Environmental Problems, Their Causes, and Sustainability 1

Ultraviolet light hits a chlorofluorocarbon(CFC) molecule, such as CFCl 3 , breakingoff a chlorine atom and leaving CFCl 2 .Cl ClCClFClSunUV radiationOnce free, the chlorine atom is offto attack another ozone moleculeand begin the cycle again.ClOOFigure 21-21 Natural capitaldegradation: simplified summaryof how chlorofluorocarbons (CFCs)and other chlorine-containing compoundscan destroy ozone in thestratosphere faster than it is formed.Note that chlorine atoms are continuouslyregenerated as they react withozone. Thus they act as catalysts,chemicals that speed up chemicalreactions without being used up bythe reaction. Bromine atoms releasedfrom bromine-containing compoundsthat reach the stratosphere also destroyozone by a similar mechanism.The chlorine atom attacksan ozone (O 3 ) molecule, pullingan oxygen atom off itand leaving an oxygen Omolecule (O 2 ).O OSummary of ReactionsCCl 3 F + UV Cl + CCl 2 FCl + O 3 ClO + O 2 RepeatedCl + O Cl + O 2 many timesClA free oxygen atom pullsthe oxygen atom offthe chlorine monoxidemolecule to form O 2.ClThe chlorine atom andthe oxygen atom join toform a chlorine monoxidemolecule (ClO).What Other Chemicals Deplete StratosphericOzone? More CulpritsA number of chemicals can end up in the stratosphereand deplete ozone there for up to several hundredyears.CFCs are not the only ozone-depleting compounds(ODCs). Others are halons and hydrobromoflurocarbons(HBFCs) (used in fire extinguishers), methyl bromide(a widely used fumigant), hydrogen chloride (emittedinto the stratosphere by space shuttles), and cleaningsolvents such as carbon tetrachloride, methyl chloroform,n-propyl bromide, and hexachlorobutadiene.The oceans and occasional volcanic eruptions alsorelease chlorine compounds into the troposphere. Butmost of these do not make it to the stratosphere becausethey dissolve easily in water and wash out of thetroposphere in rain. Bromine compounds may be lesslikely to wash out of the troposphere, but further studyis needed to confirm this possibility. Measurementsand models indicate that 75–85% of the observedozone losses in the stratosphere since 1976 are the resultof ozone-depleting chemicals released into the atmosphereby human activities beginning in the 1950s.What Happens to Ozone Levels over theEarth’s Poles Each Year? It Drops EachWinter and Spring.During four months of each year up to halfof the ozone in the stratosphere over Antarcticais depleted.OOOClIn 1984, researchers analyzingsatellite data discoveredOthat 40–50% of the ozone inOthe upper stratosphere overAntarctica disappeared duringthe Antarctic late winterand spring (August–November),especially since 1976 (Figure 21-22).Figure 21-23 (p. 486) shows the seasonal variationof ozone with altitude over Antarctica during2003. The observed loss of ozone above Antarctica oftenis called an ozone hole. A more accurate term isozone thinning because the ozone depletion varies withaltitude and location.The total area of the stratosphere above Antarcticathat suffers from ozone thinning during the peak seasonvaries from year to year and in some recent yearshas covered an area greater than that of North AmericaIn 2003, the area of thinning was the second largestsize ever.Measurements indicate that CFCs and other ODCsare the primary culprits. Each winter, steady windsblow in a circular pattern over the earth’s poles. Thiscreates a polar vortex: a huge swirling mass of very coldair that is isolated from the rest of the atmosphere untilthe sun returns a few months later.Total ozone (Dobson units)400350300250200150October monthly means1001955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005YearFigure 21-22 Mean total level of ozone for October over theHalley Bay measuring station in Antarctica, 1956–2003. (Datafrom British Antarctic Survey and World MeteorologicalOrganization)http://biology.brookscole.com/miller14485