Page 476 - Environment: The Science Behind the Stories

(a) Satellite image of a hurricane (b) Photograph of a tornado (a) Natural fire in California (b) Mount Saint Helens eruption, 1980 (c) Dust storm blowing dust from Africa
to the Americas

Figure 17.11 Wildfire, volcanoes, and dust storms are three natural sources of air pollution.

fires in recent years (p. 338). In regions like the Los Angeles Atlantic (pp. 242–243). Today, trade winds blow soil across
basin, where residential development has encroached into the Atlantic Ocean from Africa to the Americas (Figure 17.11c),
chaparral ecosystems (p. 338) that are naturally fire-prone, carrying fungal and bacterial spores that harm Caribbean coral
fires may cause extensive and costly damage. In the tropics, reefs, but also bringing nutrients to the Amazon rainforest.
many farmers set fires to clear forest for farming and graz- Strong westerlies sometimes lift soil from deserts in Mongolia
ing using a “slash-and-burn” approach (p. 239). In 1997, a and China and blow it all the way across the Pacific Ocean to
severe drought brought on by the 20th century’s strongest North America.
El Niño event caused unprecedented forest fires in Mexico,
Central America, Indonesia, and Africa. Immense swaths of We create outdoor air pollution
rainforest burned while smoke pollution sickened 20 million
Indonesians and caused a plane to crash and ships to collide. Human activity introduces many sources of air pollution. As
Scientists voice concern that global climate change (Chapter with water pollution, anthropogenic (human-caused) air pol-
18) is increasing fires as a result of drought in many regions. lution can emanate from point sources or non-point sources
Volcanic eruptions (pp. 58–60) release large quanti- (pp. 426–427). A point source describes a specific location
ties of particulate matter, as well as sulfur dioxide and other from which large quantities of pollutants are discharged, such
gases, into the troposphere (Figure 17.11b). In 2012, residents as a coal-fired power plant. Non-point sources are more dif-
of Mexico City went on alert as Popocatepetl, a volcano just fuse, consisting of many small, widely spread sources (such as
70 km (45 mi) from the city, let loose a series of moderate thousands of automobiles).
eruptions. Ash from these eruptions added to the region’s pol- Primary pollutants, such as soot and carbon monox- CHAPTER 17 • AT m os PHER i C sC i E n CE , Ai R Qu A li T y, A nd Poll u T i on Con TR ol
lution challenges. Ash from major volcanic eruptions near ide, are pollutants that can cause harm directly or that react
populated areas can ground airplanes, destroy car engines, and chemically to form harmful substances. Harmful substances
pose respiratory health dangers for millions of people. Major produced when primary pollutants interact or react with con-
eruptions may also blow matter into the stratosphere, where it stituents of the atmosphere are called secondary pollutants.
can circle the globe for months or years. Sulfur dioxide reacts Pollutants differ in the amount of time they spend in
with water and oxygen and then condenses into fine droplets, the atmosphere—called their residence time—because sub-
called aerosols, which reflect sunlight back into space and stances differ in how readily they react in air and in how
thereby cool the atmosphere and surface. The 1991 eruption quickly they settle to the ground. Pollutants with brief resi-
of Mount Pinatubo in the Philippines ejected nearly 20 mil- dence times exert localized impacts over short time periods.
lion tons of ash and aerosols and cooled global temperatures Most particulate matter and most pollutants from automo-
by 0.5°C (0.9°F). bile exhaust stay aloft only hours or days, which is why air
Winds sweeping over arid terrain can send huge amounts quality in a city like Mexico City or Los Angeles can change
of dust aloft. Dust storms occur naturally, but they are made from day to day. In contrast, pollutants with long residence
worse by unsustainable farming and grazing practices that strip times can exert impacts regionally or globally for long peri-
vegetation from the soil, promote wind erosion, and lead to ods, even centuries. The pollutants that drive global climate
desertification (pp. 241–242). Continental-scale dust storms change and those that deplete Earth’s ozone layer (two sepa-
took place in the United States in the 1930s, when soil from rate phenomena!—see FAQ, p. 490) are each able to cause
the drought-plagued Dust Bowl states blew eastward to the these global and long-lasting impacts because they persist in 475

M17_WITH7428_05_SE_C17.indd 475 12/12/14 3:22 PM