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@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]

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 man’s

activities on earth’ (George Bush, 2001-2004, 2006 ?) ‘Global warming is the most serious phenomenon affecting the world’s

security and prosperity, more so than terrorism’(David King, UK Government Chief Scientific Adviser, 2003)

Scientists ? ‘Global warming is not due to man’s 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.

CO2 concentrations and Temperature change over 1000 years

CO2 concentrations and Temperature change over ‘recent’ years

But what about the period 1780 – 1904 ??

Correlation between concentrations of O3 and ClO over Antarctic, Sept 1987 : importance of resolution

Rise in concentration of ClO anti-correlates with fall in concentration of O3

O3

ClO

Anderson et al.

J. Geophys. Res. D., (1989) 94, 11465

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.gov/products/vis/images/gallery/sphere_04_150.gif

The power of computers and the web : predictions for AD 2156

Atmospheric scientist’s triangle

Laboratorymeasurements

(low T)

Fieldmeasurements

Modellers

sourcesO reactions3

sinks

high altitude

low

alti

tude

O or 3 N O + h2

H 2+ h

Chemical cycles for HOx trace species

Rate constants (low T), products of elementary reactions, absorption cross-sections, thermochemistry

Mace Head, West Ireland

Cape Grim, TasmaniaConcentration of CHF3 / ppt

Concentration of CO2 / ppm

TOMS (Total ozone monitoring spectrometer)NASA

Development of Antarctic Ozone Hole, 1979-1997

Simple Photochemistry

Vacuum-UV 100 – 200 nm

UV 200 – 400 nm

visible 400 – 750 nm

IR 750 - 20000 nm

E = h = hc / (E as )

E, energy of photonh, Planck’s Constant, frequency of photonc, velocity of light, wavelength of photon

5780K

peak at~0 .5 m

peak at~10 m

w aveleng th / m

Sun Earth290K

0.1 0.5 1 5 10 50 100

Em

itted

en

ergy

Black body emission from the Sun (T ~ 5780 K) compared to that of the Earth (T ~ 290 K)

(Einstein, 1905)

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 intemperature, and global warming.

Earth’s atmosphere is 78% N2, 21% O2 ; neither absorb IR radiation.

Satellite data confirming trapping of IR radiation (Nimbus 4)RP Wayne, Chemistry of atmospheres (1991)

- - - - - spectrum expected for a black body at temperature T. Natural GH gases : 2 modes of CO2 (15 m), H2O (6.3 m) Enhancing GH gases : pollutants that absorb IR strongly in the range 6-25 m where CO2 and H2O do not absorb.

CO2 O3 H2O

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

Ground level clean air : main constituents

99% of earth’s atmosphere in troposphere, < 1% in stratosphere

Regions of the earth’s atmosphere

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 m. Coincidentally, CO2 absorbs at 15 m ; nature is unkind.

Long lifetime in the earth’s atmosphere ; no reaction with OH and O*(1D), or photodissociation in troposphere (300-500 nm) or stratosphere (200-300 nm).

IR spectroscopy, absolute absorption coefficientsReaction 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 man’s activity

CO2 = 1, CH4 = 23, CF2Cl2 = 10600, CF3SF5 = 18000

Properties of greenhouse gases (absorption of infra-red radiation into vibrational modes of the gas)

1 mode of CO2

4.2 x 1013 vibrations per second1388 cm-1 or 7.2 mInfra-red inactive

O C O

2 mode of CO2

2.0 x 1013 vibrations per second667 cm-1 or 15.0 m

Infra-red active

O C O

3 mode of CO2

2.0 x 1013 vibrations per second2340 cm-1 or 4.3 m

Infra-red active

O C O

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 < 106

Concn / % per year 0.45 0.60 ca. 5 ca. 6

Microscopic radiative 1.68 x 105 4.59 x 104 0.32 0.60

forcing / W m-2 ppb-1

Total radiative 1.46 0.48 0.16 7.2 x 105

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___________________________________________________________________________

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

CF3SF5 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 rHo0 (CF3SF5 CF3

+ + SF5+ e) as a route to determine the CS bond strength.

DIE (CF3SF5) = Do0(CF3SF5) + Adiabatic IE (CF3)

Fluorine

Carbon

Sulphur

Structure of CF3SF5

Sources of CF3SF5

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

F

C S F

F

FF

F

F

F

Strength of this bond ?

Infra-red absorption spectrum of CF3SF5 (Gaussian 03)(Michael Parkes)

Wavenumber / cm-1

24 vibrational modes : only 6 have any significant IR intensity

CS stretching mode3.3 x 1013 vibrations per second

1095 cm-1, 9.1 m

CS wagging mode3.8 x 1013 vibrations per second

1255 cm-1, 8.0 m

R(CF3-SF5)

EN

ER

GY

CF3 + SF5

CF3+ + SF5 + e-

Ionisation energy of CF3

Eavail

hEthermal

0

DIE

R (CF3 – SF5) 

CF3SF5+

CF3SF5

Dissociative Ionisation Energy (DIE) of CF3SF5

Do0 (CF3-SF5)

Daresbury Synchrotron Radiation Source, Cheshire

Multi-purpose coincidence apparatus (Daresbury)

PMTFilter

Holder

Lens

QuartzWindow

Mirror

FluorescenceSignal

MonochromatisedSynchrotron

Radiation

Channeltron

ThresholdElectron

Signal

MCPs

-96.5 V

-96.5 V

-20 V

-96.5 V

DriftTube

Photo-IonSignal

+20 V

+20 V

SodiumSalicylateWindow

+2 V

+10 V

+10 VElectronLenses

127 Post-analysero

PMT

PhotoexcitationFlux Signal

Paul Hatherly(Reading University)

Meas Sci Tech (1992) 3 891

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 SC -bonding orbital.

Analysis of the variation of KE with photon energy yields :

DIE (CF3SF5 CF3+ + SF5 + e)

= 12.9 0.4 eV.

Do0(CF3SF5) = 3.86 0.45 eV.

fHo0 (CF3SF5)

= 1750 47 kJ mol-1

Strong SC bond Photon Energy / eV

12 13 14 15 16

Thr

esho

ld P

hoto

elec

tron

sign

al /

arb.

uni

ts

0

Mea

n to

tal K

E r

elea

se /

eV

0.0

0.5

1.0

TPES

SF5CF3 SF5 + CF3+ + e-

(rHo = 12.9 ± 0.4 eV)

X

A

Regions of the earth’s atmosphere (again)

Rate of removal of CF3SF5 in the mesosphere is

[CF3SF5].( kion[ion] + ke[e-] + 121.6J121.6) molecules cm-3 s-1

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. 1012 cm3 molecule-1 s-1

Gate

63Ni Source

1 Bar Buffer & Sample Flow

Glass Cylinder10 M Resistors

“Forward” Flow

Collector

To quad mass spec.

SWARM electron attachment : Jarvis et al. Int. J. Mass Spectrom. (2001) 205 255

kexp(298 K) = (7.7 0.6) 108 cm3 s-1

s-wave capture gives kthermal(298 K) = 3.2 107 cm3 s-1

Kennedy and Mayhew, Int. J. Mass Spectrom., (2001) 206 i - iv

0.0 0.5 1.0 1.50.0

2.0x10-8

4.0x10-8

6.0x10-8

8.0x10-8

k a (/c

m3 m

olec

ule-1

s-1)

Mean Electron Energy (/eV)

3-5

-3535 CFSFCFSFCFSF *

e

The main product is dissociative, SF5

.

Vacuum-UV absorption apparatus(H.W. Jochims : Freie Universitat, Berlin)

Measure cross-sections in the range 10 to 10cm2

Beer Lambert Law : I = Io exp(cL)

Photon Energy / eV

5 10 15 20 25

Abs

orpt

ion

cros

s-se

ctio

n (c

m2 )

0.0

2.0e-17

4.0e-17

6.0e-17

8.0e-17

1.0e-16

1.2e-16

1.4e-16

Vacuum-UV absorption spectrum of SF5CF3 (Chem. Phys. Letts., (2003) 367 697)

photon resolution = 0.08 nm (121.6 nm) = 1.3 0.2 x 1017 cm2

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-1 121.6 / cm2 lifetime / yrs

___________________________________________________________________

CF4 < 10 < 8 x 10 > 50000

CF3SF5 7.7 x 10 1.3 x 10 ca. 1000

SF6 2.3 x 1.76 x 10 > 800 ___________________________________________________________________

CF3SF5 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.

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 : Don’t put a long-lived pollutant up into the atmosphere in the first place. Attack problem at source.

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

[O3]max =

4×1012 cm-3

at 30 km

The Ozone story : distribution in the stratosphere before ca. 1960

stratosphere

troposphere

m esosphere

therm osphere

Tem perature / K

Temperature / K

Ozone

Alti

tud

e / k

m

Catalytic Destruction of O3 in the Stratosphere by Cl atoms[Molina and Rowland, mid 1960s]

C l + O C lO + O3 2

C lO + O C l + O 2

3 2O + O 2 O

Chain Reaction

Termination Reactions : produce ‘resevoir’ compoundsCl + CH4 HCl + CH3

ClO + NO2 ClONO2

Ozone depletion is a serious issue because kI » kII

The C-Cl bond is fairly weak ; it can be broken by UV radiation from the sun.CF2Cl2 + h(200-400 nm) CF2Cl + Cl

(I)

(II)

Spring ozone hole over Antarctica, October 2000

Variation of O3 concentration with Altitude and Time of YearFormation 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.

1930s Large-scale production of chlorofluorocarbons begins

1964 First prediction by Molina and Rowland of O3 destruction

1980 First observation of ozone hole in Antarctica

1987 36 nations sign Montreal Protocol

1988 Du Pont stop production of CFCs

1992 93 nations sign Copenhagen Protocol

2006 First reports that ozone hole may be recovering ;timescale 50100 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.

Success story for atmospheric scientists ?