Page 111 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 3 earth’s Modern atmosphere 75
in 2010. Concern remains over some of the substitute compounds and a robust black market for banned CFCs. in 2007, the protocol instituted an aggressive phasedown of HCFCs, or hydrochlorofluorocarbons, one of the CFC-replacement compounds. if the protocol is fully enforced, scientists estimate that the stratosphere will return to more normal conditions in a century.
For their work, Doctors rowland (who passed away in 2012) and Molina and another colleague, Dr. Paul Crutzen, received the 1995 nobel Prize for Chemistry.
Ozone Losses over the Poles
How do northern Hemisphere CFCs become concentrated over the South Pole? Chlorine freed in the northern Hemisphere midlatitudes concentrates over antarctica through the work of atmospheric winds. Persistent cold tem- peratures during the long, dark winter create a tight atmospheric circulation pattern—the polar vortex—that remains in place for several months. Chemicals and water in the stratosphere freeze
out to form thin, icy polar stratospheric clouds.
Within these clouds, ice particle surfaces allow the chemicals to react, releasing chlorine. The chlorine cannot destroy ozone without the addition of UV light, which arrives with the spring in September (Figure 3.1.2). UV light sets off the reaction that depletes ozone and forms the ozone “hole.” as the polar vortex breaks up and temperatures warm, ozone levels return to normal over the antarctic region.
Over the north Pole, conditions are more changeable, so the hole is smaller, although growing each year. The arctic ozone hole in 2011 was the largest on record (see earthobservatory.nasa.gov/IOTD/view .php?id=49874).
Make a note to check some of the listed internet data sources for periodic updates on the stratosphere. The effects of ozone-depleting substances will be with us for the rest of this century.
TABLE 3.1.1 Significant Events in the History of Ozone Depletion
1960s 1970s
1980s
1990s Since 2000
30 25 20 15 10
5 0
• Experts express concern that human-made chemicals in the atmosphere may affect ozone.
• Scientists hypothesize that synthetic chemicals, primarily chlorofluorocarbons (CFCs), release chlorine atoms that react chemically to break down ozone.
• CFCs and aerosol propellants banned internationally in 1978 (although the ban in the United States is weakened in 1981).
• Satellite measurements confirm a large ozone “hole” above Antarctica from September through november.
• In the Arctic region, where stratospheric conditions and temperatures differ from the antarctic, ozone depletion occurs on a smaller scale.
• Scientific consensus confirms CFCs as the cause of ozone depletion, raising public awareness of the global effects of human activity on the atmosphere.
• •
• •
• •
189 countries sign the 1987 Montreal Protocol to phase out the use of ozone-depleting substances.
International organizations standardise the reporting of UV radiation to the public—the UV Index.
The largest areas of ozone depletion over Antarctica on record occur in September 2006. In 2010, the U.S. Environmental Protection Agency (EPA) bans all produc- tion of CFCs. In 2011, scientists find a significant ozone hole over the Arctic region. In 2012, scientists report that intense summer storms over the United States are increasing atmospheric water vapour in the lower strato- sphere, causing chemical reactions that deplete ozone. Climate change is driving the more frequent occurrence of these storms.
Ozone Hole Area (40°S–90°S)
Area of North America
Area of ozone < 220 (shaded area gives range)
1990–2001 2003 TOMS 2005 OMI 2006 OMI
Jun
Jul
Aug Sep Month
Oct Nov
Dec
Satellite
Southern Hemisphere Ozone 2002–2003
▲Figure 3.1.2 Timing and extent of ozone depletion over Antarc- tica. The area of ozone depletion generally peaks in late September; it reached record size in 2006. naSa’s Total Ozone Mapping Spectrometer (TOMS) instrument began measuring ozone in 1978. Since 2004, the Ozone Monitoring instrument (OMi) on board Aura has tracked ozone de- pletion. The Suomi-NPP satellite launched in 2011 carries the Ozone Map- per Profiler Suite (OMPS) to track ozone recovery. [Data from gSFC/naSa.]
Ozone hole area (million km2)