The Ozone Layer

The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone (O3). This layer absorbs 97-99% of the sun's high frequency ultraviolet light, which is potentially damaging to life on earth. Over 90% of ozone in earth's atmosphere is present here "Relatively high" means a few parts per million—much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately 15 km to 35 km above Earth's surface, though the thickness varies seasonally and geographically.

A dobson unit is the most basic measure used in ozone research.One Dobson Unit (DU) is defined to be 0.01 mm thickness at STP (standard temperature and pressure). Ozone layer thickness is expressed in terms of Dobson units, which measure what its physical thickness would be if compressed in the Earth's atmosphere. In those terms, it's very thin indeed. A normal range is 300 to 500 Dobson units, which translates to an eighth of an inch-basically two stacked pennies.

Dobson Unit

In space, it's best not to envision the ozone layer as a distinct, measurable band. Instead, think of it in terms of parts per million concentrations in the stratosphere (the layer six to 30 miles above the Earth's surface).

The unit is named after G.M.B. Dobson, one of the first scientists to investigate atmospheric ozone .

A thinning ozone layer leads to a number of serious health risks for humans. It causes greater incidences of skin cancer and cataract of the eye, with children being particularly vulnerable. There are also serious impacts for biodiversity. Increased UV-B rays reduce levels of plankton in the oceans and subsequently diminish fish stocks. It can also have adverse effects on plant growth, thus reducing agricultural productivity. Another negative effect is the reduced lifespan of certain materials.

Severe depletion of the Antarctic ozone layer was first observed in the early 1980s. The international response embodied in the Montreal Protocol. Today 191 countries worldwide have signed the Montreal Protocol which is widely regarded as the most successful Multinational Environmental Agreement ever reached to date.

Furthermore the phasing out of ozone depleting substances (ODS) has helped to fight climate change since many ODS are also powerful greenhouse gases.

HISTORY OF THE OZONE LAYER

600,000,000 B.C.	Ozone layer forms
1839	Christian Schöenbein identifies ozone in the laboratory
1845	Auguste de la Rive and Jean-Charles de Marignac suggest ozone is a form of oxygen; confirmed by Thomas Andrews in 1856
1858	Andrei Houzeau finds ozone present in natural air
1865	Jean-Louis Soret proves that ozone is O₃
1879	Marie Alfred Cornu measures solar spectrum and finds sharp cutoff in ultraviolet (UV) light
1881	Walter Hartley recognizes cutoff corresponds to UV absorption by ozone
1913	John William Strutt (Lord Rayleigh) shows absorption is not in lower atmosphere
1919	Charles Fabry makes first spectrometric measurements of "thickness" of ozone layer
1924	G.M.B. Dobson develops ozone spectrophotometer and begins regular measurements of ozone abundance (Arosa, Switzerland)
1925	Jean Cabannes and Jean Dufay show ozone is about 10 miles high
1928	Thomas Midgley synthesizes chlorofluorocarbons (CFC's)
1929	Umkehr method for Dobson instrument establishes that ozone maximum is below 15 miles altitude
1930	Sydney Chapman describes theory that explains existence of an ozone "layer"
1934	Ozonesonde (balloon) measurements establish the ozone concentration is maximum around 12 miles up
1930's	GM develops applications for CFC's
1950	David Bates and Marcel Nicolet propose catalytic (HOx) ozone destruction
1957	Global network of Dobson spectrophotometers established during the International Geophysical Year (IGY)
late 1950's	CFC market expands rapidly
early 1960's	Catalytic destruction is necessary in order to explain ozone amounts
1960's	Boeing proposes supersonic transport (SST) fleet of 800 aircraft
1969	Paul Crutzen discovers NOx catalytic cycle
1971-74	Dept of Transportation sponsors intensive program of research, The Climatic Impact Assessment Program (CIAP)
1971	Congress axes funding for the SST
1971	Johnston calculates that NOx from SST's could deplete ozone layer
1973	Rick Stolarski and Ralph Cicerone suggest catalytic capability of Cl
1973	James Lovelock detects CFC's in atmosphere
1974	Sherwood Rowland and Mario Molina warn of ozone depletion due to CFC's
March 1977	First international meeting (Washington DC) to address issue of ozone depletion held by the United Nations Environmental Programme (UNEP)
March 1978	US bans non-essential use of CFC's as aerosol propellant
1978	Total Ozone Mapping Spectrometer (TOMS) is launched aboard NIMBUS-7 spacecraft giving global coverage of ozone layer thickness
1980's	Renewed expansion of CFC market
Oct 1982	Shigeru Chubachi measures low ozone over Syowa, Antarctica (reported at Ozone Commission meeting in Halkidiki, Greece in Sept 1984)
1984	British Antarctic Survey scientists discover recurring springtime Antarctic ozone hole (published in Nature May 1985)
March 1985	Vienna Convention for the Protection of the Ozone Layer
Sept. 1987	Montreal Protocol on Substances That Deplete the Ozone Layer (Amendments - London 1990; Copenhagen 1992)
March 1988	DuPont agrees to CFC production phase-out
late 1980's	Ten years of satellite data begin to show measurable ozone depletion globally
1991	DuPont announces phase-out of CFC production by end of 1996
1992/3	Abnormally low ozone observed globally
1995	Crutzen, Rowland, and Molina win Nobel Prize in Chemistry
mid-1990's	springtime Arctic ozone dent appearing
Jan. 1996	CFC production ends in US and Europe
2000	Maximum CFC concentrations in stratosphere are reached
Today	The Ozone Layer - Global Map
	THE FUTURE
2010	CFC production ends world-wide
2030	Hydrochlorofluorocarbon (HCFC) alternatives are phased out
2040	HCFC production ends world-wide
2050	Springtime Antarctic ozone hole disappears

Credit: Bob Keesee The Upper Atmosphere A ATM 101N University Of Albany, NASA, EPA, Environment Canada