1972-1974 1972 Lovelock detects CCl 3 F (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 O 3 layer, which shields Earth from UV light. CFC + hυ Cl + CCl 2 F See absorption characteristics of molecules in UV. Calculated atmospheric lifetime of 50+ yr for CFC's.
1974 1974 Rowland & Molina's Nature paper ClO x free radical catalytic chain reaction should result in recycling of Cl: Cl + O 3 ClO + O 2 ClO + O Cl + O 2 Net: O + O 3 O 2 + O 2 Cl disperses & is eventually lost by transport. Drifts back into troposphere as HCl; but after chain length of about 10 5. 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 NAS Reports 1976 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 CH 4 or NO 2 to form temporary reservoir molecules (hours to days): Cl + CH 4 HCl + CH 3 ClO + NO 2 + M ClONO 2 + M Photolysis of ClONO 2 or reaction of HCl with OH returns Cl Rapid reactions of ClONO 2 observed in lab: HCl +ClONO 2 Cl 2 + HNO 3 H 2 O + ClNO 2 HOCl + HNO 3 But in order to be fast in stratosphere, reactions requires surfaces. Initial focus on H 2 SO 4 aerosols from volcanic sources.
1985 1985 Farman & colleagues of BAS report 35% loss of October over Halley Bay, Antarctica from 1957-58 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; ClONO 2 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; ClONO 2 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 & O 3 in situ, which had remarkable anticorrelation. 80% of O 3 in vortex at altitude of measurement disappeared in on month, as polar sunrise occurred
1987 1987 NOZE expedition provided simultaneous measurements of ClO & O 3 in situ, which had remarkable anticorrelation. 80% of O 3 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 195 o K. Heterogeneous reactions occur on surfaces in PSC's, creating HOCl & Cl 2 from reservoir molecules. Approaching spring (late August) sunlight releases the Cl & initiates O 3 loss reaction. Absence of chain terminating steps results in large losses through mid-october. NO 2 trapped as HNO 3 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 + ClO 2 Cl + Cl + O 2 Note that polar reaction (together with Cl ClO reaction above sums to 2O 3 3 O 2 & does not require O atoms. So it can operate efficiently in the lower stratosphere where O is scarce.
1987 Montreal protocol. 1987 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, CCl 4, CCl 3 CH 3 by 1996. Phase out halons by 1994; control CH 3 Br & 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 O 3 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 O 3 loss. Br is not nearly as abundant as is Cl in stratosphere. Several source molecules: CH 3 Br & fire suppressent halons (CBrF 3, CBrBlF 2, CBrF 2 CBrF 2 ).
1991 1991 Other processes confound O 3 losses. Pinatubo eruption puts more aerosols/surfaces in stratosphere, enhancing O 3 loss reactions. Reaction on sulfate aerosol ties up NO 2. NO 2 +NO 3 N 2 O 5 N 2 O 5 + H 2 O OH + 2O 2 (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 O 3 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 O 3 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.
1995-2000 1995-2000 Observations show leveling off/slight decline in CFC/FC concentrations.
1995-2000 Ozone hole continues to grow. 1998 Sept 19 Ozone hole reaches area of 27.1million km 2, a new record. Average for season ~ 25 million km 2 2000 Sep 8-9 Ozone hole reaches area of 29.5 million km 2, 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 km 2, a new record. Average for season ~ 25 million km 2 2000 Sep 8-9 Ozone hole reaches area of 29.5 million km 2, 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.