Environmental Problems, Their Causes, and Sustainability 1

tems, most scientists say that it meets the criteria for beingclassified as an air pollutant.In 2003, 12 states sued the U.S. EnvironmentalProtection Agency (EPA) for failure to regulate emissionsof carbon dioxide as allowed under the Clean AirAct. The EPA contends that the Clean Air Act does notgive the agency the power to control gases that cancause global warming. Meanwhile, some states arepassing laws that regulate carbon dioxide emissions.xHOW WOULD YOU VOTE? Should carbon dioxide beclassified as an air pollutant? Cast your vote online athttp://biology.brookscole.com/miller14.20-3 PHOTOCHEMICALAND INDUSTRIAL SMOGWhat Is Photochemical Smog? Brown-AirSmogPhotochemical smog is a mixture of air pollutantsformed by the reaction of nitrogen oxides and volatileorganic hydrocarbons under the influence of sunlight.A photochemical reaction is any chemical reaction activatedby light. Air pollution known as photochemicalsmog is formed when a mix of nitrogen oxides (NOand NO 2 , both called NO x ) and volatile organic hydrocarboncompounds from natural and human sourceschemically react under the influence of UV radiationfrom the sun to produce a mixture of more than 100primary and secondary pollutants (Figure 20-5,p. 440). In greatly simplified terms,VOCs NO x heat sunlightground level ozone (O 3 ) other photochemicaloxidants aldehydes other secondary airpollutantsThe formation of photochemical smog involves acomplex series of chemical reactions. It begins insideautomobile engines and in the boilers of coal-burningpower and industrial plants. At the high temperaturesfound there, nitrogen and oxygen in air react to producecolorless nitric oxide (N 2 O 2 2 NO). Inthe atmosphere, some of the NO is converted to nitrogendioxide (NO 2 ), a yellowish-brown gas with achoking odor (Table 20-2). NO 2 is the cause of thebrownish haze that hangs over many cities during theafternoons of sunny days, explaining why photochemicalsmog sometimes is called brown-air smog.When exposed to ultraviolet radiation from thesun, some of the NO 2 engages in a complex series ofreactions with hydrocarbons (mostly released by vegetation,motor vehicles, and other human activities) thatproduce photochemical smog—a mixture of ozone,nitric acid, aldehydes, peroxyacyl nitrates (PANs), andother secondary pollutants.Collectively, NO 2 , O 3 , and PANs are called photochemicaloxidants because they can react with and oxidizecertain compounds in the atmosphere or insideyour lungs that normally are not oxidized. Mere tracesof these oxidants (especially ozone, Table 20-2) andaldehydes in photochemical smog can irritate the respiratorytract and damage crops and trees.Hotter days lead to higher levels of ozone andother components of smog. As traffic increases on asunny day, smog builds up to peak levels by early afternoon,irritating people’s eyes and respiratory tracts.All modern cities have some photochemical smog,but it is much more common in cities with sunny,warm, dry climates and lots of motor vehicles. Examplesare Los Angeles, Denver, and Salt Lake City in theUnited States; Sydney, Australia; Mexico City; SãoPaulo, Brazil; and Buenos Aires, Argentina.What might happen if 400 million Chinese drivegasoline-fueled cars by 2050 as projected? According toa 1999 article in Geophysical Research Letters, the resultingphotochemical smog could cover the entire westernPacific in ozone, extending to the United States.How Can Trees Contribute to PhotochemicalSmog? Hydrocarbon EmittersSome hydrocarbon-emitting tree species cancontribute to the formation of photochemical smog.Trees have many environmental benefits. They emitoxygen, absorb CO 2 , provide shade (which reduces energyneeded for air conditioning), and help absorb andremove various pollutants from the air.When Ronald Reagan was running for presidentin 1980 he was ridiculed when he implied that we didnot need tougher air pollution laws because trees andplants caused 80% of all air pollution. This was certainlyan exaggeration.But recently scientists have found that somespecies of trees and plants (such as some oak species,sweet gums, poplars, and kudzu) in and around urbanareas play a larger role in smog formation than wasonce thought. They do so by emitting volatile organiccompounds (VOCs, especially hydrocarbons such asisoprene) that are ingredients in the development ofphotochemical smog.Unless forests of such trees are close to urban areas,most of their VOC emissions occur in nonurbanareas and disperse into the atmosphere. Thus they donot make a significant contribution to the formation ofphotochemical smog except in forested areas near urbanareas with large sources of NO x and plenty of sunlight.This is in contrast to cars, which along with refineriesand other sources emit most of their VOCs,NO x , and other pollutants into urban air.http://biology.brookscole.com/miller14439