Environmental Problems, Their Causes, and Sustainability 1

20-4 REGIONAL OUTDOORAIR POLLUTION FROM ACIDDEPOSITIONWhat Is Acid Deposition, and Where Does ItOccur? Acids Falling on Your HeadSulfur dioxide, nitrogen oxides, and particulatescan react in the atmosphere to produce acidicchemicals that can travel long distances beforereturning to the earth’s surface.Most coal-burning power plants, ore smelters, andother industrial plants in developed countries use tallsmokestacks to emit sulfur dioxide, suspended particles,and nitrogen oxides high into the tropospherewhere wind can mix, dilute, and disperse them.These tall smokestacks reduce local air pollution,but they can increase regional air pollution downwind.This occurs because the primary pollutants, sulfurdioxide and nitrogen oxides, emitted into the atmosphereabove the inversion layer are transported asmuch as 1,000 kilometers (600 miles) by prevailingwinds. During their trip, they form secondary pollutantssuch as nitric acid vapor, droplets of sulfuric acid,and particles of acid-forming sulfate and nitrate salts.These acidic substances remain in the atmospherefor 2–14 days, depending mostly on prevailing winds,precipitation, and other weather patterns. During thisperiod they descend to the earth’s surface in two forms.One is wet deposition as acidic rain, snow, fog, and cloudvapor with a pH less than 5.6 (Figure 3-6, p. 41). Theother is dry deposition as acidic particles. The resultingmixture is called acid deposition (Figure 20-8), sometimestermed acid rain. Most dry deposition occurswithin about 2–3 days fairly near the emission sources,whereas most wet deposition takes place within 4–14days in more distant downwind areas.Acid deposition is a regional air pollution problemin most parts of the world that are downwindfrom coal-burning facilities and from urban areas withlarge numbers of cars. Such areas include the easternUnited States (Figure 20-9) and other parts of theworld (Figure 20-10, p. 446). Look at the maps to see ifyou live in a major acid deposition area.In the United States, coal-burning power and industrialplants in the Ohio Valley emit the largestquantities of sulfur dioxide and other pollutants thatcan cause acid deposition. Mostly as a result of theseemissions along with those by other industries andmotor vehicles in urban areas, typical precipitation inthe eastern United States has a pH of 4.4–4.8 (Figure20-9). This is about 10 or more times the acidity ofnatural precipitation, which has a pH of 5.6. Somemountaintop forests in the eastern United States andeast of Los Angeles, California, are bathed in fog anddews as acidic as lemon juice, with a pH of 2.3—about1,000 times the acidity of normal precipitation.In some areas, soils contain basic compounds suchas calcium carbonate (CaCO 3 )orlimestone that can reactwith and neutralize, or buffer, some inputs of acids.WindTransformation tosulfuric acid (H 2 SO 4 )and nitric acid (HNO 3 )Nitric oxide (NO)Sulfur dioxide(SO 2 ) and NOWindborne ammonia gasand particles of cultivated soilpartially neutralize acids andform dry sulfate and nitrate saltsDry acid deposition(sulfur dioxide gasand particles ofsulfate and nitrate salts)Wet acid depostion(droplets of H 2 SO 4 andHNO 3 dissolved in rainand snow)OceanAcid fogFarmLakes in deepsoil high in limestoneare bufferedLakes in shallow soillow in limestonebecome acidicFigure 20-8 Natural capital degradation: acid deposition, which consists of rain, snow, dust, or gas with a pHlower than 5.6, is commonly called acid rain. Soils and lakes vary in their ability to buffer or remove excess acidity.444 CHAPTER 20 Air Pollution