Page 492 - Environment: The Science Behind the Stories

Addressing Acid Deposition TaBLe 17.1 impacts of acid Deposition

Acid deposition in northeastern forests has . . .
Just as stratospheric ozone depletion crosses political bounda- • Accelerated leaching of base cations (ions such as Ca , Mg ,
2+
2+
ries, so does another atmospheric pollution concern—acid NA , and K , which counteract acid deposition) from soil
+
+
deposition. As with ozone depletion, we are seeing some suc-
cess in addressing this challenge. • Allowed sulfur and nitrogen to accumulate in soil, where ex-
cess N can overfertilize native plants and encourage weeds
• Increased dissolved inorganic aluminum in soil, hindering plant
Fossil fuel pollution spreads acidic uptake of water and nutrients
substances widely • Leached calcium from needles of red spruce, causing trees to
die from wintertime freezing
Acid deposition (or acidic deposition) refers to the deposi- • Increased mortality of sugar maples due to leaching of base
tion of acidic (pp. 45–46) or acid-forming pollutants from the cations from soil and leaves
atmosphere onto Earth’s surface. This can take place either
by precipitation (commonly referred to as acid rain, but also • Acidified 41% of Adirondack, New York, lakes and 15% of
New England lakes
including acid snow, sleet, and hail), by fog, by gases, or by
the deposition of dry particles. Acid deposition is one type • Diminished lakes’ capacity to neutralize further acids
of atmospheric deposition, which refers more broadly to the • Elevated aluminum levels in surface waters
wet or dry deposition on land of a wide variety of pollutants, • Reduced species diversity and abundance of aquatic life,
including mercury, nitrates, organochlorines, and others. affecting entire food webs
Acid deposition originates primarily with the emission Source: Adapted from Driscoll, C.T., et al., 2001. Acid rain revisited.
of sulfur dioxide and nitrogen oxides, largely through fossil Hubbard Brook Research Foundation. © 2001 C.T. Driscoll. Used with
fuel combustion by automobiles, electric utilities, and indus- permission.
trial facilities. Once airborne, these pollutants can react with
water, oxygen, and oxidants to produce compounds of low nutrients such as calcium, magnesium, and potassium ions from
pH (p. 46), primarily sulfuric acid and nitric acid. Suspended the topsoil, altering soil chemistry and harming plants and soil
in the troposphere, droplets of these acids may travel days or organisms. This occurs because hydrogen ions from acid pre-
weeks for hundreds of kilometers before falling in precipi- cipitation take the place of calcium, magnesium, and potassium
tation (Figure 17.28). Depending on climate, 20% to 80% of ions in soil compounds, and these valuable nutrients leach into
all acidic compounds emitted into the atmosphere may fall the subsoil, where they become inaccessible to plant roots.
in precipitation, with the remainder falling as dry deposition. Acid precipitation also “mobilizes” toxic metal ions such as
aluminum, zinc, mercury, and copper by chemically converting
Acid deposition has many impacts them from insoluble forms to soluble forms. Elevated soil concen-
trations of metal ions such as aluminum weaken plants by damag-
Acid deposition has wide-ranging, cumulative detrimental effects ing root tissue, hindering their uptake of water and nutrients. In
on ecosystems and on our infrastructure (TaBLe 17.1). Acids leach some areas, acid fog with a pH of 2.3 (equivalent to vinegar, and

Primary pollutants Secondary pollutants

Water (H O) 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
2
Oxygen (O 2 )
and oxidants
Sulfur dioxide (SO ) Sulfuric acid (H SO )
2
4
2
Nitric oxide (NO) Nitric acid (HNO 3 )

Acid precipitation

Figure 17.28 Acid deposition can have consequences far downwind from its source. Sulfur dioxide
and nitric oxide emitted by industries and utilities are transformed into sulfuric acid and nitric acid through
chemical reactions in the atmosphere. These acidic compounds descend to Earth’s surface in rain, snow, fog,
and dry deposition. 491

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