Structure of the Atmosphere

1
Structure of the Atmosphere
99 of atmospheric mass
2
Water Vapor Concentration Atmospheric Depth
Profile
H2O - Source of hydroxyl radical
International Atomic Energy Agency - UN
http//www.iaea.org/programmes/ripc/ih/volumes/vol
_two/ChT_II_01.pdf
3
Ozone O3
Ozone is a stable molecule composed of three
oxygen atoms
While stable, it is highly reactive. The Greek
word ozein means to smell and O3 has a strong
pungent odor. Electric discharges in air often
produce significant quantities of O3 and you may
have smelled O3 near these sources.
4
Dobson Units (DU)
A DU is a measure of gas thickness A DU
represents the amount of ozone in a column of the
atmosphere 1 DU 0.01 mm thickness at standard
temperature and pressure (STP) STP is 0C and 1
atm Remember the ideal gas law PV nRT given
STP, we know P (1 atm) and T (273 K) R is the
ideal gas constant 0.0821 LatmK-1mol-1 DU
is a unit of length, thus units of DUarea gives
volume (V) the number of moles, n, can be
solved How many moles of ozone reside in the
atmosphere over a 1 square meter land area where
the ozone thickness was measured to be 290 DU?
(this is equivalent to a 2.9 mm band of ozone
just above the surface of this section of
land) (290 DU)(0.001 cm/DU)(1 m2)(100
cm/m)2(0.001 L/cm3)(1 mol/22.4 L) 0.129 mol
ozone (about 6.2 g ozone from the ground to
space over this 1 m2 of land!)
5
UV Absorption by Ozone
absorption of the UV photon results in
photodissociation of ozone molecules O3 hv ?
O2 O the UV photon is blocked from reaching
the Earths surface
UV radiation is high energy - can promote bond
dissociation and photochemical reactions -
harmful to biological organisms, causes skin
cancers and eye damage
6
Chapman Cycle
a) O2 hv (lt242nm) -gt 2O b) OO2M -gt O3M c) O3
hv (lt320nm)? O O2 d) O O3?2O2 M is a
molecule of air
Knowns Unknowns M concentration of
air O concentration of atomic oxygen O2
concentration of O2 O3 concentration of
ozone We will use kinetics to make an equation
that expresses ozone concentration
7
Note the elevated temperature above the ozone
layer, and lower temperature within. Why?
8
Kinetics of Reactions
consider the reaction A B ? C D the rate
of the reaction is rate kAaBb (this is
the rate equation) k is the rate constant this
is a value that has to be measured
experimentally A is the concentration of A B
is the concentration of B a is the order of
reaction in reactant A measured experimentally b
is the order of reaction in reactant B
measured experimentally the rate can be
expressed from the perspective of any of the
reactants or products rate -dA/dt
-dB/dt dC/dt dD/dt kAaBb (
dX/dt is the change in concentration of X
divided by change in time )
9
Steady state ozone concentration
We will generate two equations to solve for the
two unknowns O, O3
Each reaction has a rate equation Each rate
equation can be written from the perspective of
the unknowns O3 expressions O
expressions ratea dO/dt kaO2 rateb
dO3/dt kbOO2M rateb -dO/dt
kbOO2M ratec -dO3/dt kcO3 ratec
dO/dt kcO3 rated -dO3/dt
kdOO3 rated -dO/dt kdOO3
1) ozone is formed in one reaction step and
consumed in two reaction steps in the
steady-state, the rate of formation of ozone
rate of destruction of ozone dO3/dt
-dO3/dt rateb (ratec rated) 2) atomic
oxygen is formed in two steps, and consumed in
two steps in the steady-state, the rate of
formation of O rate of consumption dO/dt
-dO/dt 2ratea ratec rateb rated
10
Steady state ozone concentration
Our two equations, and two unknowns (O),
(O3) 1) kb(O)(O2)(M) kc(O3)
kd(O)(O3) 2) 2ka(O2) kc(O3) kb(O)(O2)(M)
kd(O)(O3) Since we are in steady state
conditions, both expressions 0, so we can add
or subtract these equations for our
convenience. Can we develop equations that
contain only one unknown? yes Add equation 1 and
2 kb(O)(O2)(M) 2ka(O2) kc(O3) kc(O3)
kd(O)(O3) kb(O)(O2)(M) kd(O)(O3) 2ka(O2)
2kd(O)(O3) Subtract 1 from 2 2ka(O2) kc(O3)
kb(O)(O2)(M) kb(O)(O2)(M) kd(O)(O3) kc(O3)
kd(O)(O3) 2ka(O2) 2kb(O)(O2)(M)
2kc(O3) ka(O2) kb(O)(O2)(M)
kc(O3) kb(O)(O2)(M) ka(O2) kc(O3)
11
Steady state ozone concentration
Carry over the equations developed from the last
slide
2ka(O2) 2kd(O)(O3)
kb(O)(O2)(M) ka(O2) kc(O3)
next, use the assumption ka(O2) ltlt kc(O3)
why? O3 photodissociates much more easily than
does O2 (fewer high energy UV photons penetrate
the lower stratosphere) so, kb(O)(O2)(M)
ka(O2) kc(O3) becomes kb(O)(O2)(M)
kc(O3) and rearranges to (O)
kc(O3)/kb(O2)(M) substitute, using (O) 2ka(O2)
2kd(O)(O3) 2ka(O2) 2kdkc(O3)2/kb(O2)(M) rearr
ange to (O3)2/(O2)2 kakb(M)/kckd
Steady-state O3 concentration
12
Altitude - effect on O3
10s of km
a) O2 hv (? lt 242nm) -gt 2O b) OO2M -gt O3M c)
O3 hv (? lt 320nm)? O O2 d) O O3?2O2
variable factors that change with increasing
altitude M decreases ka, kc increase (higher
hv flux) temperature profile changes (higher
temperatures lead to higher rates) Lets use
known data to calculate how much ozone is up
there!
13
Steady state ozone concentration
Known data (from experiments) ka 10-11 kb
10-33 kc 10-3 kd 10-15 M 1018 molecules /
cm3 (at 30 km) Calculate O3/O2
10-4 Even in the ozone layer, there are many
more O2 molecules than O3 molecules O2 in the
atmosphere is 209,400 ppm, calculate the
O3 O3 10-4 209,400 ppm O3 21
ppm Experimentally, the O3 ? 10 ppm What is
the source of the difference?
14
Catalytic Ozone Destruction

• Hydroxy radical (OH)
• OH O3 HO2 O2
• HO2 O OH O2
• Net O O3 2 O2
• Chlorine and bromine (Cl and Br)
• Cl O3 ClO O2
• ClO O Cl O2
• Net O O3 2 O2
• Nitric oxide (NO)
• NO O3 NO2 O2
• NO2 O NO O2
• Net O O3 2 O2

HOx cycle
ClOx cycle
NOx cycle
15
(No Transcript)
16
(No Transcript)
17
Hydroxyl radical

• Accounts for nearly one-half of the total ozone
  destruction in the lower stratosphere (16-20 km)
• Sources
• O3 hv (lt325nm) ? O2 O
• O H2O 2 OH (major)
• O CH4 OH CH3 (minor)
• H2O hv ? H HO (minor)
• Termination reactions
• OH NO2 ? HNO3
• 2 OH ? H2O2
• OH SO2 ? HSO3 ?? H2SO4
• H2SO4, H2O2 and HNO3 accumulate in water droplets
  and are washed out in precipitation

Increase of H2O concentration in the stratosphere
can have a devastating impact on ozone
concentration
18
Halogen Radicals
Halogen radicals generally do not exist in the
stratosphere due to natural causes As chemical
technology improved in industrialized countries,
new classes of materials called
chlorofluorocarbons (CFCs) were found to be
useful refrigerants propellants
solvents Other compounds, known as halons,
contain heavy halogen atoms bound to carbon, and
are non-flammable. These have applications
as fire extinguishing materials These types
of molecules were used over several decades and
released into the atmosphere. 1974 (UC -
Irvine) Rowland and Molina predict that CFCs
would form chlorine radicals (Cl) when exposed
to UV radiation, and that the Cl would result in
catalytic destruction of ozone. Research and
experimentation continued.
19
CFCs What are they?
CFC chlorofluorocarbon CFC- 90 1st
of carbons 2nd of hydrogens 3rd
of fluorines the valences of the unsaturated CFC
are balanced by chlorines CFC-11 11 90 101 1
C, 0 H, 1 F, 3 Cl CFCl3 CFC-113 113 90
203 2 C, 0 H, 3 F, 3 Cl C2F3Cl3 if there are
hydrogens in the CFC, it is called an
HCFC HCFC-141 141 90 231 2 C, 3 H, 1 F, 2 Cl
C2H3FCl2
20
CFCs - Halogen Radical Source
CF2Cl2 hv ? CF2Cl Cl initiation Cl O3
? ClO O2 ClO O ? O2
Cl Remember O3 hv ? O2 O It is
estimated that each Cl involved in a chain
reaction has the potential to destroy 100,000 O3
molecules Halons, (ex CF3Br, CH3Br, etc.),
form radicals even more easily (C-Br) bond is
weaker, Br is more stable. These are in lower
concentration, but potentially even more
destructive.

propagation
21
Nitric oxide
Nitrogen oxides are products of lightning,
nitrogen fixation, and combustion Some nitrogen
oxides nitric oxide (NO), nitrous oxide (N2O),
dinitrogen pentoxide (N2O5), nitrogen dioxide
(NO2) NO readily reacts with O2 NO O2 ?
NO2 NO2 is eventually washed out by hydroxyl
radical NO2 OH ? HNO3 and the nitrate is
removed in precipitation These reactive nitrogen
oxides do not migrate from the troposphere to the
stratosphere however, much less reactive nitrous
oxide does. Nitrous oxide is then the source of
stratospheric nitric oxide N2O hv ? N2
O O N2O ? 2 NO
22
Nitric oxide reactions in the stratosphere
NO O3 ? NO2 O2 NO2 O ? NO O2 net O
O3 ? 2 O2 stratospheric NO2 leads to still
other reactions OH NO2 ? HNO3 (nitric
acid) ClO NO2 ? ClONO2 (chlorine
nitrate) nitric acid and chlorine nitrate
participate as reservoirs of NO2, OH, ClO HONO2
hv ? HO NO2 ClONO2 hv ? ClO NO2 As
long as the reservoir molecules are in the
atmosphere, they can be activated by UV light to
initiate catalytic ozone destruction however,
these molecules can be sequested in water/ice
droplets and precipitated away.
23
Antarctic Ozone Hole
Seasonal phenomenon that occurs every Antarctic
spring
Some different atmospheric chemistry is
responsible for this
24
Polar Stratospheric Clouds
Arctic stratospheric clouds
In winters, there is very little light.
Temperatures become severely cold at the poles.
During such cold (190K -83C -117F ), water
can condense out of the stratosphere. As this
water condenses, it takes a few things with it.
25
Polar Stratospheric Clouds
The stratospheric clouds consist of ice crystals
and hydrated nitric acid The surfaces of the ice
crystals absorb nitric acid (HNO3) , chlorine
nitrate (ClONO2) , and hydrochloric acid
(HCl). Reaction occur on the surfaces of the ice
crystals HCl ClONO2 ? Cl2 HNO3 H2O
ClONO2 ? HOCl HNO3 These reactions do not
occur in the gas phase, but are accelerated
during formation of the stratospheric clouds,
since the reactants are concentrated on the ice
crystals. The nitric acid molecule is also
stabilized by extensive hydrogen bonding to the
ice crystal surface, which provides additional
driving force for the reaction. Cl2 escapes as
gas, and the ice clouds become enriched in
HNO3 In effect a time bomb is waiting to go off
26
Seasonal Ozone Destruction Event
As spring arrives, the poles emerge from the
dark, now photochemical reactions are possible
again All of the Cl2 and HOCl that has been
building up during winter on the polar
stratospheric clouds is now bombarded by
photons Cl2 hv ? 2 Cl HOCl hv ? HO
Cl As the polar stratospheric clouds
dissipate, the reservoir molecules build up
again HNO3 and ClONO2

massive initiation event (Spring)
27
(No Transcript)
28
See the diseasonal pattern of polar ozone
depletion?
Largest ozone hole ever recorded September 2006
29
Antarctic Ozone hole lowest reported thicknesses
30
(No Transcript)
31
Montreal Protocol
International treaty for the purpose of
protecting the ozone layer Phases out a number
of substances believed to be responsible for
ozone depletion, especially CFCs other
substances are addressed as well Substances in
Group I of Annex A to be phased out CFCl3
(CFC-11) CF2Cl2 (CFC-12) C2F3Cl3 (CFC-113)
C2F4Cl2(CFC-114) C2F5Cl (CFC-115)
from 1991 to 1992 its levels of consumption and
production of the controlled substances in Group
I of Annex A do not exceed 150 per cent of its
calculated levels of production and consumption
of those substances in 1986 from 1994 its
calculated level of consumption and production of
the controlled substances in Group I of Annex A
does not exceed, annually, twenty-five per cent
of its calculated level of consumption and
production in 1986. from 1996 its calculated
level of consumption and production of the
controlled substances in Group I of Annex A does
not exceed zero
32
An international success!
33
CFC Substitutes
Trade name Chemical Market Atmospheric 100
yr ODP Formula lifetime (yr) GWP CFC-11 C
Cl3F Blowing agent 50 4000 1 CFC-12 CCl2F2 Ref
rigerant 102 8500 1 CFC-113 CCl2FCClF2 Cleaning
agent 85 5000 0.8 HCFC-22 CHF2Cl Refrigerant 1
2.1 1700 0.055 Blowing agent HCFC-141b CH3CFC
l2 Blowing agent 9.4 630 0.11 HCFC-123 CF3FCHCl2
Blowing agent 1.4 93 0.02 HFC-134a CH2FCF3 Refr
igerant 14.6 650 0 Blowing
agent HFC-23 CHF3 Fire extinguisher 260 11700 0
HFC-227ea C3HF7 Fire extinguisher 36.5 2900 0 H
FC-245-fa C3H3F5 Blowing agent 6.6 790 0