
2H2S + 3O2  2SO2 + 2H2O
2SO2 + O2  2SO3
SO3 + H2O  H2SO4
brown haze
Aldehydes, ketones
peroxyacl nitrates
restarts #3
Toxic Gas
Toxic Gas
“Acid Rain”
Solar Radiation
UV-C
UV-B
UV-A
200 - 280 nm
280 - 315 nm
315 - 400 nm
Thermosphere
O2 + hv (  242nm)  O + O
- Oxygen atoms are produced by cleaving O2
molecules (endothermic)
- Oxygen atoms are highly reactive
O + O  O2 (exothermic)
- This is an energy conversion process:
UV-C (high energy)  vibrational energy(low energy) + (heat)
This can occur since (O2) molecules in thermosphere is
very low and (UV-C) is great.
OZONE
- Allotropic form of oxygen
O2 is odorless
O3 is smelly
light blue liquid
dark blue liquid
at - 183º C
at -112ºC
- Commercially produced by passing UV radiation or
an electrical discharge through O2.
- Commercially used as a water disinfectant and as a
bleach
UV
3O2

2O3
ele
OZONE
O3
- Detectable around electrical equipment,
faulty fluorescent lamps, or
thunderstorms in the troposphere
- Ozone is stable at lower elevations (low
temperatures) but reactive enough to
be a key component in smog.
- Ozone is unstable at higher elevation
(high temperature or if a catalyst is used)
and breaks down.
2 O3

3 O2
Ozone and Oxygen
• Oxygen molecule is O2.
• Ozone is O3.
• Short wavelength UV
dissociates O2.
• O atoms are reactive.
• O3 absorbs longer
wavelength UV recreating
O2 and O atoms.
• Equilibrium.
uv242 nm
O2 
O  O
O  O2  O3
uv320nm
O3 
O2  O
uv
O2  O 
O3
PROTECTIVE UMBRELLA
One of the most important functions of the earth’s
atmosphere is to shield the planet from harmful UV
radiation from the sun.
High energy radiation from the sun, travels through
space until it enters the stratosphere. The gases
present in the stratosphere absorb the high energy
photons
O3 + hv  O2* + O*
N2 + hv  2N* (hv=UV-B)
O2 + hv  2O*
Chapman mechanism (1930’s)
O2 + hv (< 242 nm)  O + O
O* + O2* M O3
O3 + hv (< 325 nm)  O2* + O*
O3 + O*  2O2
Good Refrigerants Are Not
Necessarily Good For the Air
• Water is not a good
refrigerant because of
its high boiling point.
• Chlorofluorocarbons
(CFCs) are excellent
refrigerants.
• AKA Freons:
– Non-toxic, nonflammable, chemically
inert.
Ozone Depletion and CFCs
• The ozone layer in the
stratosphere absorbs
harmful UV radiation.
• Reduction in the ozone
layer was linked to
presence of CFCs.
A hole over Antarctica
Theorized in 1974,
experimentally
detected in 1984.
Upsetting the Applecart
• Even the stable CFC molecule is
cleaved by UV.
• Reactive Cl radicals catalyze
Cl 
decomposition of O3.
O3 
O2  O
DEPLETION OF OZONE (O3) IN THE STRATOSPHERE
Catalyst
A substance which increases the rate of reaction but
is not consumed in the reaction. A catalyst lowers
the barrier thus allowing the reaction to proceed
faster
Catalytic Cycle
x = catalyst
X• + O3  XO + O2
XO + O  X• + O2
O3 + O  2O2 Net Rx
1. X is not dissipated
2. O2 is the dominate reactive species
3. XO is not reactive with O2
4. XO requires O atoms to complete the cycle
5. Most of the O atoms are in the upper stratosphere
6. Net result still has more O3 in lower stratosphere
X• free radical (unpaired electron) Cl•
CFC CATALYSIS
CFC  CH3Cl, CCl4, CH3CCl3, CFCl3 (Freon 11)
CF2Cl2 (Freon 12)
CF2Cl2 example
CF2Cl2 + hv  CF2Cl• + Cl•
Cl• + O3  ClO• + O2
ClO• + O  Cl• + O2
The single Cl• radical can undergo step 2 around
100,000 time before being terminated
(Cl• + CH4  HCl + CH3)
(ClO• + NO2  ClONO2)
HNO3
Around 200 reactions have been identified as possible
participants in ozone depletion.
All is Not Lost
• The Montreal Protocol
mandated cessation of
CFC use by 1997:
– 57 nations signed the
document.
• Modified CFCs with
some of the F or C
atoms replaced by H
atoms do not threaten
the ozone layer but…
GREENHOUSE EFFECT
STEP 1
Solar radiation reaches the atmosphere primarily as visible light. A smaller
portion of the solar spectrum is infrared. 30% of this light is reflected back
in to space while the remaining amount is absorbed by molecules on the
surface of the planet. Visible light travels past the atmosphere and collides
with surface molecules like sand and vegetation. The surface molecules
absorb the visible radiation; some of this energy is lost as vibrational energy
and the remaining energy is re-emitted as infrared (lower energy).
N2 and O2 are transparent to visible light, these components of the
atmosphere absorb predominantly UV radiation and do not participate in
the greehouse effect. The primary greenhouse gases are H2O, CO2, and O3.
Secondary gases are CH4, CFC’s and NO. These gases are also transparent
to visible light and absorb infrared.
GREENHOUSE EFFECT
STEP 2
The infrared radiation produce by the interaction of visible light with the
surface molecules and that emitted by the sun is absorbed by the greenhouse
gases in the atmosphere. These gases have several vibrational modes so they
are capable of absorbing lr light. The energy absorbed by these gases is
radiated out in all directions as infrared radiation. Some of the re-emitted
light is released into space and the rest is directed back down to earth and the
processes repeats itself.
The overall effect is the accumulation of large quantities of lr radiation in the
atmosphere, resulting in an increase in the global temperature.
CO2
Things Are Heating Up
• Since 1880 the
average temperature
on earth has increased.
• Greenhouse gases:
–
–
–
–
–
–
Carbon dioxide
CFCs
Sulfur hexafluoride
Nitrous oxide
Perfluorocarbons
Methane
The Kyoto Protocol: A Step in the
Right Direction
• Evidence for global warming has prompted
world-wide action.
• The Kyoto Protocol 1997 calls for reduction
of greenhouse gases to 8 % below 1990
levels.
• Ratified by most industrial nations – except
U.S.A.