1972 - 1974
1972 Lovelock detects CCl3F (CFC-11) in the
atmosphere of both hemispheres.
1974 Rowland & Molina's Nature paper
• Review suggests that tropospheric sinks for CFC's (e.g.
oxidation, dissolution in raindrops) not important.
• Only significant sink for CFC's UV photolysis in middle
stratosphere, 25-30 km. CFC molecules transparent above
230 nm, so need to be above O3 layer, which shields Earth
from UV light.
CFC + hυ → Cl + CCl2F
• See absorption characteristics of molecules in UV.
• Calculated atmospheric lifetime of 50+ yr for CFC's.
1974
1974 Rowland & Molina's Nature paper
• ClOx free radical catalytic chain reaction should result in
recycling of Cl:
Cl + O3 → ClO + O2
ClO + O → Cl + O2
Net: O + O3 → O2 + O2
• Cl disperses & is eventually lost by transport. Drifts back into
troposphere as HCl; but after chain length of about 105.
• Annual global release of CFC's doubling every 6-7 yr.
1974 - 1975
[1974] Rowland & Molina's ACS presentation & press
conference
• Estimated future steady state depletion of 7-13%
• Recommend complete CFC ban
• Cicerone-Stolarski & McElroy-Wofsey work supportive
1975 Legislative hearings in U.S. & elsewhere
• 2/3 of CFC use as propellents
• Also in air conditioners, blowing agents, etc.
• Lack of broad scientific participation
1975
1975 NCAR balloon sampling of stratospheric air,
which contained CFC-11
• Experimental evidence.
• Scientists had questioned that a heavier-than-air molecule
could be transported to the stratosphere.
1975 Oregon passes CFC use as propellants as partly
symbolic measure, but serves as test site for CFC
substitutes.
1976
1976 NAS Reports
• Panel on Atmospheric Chemistry essentially agreed with
original paper
• Committee on Impacts of Stratospheric Change recommended
additional 2-yr study before putting emissions controls in
place.
• Two federal agencies issues proposed rules banning CFC's as
aerosol propellants anyway.
1977 - 1984
1977 Several manufacturers initiate shift away from
CFC propellents.
but then….
1981 Reagan years
• U.S. regulatory actions stop during Reagan administration.
• CFC use as cleaning agent in microelectronics increases.
• All through 1980's there was a rapid rise in CFC use (see
graph).
• Canada, Norway & Sweden initiate regulation.
1985
1985 Role of surface reactions in recycling of Cl
examined.
• ClO chain interrupted by CH4 or NO2 to form temporary
reservoir molecules (hours to days):
Cl + CH4 → HCl + CH3
ClO + NO2 + M → ClONO2 + M
• Photolysis of ClONO2 or reaction of HCl with OH returns Cl
• Rapid reactions of ClONO2 observed in lab:
HCl +ClONO2 → Cl2 + HNO3
H2O + ClNO2 → HOCl + HNO3
• But in order to be fast in stratosphere, reactions requires
surfaces. Initial focus on H2SO4 aerosols from volcanic
sources.
1985
1985 Farman & colleagues of BAS report 35% loss
of October over Halley Bay, Antarctica from 195758 to 1984.
1985
1985 Farman & colleagues of BAS report 35% loss of
October over Halley Bay, Antarctica from 1957-58
to 1984.
• Unit of measure is Dobson Unit (DU), which is about a ppbv.
Integrated over atmosphere.
• Concentrations as low as 85-95 DU reported in 1993 at some
Antarctic stations.
• Losses confirmed by data from Total Ozone Mapping
Spectrometer (TOMS), on NASA's Nimbus 7 satellite.
• Atmospheric chemistry models could not explain such
dramatic losses.
1985
1985 Several explanations offered for proposed
losses over Antarctica.
• HCl accumulates on ice crystal surfaces; ClONO2 reacts with
high efficiency upon collision.
• Suggestions of previously unknown cyclic events based on
solar cycle, or changes in meteorology largely set aside by
Solomon's 1986 National Ozone Expedition (NOZE) to
McMurdo.
1985 - 1987
1985 Several explanations offered for proposed
losses over Antarctica.
• HCl accumulates on ice crystal surfaces; ClONO2 reacts with
high efficiency upon collision.
• Suggestions of previously unknown cyclic events based on
solar cycle, or changes in meteorology largely set aside by
Solomon's 1986 National Ozone Expedition (NOZE) to
McMurdo.
1987 NOZE expedition provided simultaneous
measurements of ClO & O3 in situ, which had
remarkable anticorrelation. 80% of O3 in vortex at
altitude of measurement disappeared in on month,
as polar sunrise occurred
1987
1987 NOZE expedition
provided simultaneous
measurements of ClO &
O3 in situ, which had
remarkable anticorrelation. 80% of O3 in
vortex at altitude of
measurement
disappeared in on month,
as polar sunrise occurred
Aug 23
Sep 21
1987
1987 Stratospheric chemistry of polar spring becomes much
clearer.
• In midwinter, polar stratospheric clouds (PSC's) drop below 195oK.
• Heterogeneous reactions occur on surfaces in PSC's, creating HOCl &
Cl2 from reservoir molecules.
• Approaching spring (late August) sunlight releases the Cl & initiates
O3 loss reaction.
• Absence of chain terminating steps results in large losses through
mid-October. NO2 trapped as HNO3 in PSC's.
• ClO radicals form dimer, which is broken apart by sunlight to again
release Cl.
ClO + ClO + M → ClOOCl + M
ClOOCl + hυ → Cl + ClO2 → Cl + Cl + O2
Note that polar reaction (together with Cl → ClO reaction above
sums to 2O3 → 3 O2 & does not require O atoms. So it can
operate efficiently in the lower stratosphere where O is
scarce.
1987
1987 Montreal protocol.
• Framework convention (treaty) in 1985 in Vienna, under united
Nations Environment Program (UNEP).
• Montreal protocol in 1987 called for 50% cutback in yearly CFC
production by year 2000.
• 1989 Helsinki meeting: declaration calling for CFC & halon phaseout
by 2000. Antarctica findings spurred 1990 London protocol calling
for complete ban on CFC production by year 2000.
• European regulation 594/91 called for phase out of CFC's within EC
by 1997.
• Protocol strengthened in 1992 in Copenhagen to call for complete
production ban on CFC's, CCl4, CCl3CH3 by 1996. Phase out halons
by 1994; control CH3Br & HCFC's. Followed U.S. unilateral move to
1996 phase out CFC's.
• Agreement in 1993 that no essential uses for halons in 1994.
• Montreal protocol a good model for handling of global environmental
problems.
1986
1986 Ground-based stations in mid-latitudes show O3
losses.
• Examination of ground-based data for Arosa Switzerland
showed decreases for 1931-69 vs. 1970-86.
• Losses also at U.S. sites for 1963-86 period.
• Later examination of all 18 long-record DU stations in
northern hemisphere showed winter losses.
• Nimbus data, since 1978, show global losses. Calibrated for
long-term drift using DU stations.
• Arctic losses not as large as Antarctic losses due to milder
winters & less stable vortex. Result has been breakup of
conditions for large losses before losses can occur.
Examination of ground-based data for Arosa Switzerland showed
decreases for 1931-69 vs. 1970-86.
1988
1988 U.S. manufacturers DuPont, Allied Signal,
others announce rapid discontinuation of
production of CFCs.
• Undertook crash programs to resume development of
substitutes.
• Substitutes are hydrochloroflurocarbons (HCFC's) &
hydrofluorocarbons (HFC's), which degrade faster in
troposphere.
1990 Stratospheric Br 50 times as efficient as Cl in
O3 loss.
• Br is not nearly as abundant as is Cl in stratosphere.
• Several source molecules: CH3Br & fire suppressent halons
(CBrF3, CBrBlF2, CBrF2CBrF2).
1991
1991 Other processes confound O3 losses.
• Pinatubo eruption puts more aerosols/surfaces in
stratosphere, enhancing O3 loss reactions.
• Reaction on sulfate aerosol ties up NO2.
NO2+NO3 ↔ N2O5
N2O5 + H2O → OH + 2O2 (occurs on sulfate
aerosol)
• Note that there are also natural loss mechanisms involving
NO & OH.
1993
1993 Global lowest in period of record, measured
from space.
• January 1993 O3 13-14% below normal.
• Previous loss about 3% per decade.
• 1993 UARS map of ClO further closes points for critics to
question.
1994
1994 We have a CFC-free economy, but Cl persists.
• Note figure showing persistence well into next century, even
under 192 protocol revisions.
• Current evaluation places emphasis on ozone depletion
potential (ODP).
• Climate effects involve lowering of stratospheric temperature
( 1.7 ± 1.0°C in decade, since less O3 to absorb radiation.
Shifts in wind patterns?
• Biological effects to humans are more skin cancer, more eye
cataracts, partial immune suppression. Nonhuman effects,
e.g. tissue damage, not well studied.
• Denial of problem widespread in popular press.
19952000
1995-2000 Observations show
leveling off/slight decline in
CFC/FC concentrations.
19952000
Ozone hole
continues to
grow….
1998 Sept 19 Ozone hole reaches area of 27.1million km2, a
new record. Average for season ~ 25 million km2
2000 Sep 8-9 Ozone hole reaches area of 29.5 million km2, a
new record
1995-2000
1995-2000 Observations show leveling off/slight
decline in CFC/FC concentrations.
Ozone hole continues to grow….
1998 Sept 19 Ozone hole reaches area of 27.1million
km2, a new record. Average for season ~ 25
million km2
2000 Sep 8-9 Ozone hole reaches area of 29.5
million km2, a new record
• Hole began forming several weeks earlier than usual.
• Exposed the city of Punta Arena, Chile, a city of 120,000
people, the first time a population center has been exposed.
• Prediction that possible argentine, tip of South Africa,
Australia & New Zealand may be at risk in future.