Cecie Starr, Ralph Taggart, Lisa Starr. Biology: The Unity and Diversity of Life. 9th ed. UnitedStates of America: Thomson Learning, Inc. Brooks/ Cole, 2001.
Air Pollution pg926Pollutants are substances with which ecosystems have had noprior evolutionary experience, in terms of kinds or amounts, so adaptivemechanisms that might deal with them are not in place. From the humanperspective, they are substances that accumulate to levels that have adverseeffects on our health, activities, or survival.Air pollutants are prime examples. As listed in the table,they include carbon dioxide, oxides of nitrogen and sulphur, andchlorofluorocarbons. Also among them are photochemical oxidants formed when thesun’s rays interact with certain chemicals.
The United Sates alone releases700,000 metric tons of air pollutants each day. Whether these remainconcentrated at the source or are dispersed during a given interval depends onthe local climate and topography, as you will now see. Major Classes of Air Pollutants Carbon Oxides Carbon monoxide (CO), Carbon dioxide (CO2) Sulphur Oxides Sulphur dioxide (SO2), Sulphur trioxide (SO3) Nitrogen Oxides Nitric oxide (NO), Nitrogen dioxide (NO2), Nitrous oxide (N2O) Volatile Organic Compounds Methane (CH4), Benzene (C6H6), Chlorofluorocarbons (CFCs) Photochemical Oxidants Ozone (O3), Peroxyacyl nitrates (PANs), Hydrogen peroxide (H2O2) Suspended Particles Solids (dust, soot, asbestos, lead, etc.) and droplets (sulphuric acid, oils, pesticides, etc.) Smog pg 926When weather conditions trap a layer of cool, dense airunder a layer of warm air, this is a thermal invasion. If the trapped air holdspollutants, winds can’t disperse them, and they might accumulate to dangerouslevels. Thermal inversions have been key factors in some of the worst airpollution disasters, because they intensify an atmospheric condition calledsmog.Two types of smog- gray air and brown air- form in majorcities.
Where winters are cold and wet, industrial smog develops as a gray hazeover industrialized cities that burn coal and other fossil fuels formanufacturing, heating, and generating electric power. Burning releasesairborne pollutants, such as dust, smoke, soot, ashes, asbestos, oil, bits oflead and other heavy metals, and sulphur dioxides. If not dispersed by windsand rain, the emissions may reach lethal concentrations. Industrial smog caused4,000 deaths during the1952 air pollution disaster in London. Before coalburning was restricted, New York, Pittsburgh, and Chicago also were gray-aircities.
Today, most industrial smog forms in the cities of China, India, andother developing countries, as well as cities of coal-dependent countries ofeastern Europe.In warm climates, photochemical smog develops as a brownhaze over large cities. It becomes concentrated when the surrounding land formsa natural basin, as it does around Los Angeles and Mexico City. The key culpritis nitric oxide. After it is released from vehicles, it reacts with oxygen inair to form nitrogen dioxide When exposed to sunlight, nitrogen dioxide reactswith hydrocarbons; photochemical oxidants result. Most of the hydrocarbons arefrom spilled or partly burned gasoline. Key oxidants are ozone and PANs(peroxyacyl nitrates). Even traces of PANs sting eyes, irritate luns, anddamage crops.
Acid Deposition pg 926Oxides of sulphur and nitrogen are among the worst airpollutants. Coal-burning power plants, metal smelters, and factories emit mostsulphur dioxides. Motor vehicles, power plants that burn gas and oil, andnitrogen-rich fertilizers produce nitrogen oxides. In dry weather, fineparticles of oxides may briefly stay airborne, then fall to Earth as dry aciddeposition. When dissolved in atmospheric water, they form weak solutions ofsulphuric and nitric acids. Winds may disperse them over great distances. Ifthey fall to Earth in rain and snow, we call this wet acid deposition, or acidrain.
The pH of normal rainwater is about 5. Acid rain can be 10 to 100 timesmore acidic, even as potent as lemon juice! The deposited acids eat away atmarble buildings, metals, rubber, plastic, nylon stockings, and many othermaterials. They significantly disrupt the physiology of organisms, thechemistry of ecosystems, and biodiversity.Depending on their soil type and vegetation cover, someregions are far more sensitive than others to acid deposition. Highly alkalinesoil neutralizes acids before they enter streams and lakes of watersheds. Waterhaving a high carbonate content also neutralizes acids.
However, in manywatersheds of northern Europe and southeastern Canada, and in regionsthroughout the United Sates, thin soils overlie solid granite. These soilscannot buffer much of the acidic inputs.Rain in much of eastern North America is thirty to fortytimes more acidic than it was several decades ago. Crop yields are diminishing.Fish populations have already vanished from more than 200 lakes in New York’sAdirondack Mountains. By some predictions, fish will disappear from 48,000lakes in Ontario, Canada, in the next two decades. Pollution from industrialregions also is changing rainfall acidity enough to contribute to the declineof forest trees and mycorrhizae that support new growth.
Researchers confirmed long ago that emissions from powerplants, factories, and vehicles are the key sources of air pollutants. In 1995,researchers from the Harvard School of Public Health and Brigham YoungUniversity reported this finding from a comprehensive air quality study: Whacka year or so off your life span if you live in cities with the dirtiest air-especially air with fine particles of dust, soot, smoke, or acid droplets.Smaller particles are more easily inhaled and can damage lung tissue.
At one time the world’s tallest smokestack, in the Canadianprovince of Ontario, accounted for 1 percent by weight of the annual worldwideemissions of sulphur dioxide. But Canada receives more acid deposition fromindustrialized regions of the midwestern United States than it sends across itssouthern border. Most of the air pollutants in Scandinavian countries, theNetherlands, Austria, and Switzerland arise in industrialized regions ofwestern and eastern Europe. Prevailingwinds- hence air pollutants- do not stop at national boundaries.Pollutants aresubstances with which ecosystems have had no prior evolutionary experience, interms of kinds or amounts, so adaptive mechanisms are not in place to deal withthem.Accumulatedpollutants can harm organisms, as when they reach levels that adversely affecthuman health.
Smog formation andacid deposition are two examples of air pollution in specific regions, althoughprevailing winds often distribute pollutants beyond regional boundaries.Smog forms mainly asa result of fossil fuel burning in urban and industrialized regions. Airborneacidic pollutants drift down as dry particles or as components of acid rain.Ozone Thinning pg928Now think about the ozone layer, almost twice as high abovesea level as the top of Mount Everest, the highest place on Earth. EachSeptember through mid-October, the layer becomes thinner at high latitudes.
Theseasonal ozone thinning is so pronounced, it was once called an “ozone hole”.In 1998, ozone thinning over Antarctica was the most extensive ever recorded;it covered an area larger than North America. In both 1997 and 1999, theseasonal loss at high northern latitudes was 60 percent- the lowest everrecorded.
Why is ozone reduction alarming? It lets more ultravioletradiation reach the Earth, which is triggering far more skin cancers,cataracts, and weakened immune systems. Ultraviolet radiation also affectsphotosynthesis. Substantial declines in the oxygen-releasing activity ofphytoplankton alone could alter the composition of the atmosphere, given theirstaggering numbers.Chlorofluorocarbons (CFCs) are the major factors in theozone reduction. These compounds of chlorine, fluorine, and carbon are odourlessand invisible. You find them in refrigerators (as the coolants), solvents, andplastic foams. CFCs slowly escape into air and resist breakdown. When a freeCFC molecule absorbs ultraviolet light, it gives up one chlorine atom.
Ifchlorine reacts with ozone, this yields oxygen and chlorine monoxide- which canreact with free oxygen to release another chlorine atom. Each released chlorineatom can convert 10,000 ozone molecules or more to oxygen!The chlorine monoxide levels above Antarctica are 100 to 500times higher than at midlatitudes. Why? During the winter, high-altitude iceclouds form there.
Winds rotate around the South Pole for most of the winter,like a moving fence that keeps the ice clouds from spreading to otherlatitudes. This also happens on a lesser scale in the Arctic. The ice crystalsserve as surfaces on which chlorine compounds can be swiftly degraded. When airwarms in the spring, chlorine is tree to destroy ozone- hence the ozonethinning.
CFCs aren’t the only ozone eaters. Methyl bromide, afungicide, is better at it. It persists only briefly in the atmosphere, but itwill account for about 15 percent of the thinning in future years unlessproduction stops.Some scientists dismiss the threat of the chemicals thatcontain chlorine and bromine.
But the vast majority of those who have studiedecosystem modelling programs conclude that these chemicals so pose long-termthreats- not only to human health and certain crops but also to animal life ingeneral. Substitutes are now available for most applications of CFCs, andothers are being developed.Under international agreement, CFC production has beenphased out in the developed countries. By 2010, developing countries will have phasedout CFCs. Methyl bromide production will stop by 2010. By recent computermodels, the area of thinning should not become any larger. Assuminginternational goals are met, it still will be about 50 years before the ozonelayer is restored to 1985 levels and another 100 to 200 years to total recovery,to pre-1950 levels.
Meanwhile, you and all the children and grandchildren offuture generations will be living with the destructive effects of airpollutants. And if levels of greenhouse gases continues to rise, thestratosphere could cool enough to increase the size and duration of seasonalozone depletions over the poles.Air pollution canhave global repercussion, as when CFCs and other compounds contribute to athinning of the ozone layer that shields life from the sun’s ultravioletradiation.