Ozone Milestones



Ozone is discovered by C. F. Schönbein in a paper entitled "Research on the nature of the odor in certain chemical reactions".  Ozone is a pungent gas that can often be detected by its characteristic smell after lightning storms or arcing of electrical equipment.  Schönbein was not able to characterize the chemical structure of ozone.  Most interestingly, he named it after the Greek word "ozein", to smell.  J. L. Soret works out the molecular structure of ozone a few years later.
Observations of the abundance of surface ozone started at numerous locations.

The absorption of solar radiation between wavelengths of 200 and 320 nm is attributed to ozone by W. N. Hartley and A. Cornu.  They correctly deduced that most of the ozone must be in the upper atmosphere, rather than near the surface.
The first measurements of total column ozone obtained by G. M. B. Dobson reveal, unexpectedly, maximum abundances at high-latitude.  Dobson correctly concludes that stratospheric winds must play a role in transporting ozone.
T. Midgley, an industrial chemist working at General Motors, invents CFC-12 as a nonflammable, nontoxic compound to replace the hazardous materials then in use in home refrigerators.  Midgley routinely blows out a candle with inhaled vapors of CFC-12 to demonstrate that the chemical is neither toxic nor flammable.  Prior to 1928, a number of deaths had been attributed to the leakage of toxic ammonia from home refrigerators in the U.S.
The Umkehr method for measuring the vertical distribution of  ozone is invented.  It is shown that the concentration of ozone reaches a maximum below an altitude of 25 km.


S. Chapman establishes the photochemical theory of stratospheric ozone, based on formation from photolysis of molecular oxygen and loss by the reaction of atomic oxygen and ozone.

Measurements from a balloon-borne ozone-sonde establish that the maximum concentration of ozone is at about 20 km altitude.
Most home refrigerators in the U.S. use CFC-12.
CFCs are used increasingly as blowing agents to manufacture styrofoam.

Modern Theory and Observation


Worldwide network of stations developed, as part of the International Geophysical Year, to measure ozone profiles and the total column abundance of ozone using a standard quantitative procedure pioneered by Dobson.  The station installed by the British Antarctic Survey at Halley Bay, Antarctic, will later play a critical role in establishing the link between human activity and ozone depletion.
Photochemical theory of ozone destruction by hydrogen radicals is proposed by D.R. Bates and M. Nicolet.
The first observations of ozone from a satellite are obtained by the Infrared Interferometer Spectrometer (IRIS) on Nimbus 3.
Ozone destruction by nitrogen radicals is proposed.  H. Johnston raises the concern that nitrogen oxides released as exhaust from a proposed fleet of supersonic aircraft could damage the ozone layer. P. Crutzen shows how decomposition of nitrous oxide (N2O) supplies nitrogen oxides to the stratosphere.
J. McDonald shows a quantitative connection between decreases in the abundance of ozone and increases in the incidence rate of skin cancer.

CFCs and Ozone Depletion Linked


Photochemical theory of ozone destruction by chlorine radicals is proposed.  Initial concern focuses on ozone loss due to chlorine emitted as exhaust from the proposed space shuttle.  F. S. Rowland and M. Molina suggest that CFCs will reach the stratosphere, releasing chlorine that destroys ozone.

Ironically, it is the extreme biological and chemical inertness of CFCs that allow these gases to reach the stratosphere.  Most pollutants decompose in the lower atmosphere (troposphere) due to reaction with oxidants such as the hydroxyl radical.  CFCs are unreactive and insoluble.  They are eventually carried by the winds to the stratosphere, where the extreme ultraviolet environment of this region causes their decomposition, releasing chlorine that causes ozone depletion.
About half of CFCs produced in the U.S. were used as aerosol propellants for personal care items such as hair sprays and deodorants.
The first international assessment of the state of the ozone layer is conducted by the World Meteorological Organization (WMO).  Scientific assessments of the ozone layer later issued in 1981, 1985, 1988, 1991,1994, and 1998 by WMO in collaboration with the United Nations Environment Programme and national research agencies play a key role in establishing scientific consensus on matters related to ozone depletion.
Early measurements and computer simulations support the hypothesis of Rowland and Molina that industrial CFCs lead to ozone destruction. However, large uncertainties remain in quantifying the effect of CFCs on ozone.
The use of CFCs as aerosol propellants is banned first in the U.S., then in Canada, Norway, and Sweden.  No other nation will enact legislation banning the use of CFCs until the historic Montreal Protocol of 1985.
Measurements of ozone by the NASA TOMS (Total Ozone Mapping Spectrometer) satellite instrument are first obtained.  Space-borne observations of the global distribution of ozone will continue nearly continuously until the present time.  This data record will ultimately prove to be an important quantitative measure of the effect of human activity on atmospheric composition.

Ozone Holes and the Ban of CFCs


Unusually low (~200 DU) total ozone reported at Halley Station, Antarctica during spring by J. Farman and colleagues. A time series of observations extending from 1957 to 1984 shows a steady decline in overhead ozone during October, which coincides with a rise in the atmospheric abundance of CFCs.
Vienna Convention for the Protection of the Ozone Layer concluded "ozone depletion is a real issue" and establishes an agreed-upon process for a "global commitment to addressing the problem."
Montreal Protocol on Substances that Deplete the Ozone Layer sets the following limits on emissions of CFCs 11, 12, 113, 114, 115:
Phase down 1986 levels of emission by: 20% at the end of 1994
50% at the end of 1999


Documentation of a decrease in ozone concentrations by decade in the  middle and high latitude lower stratosphere during winter.

It is established that active chlorine and bromine by-products of human activity are the cause of the Antarctic ozone hole.

London Amendment to the Montreal Protocol phases out all CFC production and consumption by the year 2000:
Substance Action Amount Year
CFCs  Phase down 1989 levels by 20% 1993
85% 1997
100% 2000
Halons Phase down 1989 levels by 50% 1995
100% 2000
Carbon tetrachloride Phase down 1989 levels by 85% 1995
100% 2000
Methyl Chloroform Production freeze  1993 
Phase down 1989 levels by 30% 1995
70%  2000
100% 2005

1989 to 1992:

Documentation of declines in lower stratospheric ozone year round at all latitudes (e.g., both hemispheres) except the tropics.

Observation of enhanced concentrations of ClO in the Arctic confirms the potential for large ozone declines in this region.

Demonstration of the importance of aerosol mediated processes for suppression of NOx and enhancement of ClO in the mid-latitude stratosphere.

Worsening of the Antarctic ozone hole.

Copenhagen Amendment to the Montreal Protocol further strengthens the phase out of ozone depleting compounds:
Substance Action Amount Year
CFCs Phase out 100% 1996
Halons Phase out 100% 1994
Carbon tetrachloride Phase out 100% 1996
Methyl Chloroform Phase out 100% 1996
Methyl Bromide Freeze at 1991 levels by 1995
HCFCs Phase down 1989 levels by 35% 2005
90% 2020
100% 2030

1992 to 1997:

Unabated continuation of Antarctic ozone losses: extremely low (~100 DU) values of total ozone reported for large areas of the Antarctic vortex.

Observation of record low ozone values (exceeding 25% below long term average) over Siberia and a large part of Europe during January to March, 1995.

Establisment of an association between enhanced chemical depletion of Arctic ozone due to chlorine and bromine for winter/springs with cold, persistent vortex circulations.  Measurements of ClO from the JPL MLS (Microwave Limb Sounder) satellite instrument and the NASA ER-2 high-altitude research aircraft play a key role in this finding.

1995 to 1999:
Vienna, Montreal, and Beijing Adjustments to the Montreal Protocol accelerate the phase out of ozone depleting compounds:

Industrialized countries strengthen HCFC phase out schedule and adopt 2010 phase out for methyl bromide.

Developing countries formally accept delayed reduction schedules for all controlled substances.

First decisions adopted for remedial actions in cases of noncompliance.

An Ozone Depletion/Greenhouse Gas Connection?

1998 to 2000:

Possible links between ozone depletion and climate change become an increasing focus of measurement and modeling activities.

It was once believed that "ozone depletion" and "greenhouse warming" were largely unrelated ecological problems.  At present, scientists are concerned that the build-up of greenhouse gas concentrations may lead to colder, more persistent vortex circulation patterns at high-latitude during winter that promote chemical loss of ozone by reactions occurring on the surface of polar stratospheric clouds.  Also, scientists are concerned that climate change may lead to higher abundances of stratospheric water vapor, which leads to conditions more favorable for ozone loss by halogens at both mid-latitudes and high-latitudes and also leads to increased rates of ozone destruction by hydrogen radicals.

Understanding the effect of greenhouse gases on stratospheric  circulation and the precise physical mechanism(s) that regulate stratospheric water vapor are the focus of a number of current and future research activities at JPL as well as many other institutes.

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Author: Ross J. Salawitch
Page Design:  Aaron B. Milam

Primary Sources:

                            The Changing Ozone Layer by Rumen D. Bojkov (WMO and UNEP, 1995).
                            Ozone Discourses by Karen T. Litfin (Columbia University Press, 1995).
                            Ozone Diplomacy: New Directions in Safeguarding the Planet by Richard E. Benedick
                                (Harvard University Press, 1998).
                            Health Sciences and Technology Academy, State of West Virginia webpage