The greenhouse effect, global warming

1
The greenhouse effect, global warming and ozone
depletion facts and myths or the agnostics
views of a chemical physicist ! Professor
Richard Tuckett (School of Chemistry) r.p.tuckett
_at_bham.ac.uk RSC West Midlands ChemNet, 12th
December 2006 Specialist details in
Trifluoromethyl sulphur pentafluoride CF3SF5
the ultimate greenhouse gas, and its
contribution to global warming Adv. Fluorine
Science (Elsevier) 1 (2006) chapter 3 Thanks
to Dr Peter Barnes (Warwick University) for
technical help

2

• Myths of atmospheric science ?
• ? The greenhouse effect and ozone depletion
  have the same scientific causes (John Selwyn
  Gummer, 1994 Today programme)
• The Greenhouse Effect is all bad news
  detrimental to life on earth
• Facts ?
• ? The ozone depletion issue is slowly getting
  better. The stratosphere may recover within
  50-100 years. The science is well understood.
• ? The planet is warming up. The chemistry of
  the troposphere, where greenhouse warming occurs,
  is poorly understood.
• Opinions ?
• ? Global warming is not occurring. Even if it
  is, it is unrelated to mans activities on
  earth (George Bush, 2001-2004, 2006 ?)
• ? Global warming is the most serious
  phenomenon affecting the worlds security and
  prosperity, more so than terrorism
• (David King, UK Government Chief Scientific
  Adviser, 2003)
• Scientists ?
• ? Global warming is not due to mans activities
  since the Industrial Revolution, but to a
  natural cycle of ice ages with warm periods in
  between. Most scientists from chemistry and
  physics backgrounds disagree. Some geologists /
  geographers see the situation differently.

3
CO2 concentrations and Temperature change over
1000 years
4
CO2 concentrations and Temperature change over
recent years
But what about the period 1780 1904 ??
5
Correlation between concentrations of O3 and ClO
over Antarctic, Sept 1987 importance of
resolution
O3
Anderson et al. J. Geophys. Res. D., (1989) 94,
11465
ClO?
Rise in concentration of ClO? anti-correlates
with fall in concentration of O3
6
The power of computers and the web predictions
for AD 2156
Computer climate model in which the atmospheric
concentration of CO2 increases at a compound rate
of 1 per annum, i.e. the concentration doubles
in 70 years. The CO2 concentration then remains
constant for 80 years. http//www.gfdl.noaa.go
v/products/vis/images/gallery/sphere_04_150.gif
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Atmospheric scientists triangle
Laboratory measurements (low T)
Field measurements
Modellers
8
Chemical cycles for HOx trace species
Rate constants (low T), products of elementary
reactions, absorption cross-sections,
thermochemistry
9
Mace Head, West Ireland
Concentration of CO2 / ppm
Cape Grim, Tasmania
Concentration of CHF3 / ppt
10
TOMS (Total ozone monitoring spectrometer) NASA
11
Development of Antarctic Ozone Hole, 1979-1997
12

• Simple Photochemistry
• Vacuum-UV 100 200 nm
• UV 200 400 nm
• visible 400 750 nm
• IR 750 - 20000 nm

E hn hc / l (E ? as l ?) E, energy of
photon h, Plancks Constant n, frequency of
photon c, velocity of light l, wavelength of
photon
13
Black body emission from the Sun (T 5780 K)
compared to that of the Earth (T 290
K) (Einstein, 1905)
14
Energy balance UVin IRout
Tearth should be 256 K (-17 oC) Absorption of IR
radiation emitted by the earth by gases in the
troposphere. Radiation is trapped, like a
greenhouse. Some reflected back to earth. Leads
to an increase in temperature, and global
warming. Earths atmosphere is 78 N2, 21 O2
neither absorb IR radiation.
15
Satellite data confirming trapping of IR
radiation (Nimbus 4) RP Wayne, Chemistry of
atmospheres (1991)
CO2 O3 H2O

• - - - - spectrum expected for a black body
  at temperature T.
• ? Natural GH gases n2 modes of CO2 (15 mm),
  H2O (6.3 mm)
• ? Enhancing GH gases pollutants that absorb IR
  strongly in the range 6-25 mm where CO2 and H2O
  do not absorb.

16
Ground level clean air main constituents
Molecule Mole fraction ppmv (parts per million by volume)
N2 0.78 or 78 780900
O2 0.21 or 21 209400
H2O 0.03 (25 oC,100 humidity) 0.01 (25 oC, 50 humidity) 31000 16000
Ar 0.01 or 1 9300
CO2 3.8 10-4 or 0.038 380
Ne CH4 O3 1.8 10-5 1.5 x 10-6 Trace gases 2.0 x 10-8 18 1.5 0.02
17
Regions of the earths atmosphere
99 of earths atmosphere in troposphere, lt 1 in
stratosphere
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Properties of greenhouse gases (absorption of
infra-red radiation into vibrational modes of the
gas)

• Vibration must change the dipole moment of the
  molecule
• N2 and O2 (99 of atmosphere) play no role.
• ? Molecule must absorb in the range 5-25 mm.
  Coincidentally, CO2 absorbs at 15 mm nature is
  unkind.
• ? Long lifetime in the earths atmosphere no
  reaction with OH? and O(1D), or
  photodissociation in troposphere (300-500 nm) or
  stratosphere (200-300 nm).
• IR spectroscopy, absolute absorption coefficients
• Reaction kinetics of greenhouse gas with OH? and
  O(1D)
• Photodissociation of greenhouse gas with UV /
  visible radiation (200-500 nm)
• Greenhouse Potential (GHP) or Global Warming
  Potential
• A molecule with a large GHP is one with strong IR
  absorption, long
• lifetimes, and concentrations rising rapidly due
  to mans activity
• CO2 1, CH4 23, CF2Cl2 10600, CF3SF5
  18000

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n1 mode of CO2 4.2 x 1013 vibrations per
second 1388 cm-1 or 7.2 mm Infra-red inactive
O C O
20
n2 mode of CO2 2.0 x 1013 vibrations per
second 667 cm-1 or 15.0 mm Infra-red active
O C O
21
n3 mode of CO2 2.0 x 1013 vibrations per
second 2340 cm-1 or 4.3 mm Infra-red active
O C O
22
Examples of greenhouse gases, and their
importance to global warming IPCC
2001 ____________________________________________
______________________________ Greenhouse
gas CO2 CH4 CF2Cl2 CF3SF5
_________________________________________________
_________________________ Concentration /
ppm 380 1.75 0.0003 lt
10-6 DConcn / per year 0.45 0.60
ca. 5 ca. 6 Microscopic radiative
1.68 x 10-5 4.59 x 10-4 0.32 0.60 forcing
/ W m-2 ppb-1 Total radiative 1.46 0.48
0.16 7.2 x 10-5 forcing / W m-2 Lifetime
/ years 50-200 12
100 ca.1000 GHP (100 year projection)
1 23 10600
18000 Contribn to GH effect / 52
17 ca. 6 0.1 ________________
__________________________________________________
_________
23
CF3SF5 atmospheric background
? Sturges et al. Science (2000) 289, 611
report observation of CF3SF5 in the Antarctic
from ice firn data. Anthropogenic. ? Highest
radiative forcing per molecule of any greenhouse
gas. ? Current concentrations are low (0.12
pptv), but growing at 6 per annum. ?
Stratospheric profiles suggest it is long-lived.
The value of the CF3-SF5 bond strength is
important to determine if this greenhouse gas can
be photolysed in the stratosphere. or is the
sink route determined by ionic processes in the
mesosphere ? ? Measure DrHo0 (CF3SF5 ? CF3
SF5 e-) as a route to determine the C-S bond
strength. DIE (CF3SF5) Do0(CF3?SF5)
Adiabatic IE (CF3)
24
Structure of CF3-SF5
Strength of this bond ?
Sources of CF3-SF5 Anthropogenic. Trends of SF6
and CF3SF5 track each other. SF6 a dielectric in
high-voltage applications. By-product of CF3?
(from fluoropolymers) reacting with SF5?
25
Infra-red absorption spectrum of CF3SF5
(Gaussian 03) (Michael Parkes)
24 vibrational modes only 6 have any
significant IR intensity
Wavenumber / cm-1
26
C-S stretching mode 3.3 x 1013 vibrations per
second 1095 cm-1, 9.1 mm
27
C-S wagging mode 3.8 x 1013 vibrations per
second 1255 cm-1, 8.0 mm
28
Dissociative Ionisation Energy (DIE) of CF3-SF5
CF3-SF5
Eavail
CF3 SF5 e-
DIE
Ionisation energy of CF3
hn Ethermal
CF3 SF5
CF3-SF5
Do0 (CF3-SF5)
0
R (CF3 SF5)
 
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Daresbury Synchrotron Radiation Source, Cheshire
30
Multi-purpose coincidence apparatus (Daresbury)
Paul Hatherly (Reading University) Meas Sci Tech
(1992) 3 891
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Results for CF3 / CF3SF5 (J. Phys. Chem. A.,
(2001) 105, 8403)

• No parent ion is observed CF3SF5 behaves
  similar to CF4 and SF6. But, lower ionisation
  since the HOMO of CF3SF5 is S?C s-bonding
  orbital.
• Analysis of the variation of KE with photon
  energy yields
• DIE (CF3SF5 ? CF3 SF5 e-)
• 12.9 ? 0.4 eV.
• Do0(CF3?SF5) 3.86 ? 0.45 eV.
• DfHo0 (CF3?SF5)
• ?1750 ? 47 kJ mol-1
• ? Strong S-C s bond

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Regions of the earths atmosphere (again)
Rate of removal of CF3SF5 in the mesosphere is
CF3SF5.(S kionion kee- s121.6J121.6)
molecules cm-3 s-1
33
Selected Ion Flow Tube (SIFT) Smith and Adams
(1980s) now Chris Mayhew et al. (School of
Physics, Birmingham)

Determines rate constants and product ions for
the reactions A or A- B ? C or C- D k
has to be faster than ca. 10-12 cm3 molecule-1 s-1
34
Glass Cylinder
10 M? Resistors
63Ni Source
Collector
Gate
1 Bar Buffer Sample Flow
To quad mass spec.
Forward Flow
SWARM electron attachment Jarvis et al. Int.
J. Mass Spectrom. (2001) 205 255
35
Kennedy and Mayhew, Int. J. Mass Spectrom.,
(2001) 206 i - iv
kexp(298 K) (7.7 ? 0.6) ? 10-8 cm3 s-1
s-wave capture gives kthermal(298 K) 3.2 ?
10-7 cm3 s-1
The main product is dissociative, SF5-.
36
Vacuum-UV absorption apparatus (H.W. Jochims
Freie Universitat, Berlin)
Measure cross-sections in the range 10-19 to
10-16 cm2 Beer Lambert Law I Io exp(-scL)
37
Vacuum-UV absorption spectrum of SF5CF3
(Chem. Phys. Letts., (2003) 367 697)
photon resolution 0.08 nm s (121.6 nm) 1.3 ?
0.2 x 10-17 cm2
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Thermal electron attachment rate constants,
absorption cross-sections at 121.6 nm, and
atmospheric lifetimes for CF4, SF6 and
CF3SF5  __________________________________________
______________________   Perfluoro compound ke
(298 K) / cm3 s-1 s121.6 / cm2 lifetime /
yrs ______________________________________________
_____________________ CF4 lt 10-16
lt 8 x 10-22 gt 50000
CF3SF5 7.7 x 10-8 1.3 x 10-17 ca.
1000 SF6 2.3 x 10-7 1.76 x
10-18 gt 800 ________________________________
___________________________________ CF3SF
5 is behaving as a perturbed SF6, not as a
perturbed CF4, molecule. For SF6, the dominant
process for its removal is electron attachment in
the mesosphere to form SF5-. Assume the same is
true for CF3SF5.
39
Ravishankara and Lovejoy, JCS Faraday Trans.,
(1994) 90, 2159 written six years before the
CF3SF5 story began When CFCs were invented and
released into the atmosphere, their deleterious
effects were not known. Fortunately, CFCs are
relatively short-lived (ca. 100 years) compared
to perfluorocarbons, CxFy (ca. 1000 years) it
will take only about a century for CFCs to be
removed from the atmosphere once their emissions
are curtailed. The release of any very
long-lived species into the atmosphere should be
viewed with great concern. PFC (and CF3SF5)
lifetimes, though long on historical timescales,
are short compared to evolutionary timescales.
Life on Earth may not be able to adopt to the
changes these emissions may cause.
Thus, it seems prudent to ask if a long-lived
molecule should be considered guilty, unless
proven otherwise. or my view Dont put a
long-lived pollutant up into the atmosphere in
the first place. Attack problem at source.
40
Influence of enviromental issues on UK policy
? Energy, nuclear (already happened) Transport,
obvious (within 1-2 years) Individual carbon
allowances (coming soon) Retail, Sunday trading
(?) On international policy ? Carbon trading,
morality ? Population, realistic ? But time is
short 10-20 years only The stratospheric
ozone story may give us optimism
41
The Ozone story distribution in the
stratosphere before ca. 1960
O3max 41012 cm-3 at 30 km
42
Catalytic Destruction of O3 in the Stratosphere
by Cl atoms Molina and Rowland, mid 1960s
The C-Cl bond is fairly weak it can be broken
by UV radiation from the sun. CF2Cl2 hn
(200-400 nm) ? CF2Cl? Cl?
Chain Reaction
(I)
Termination Reactions produce resevoir
compounds
Cl CH4 ? HCl CH3 ClO NO2 ? ClONO2 Ozone
depletion is a serious issue because kI kII
(II)
43
Spring ozone hole over Antarctica, October 2000
44
Variation of O3 concentration with Altitude and
Time of Year Formation of Polar stratospheric
clouds in Antarctic winter Time resolution
months
Huge drop in O3 concentration in October, the
Spring in Antarctic, when there is sunlight after
6 months of darkness. Reactions in the gas phase
cannot explain the extent of the Ozone hole over
the Antarctic. Polar stratospheric clouds.
45
Success story for atmospheric scientists ?

• 1930s Large-scale production of
  chlorofluorocarbons begins
• 1964 First prediction by Molina and Rowland of
  O3 destruction
• First observation of ozone hole in Antarctica
• 36 nations sign Montreal Protocol
• Du Pont stop production of CFCs
• 93 nations sign Copenhagen Protocol
• First reports that ozone hole may be
  recovering
• timescale 50-100 years (e.g. Nature (2006)
  441, 39-45)
• 
  
• Cause for pessimism CFCs do not affect the
  standard of most
• peoples lives. Reduction in CO2 and CH4
  concentrations may.