O-Three, Can You See?
Global ozone concentration map for January 29, 2002. The
map was created from data acquired by the Earth Probe - Total Ozone Mapping
Spectrometer (EP-TOMS) instrument.
If they hear the word "ozone", the next word many people will think of is
"hole", as in "ozone hole". The phrase "ozone hole" refers to the depletion of
stratospheric ozone over the continent of Antarctica every winter, caused by
the catalytic interaction of chlorofluorocarbons (CFCs) and ozone on the
surfaces of ice particles high in the Antarctic polar vortex. The reason that
numerous countries around the world are signatories to the Montreal Protocol
which eliminates the use of CFCs is concern over the reduction of ozone
concentrations in the stratosphere.
But when ocean color scientists hear "ozone", the next word they likely
think of is "absorption", as in the absorption of light by ozone molecules.
The presence of ozone in the atmosphere is one factor that must be corrected
for in the process of producing accurate ocean color data.
Natural ozone (as opposed to ozone that forms from pollution in the
troposphere) is formed when ultraviolet (UV) light from the sun provides energy
that breaks a normal molecule of oxygen (O2) into two oxygen
atoms. These two oxygen atoms are highly reactive, and they can combine with
another O2 molecule to form a molecule of ozone, O3.
The chemical equations for the formation of ozone are thus (where hv
represents a photon of UV light):
O2 + hv  O + O
O2 + O O3
Ozone in the stratosphere is particularly important for one reason: ozone
molecules strongly absorb UV light, which is the light that causes sunburn on
unprotected human skin. Ozone absorbs UV light by the "reverse" reaction:
O3 + hv O2 + O
Below is a graphic illustration of the process, courtesy of Vito Ilacqua of
the Rutgers University Institute of Marine and Coastal Sciences. The red
spheres represent oxygen atoms.
Illustration of the Chapman mechanism for ozone
destruction and formation in the stratosphere.<
UV light can cause damage to many other organisms, both plants and animals,
and the potential for damage is what makes the protective ozone in Earth's
stratosphere so important. The absorption of UV light by ozone also makes
it fairly easy to measure its concentration in the atmosphere, which is what
satellite instruments such as the Total Ozone Mapping Spectrometer (TOMS) and
the TIROS Operational Vertical Sounder (TOVS) do quite well.
Perhaps overlooked in the emphasis on the absorption of UV light by ozone
lesser-known fact that ozone also absorbs light in the visible portion of the
electromagnetic spectrum. Even though ozone doesnt absorb much visible
light, it is still necessary to correct for the absorption of light due to
ozone in the atmospheric correction process for ocean color data (and for other
varieties of remote sensing data).
In 1881, a few years after the discovery of ozone, W.N. Hartley hypothesized
that the observed spectrum of UV light from the sun was influenced by the
presence of ozone in the stratosphere. This spectrum was known as the "UV
cutoff", because the spectrum of UV light is sharply attenuated. Hartley's
supposition was confirmed by an individual named Chappuis, who also discovered
the absorption of light by ozone in the visible range of the spectrum. The
wavelength where ozone absorbs visible light is at 602 nm, and is called the
"Chappuis band". (Unfortunately, thats about all we can learn about Mr.
Chappuis!) The spectrum below, obtained by the Global Ozone Monitoring
Experiment (GOME) instrument on the ERS-2 satellite, shows both the ozone
cutoff and the Chappuis absorption band. (Spectrum courtesy of Dr. R.B.A.
(Robert) Koelemeijer, Royal Netherlands Meteorological Institute.)
GOME data for gaseous absorption in the atmosphere. The Chappuis ozone absorption band appears at 602 nm.
The presence of the ozone hole in the Antarctic winter stratosphere is a
drastic demonstration of how much ozone concentrations can vary. Another
example, of changes in the concentration of ozone over the Bering Sea, is shown
below. Ozone concentrations are expressed in "Dobson units", after Dobson
(from the Clarendon Laboratory at Oxford University), who invented one of the
first instruments for measuring ozone concentration based on its absorption of
UV light.
Graph of ozone concentration (in Dobson units)
variability over the Bering Sea during the CZCS mission period, October 1978- September 1986.
The significant variability of ozone concentration makes the atmospheric
correction process a bit more difficult. Although the absorption of light by
ozone in the visible range peaks at 602 nm, the presence of ozone influences
both Rayleigh scattering and aerosol scattering of light in the atmosphere.
These quantities are calculated in the atmospheric correction process, so if
the value for ozone concentration used in the calculation is in error by a
significant amount, the entire atmospheric correction process will be affected.
In order to perform the best atmospheric correction for ozone, the SeaWiFS
Project uses daily data for ozone concentration which is acquired by the
orbiting TOMS and TOVS instruments. The global ozone map shown at the top of
this page illustrates what this data looks like. Because UV light destroys
ozone, ozone concentrations will be higher near the poles during their
respective winter months. One interesting note: most of the ozone found at
high latitudes isn't formed there; it is actually formed near the Equator and
transferred 'poleward' by high-altitude winds.
In the map above, high ozone concentrations are seen over Canada and
Siberia. Ozone concentrations are lower in the tropics due both to atmospheric
transport (described above) and because of the greater intensity of sunlight
near the equator, which destroys ozone at a higher rate.
TOMS has been deployed on several satellites, such as Nimbus-7 (which also
carried the Coastal Zone Color Scanner), the Upper Atmosphere Research
Satellite (UARS), and the Earth Probe - TOMS (EP-TOMS) satellite, but the most
recent attempt to put another TOMS instrument into orbit failed, and the
EP-TOMS satellite is currently experiencing some problems. Fortunately, the
TOVS instrument is one of the basic payload instruments on NOAA polar-orbiting
environmental satellites, and this data should continue to be available if TOMS
data is not. However, the importance of ozone data to the process of producing
accurate ocean color data highlights the interdependence of many satellite
remote sensing missions.
Acknowledgments: We would like to thank Drs. Jay Herman and
Charles McClain and Ms. Maria Tzortziou for helpful reviews of this
Science Focus! article.
Links:
References:
McClain, C.R., and Yeh, E-N., 1994: "SeaWiFS Ozone Data Analysis Study".
Chapter 2 in Volume 13 of the SeaWiFS Prelaunch Technical Memorandum Series,
Case Studies for SeaWiFS Calibration and Validation, Part 1, NASA
Technical Memorandum 104566, S.B. Hooker and E.R. Firestone, Eds., pages 9-14.
Vasilkov, A., Krotkov, N., Herman, J., McClain, C.R., Arrigo, K., and
Robinson, W., 2001: Global mapping of underwater UV irradiances and DNA-
weighted exposures using Total Ozone Mapping Spectrometer and Sea-viewing Wide
Field-of-view Sensor data products. J. Geophys. Res.,, 106 (C11), 27,205-27,219.
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