Acid Deposition - Air Pollution and Global Warming: History, Science, and Solutions - page 228

When SO 2 (g) dissolves in a drop to form H 2 SO 3 (aq),
the H 2 SO 3 (aq) reacts to form SO 4 2 − ,forcing more
SO 2 (g) to be drawn into the drop to replace the lost
H 2 SO 3 (aq). The more SO 2 (g) that dissolves and reacts,
the more SO 4 2 − that forms. In cloud drops, dissolu-
tion and aqueous reaction can convert 60 percent of
SO 2 (g) molecules to SO 4 2 − molecules within 20 min-
utes (Liang and Jacobson, 1999).

varieties of fish (including trout and salmon), inver-
tebrates, and microorganisms. Most aquatic insects,
algae, and plankton live only above pH levels of 5. The
reduction in lake pH below 5 kills off these organisms
and fish eggs, causing starvation at higher levels of the
food chain. Low pHs (less than 5.5) in lakes have also
been associated with reproductive failures and muta-
tions in fish and amphibians.
Lake acidification has particularly been a problem in
Scandinavian countries. Most damage occurred during
the 1950s and 1960s, when the average pH of Swedish
lakes fell by 1 pH unit. By the end of the 1970s, about
25,000 of Sweden's 90,000 lakes were so acidified that
only acid-resistant plants and animals could survive. Of
the acidified lakes, about 8,000 were naturally acidic,
suggesting that 17,000 had been acidified anthropogeni-
cally. Today, many lakes in Sweden and in other coun-
tries have been restored.
Figure 10.4 shows rainwater acidity in the United
States in 1994 and 2009. In both years, most acid
rain occurred in Ohio, West Virginia, Pennsylvania,
New York, Indiana, Michigan, Maryland, and parts
of Florida. However, minimum and average rainwa-
ter acidity decreased (pH increased) markedly between
1994 and 2009 due to reductions in sulfur emissions
during this period (Section 10.7.2).
The effects of acid rain on lakes are most pronounced
after the first snowmelt of a season. Because acids accu-
mulate in snow, runoff from melted snow can send a
shock wave of acid to a lake. In some cases, the acid-
ity of meltwater is ten times greater than is that of the
original snowfall.

10.4. Nitric Acid Deposition
Nitric acid deposition occurs in and downwind of urban
areas and is enhanced by the presence of clouds or fog.
The origin of nitric acid is usually nitric oxide, emit-
ted from vehicles and power plants. In the air, NO(g)
is oxidized to NO 2 (g), some of which is also directly
emitted. NO 2 (g) is oxidized to nitric acid by

M
→ HNO 3 (g)
Nitric
acid

OH(g)
Hydroxyl
radical

NO 2 (g)
Nitrogen
dioxide

+ (10.14)

Gas-phase nitric acid dissolves into aerosol particles or
fog drops to form HNO 3 (aq), which dissociates to a pro-
ton [H + ] and the nitrate ion [NO 3 − ]byReaction 10.7.
Thus, the addition of nitric acid to cloud water decreases
the pH and increases the acidity of the water. Gas-phase
nitric acid also deposits directly to the ground, where it
can cause environmental damage.

10.5. Effects of Acid Deposition
The most severe pollution episode in the twentieth cen-
tury involving sulfuric acid-containing fog was proba-
bly that in London in 1952, discussed in Section 4.1.6.1.
During that episode, coal burning combined with a
heavy fog resulted in more than 4,000 excess deaths.
Although other pollutants were also responsible, the
acidified fog contributed to the disaster.
Acid deposition affects lakes, rivers, forests, agri-
culture, and building materials. The regions of the
world that have been affected most by acid deposi-
tion include provinces of eastern Canada, the northeast-
ern United States (particularly the Adirondack Moun-
tain region), southern Scandinavia, middle and eastern
Europe, India, South Korea, Russia, China, Japan, Thai-
land, and South Africa.

10.5.2. Effects on Biomass
Acids damage plant and tree leaves and roots. When sul-
furic acid deposits onto a leaf or needle, it forms a liquid
film of low pH that erodes the cuticle wax, leading to
the drying out (desiccation) of, and injury to, the leaf or
needle. When acid gases, aerosol particles, or raindrops
enter forest groundwater, they damage plants at their
roots in two ways. First, sulfuric and nitric acid solutions
dissolve and carry away important mineral nutrients,
including calcium, magnesium, potassium, and sodium.
Second, in acidic solutions, hydrogen ions [H + ]react
with aluminum- and iron-containing minerals, such as
aluminum hydroxide [Al(OH) 3 (s)] and iron hydrox-
ide [Fe(OH) 3 (s)], releasing Al 3 + and Fe 3 + ,respectively.
At high enough concentrations, these metal ions are
toxic to root systems (Tomlinson, 1983).

10.5.1. Effects on Lakes and Streams
Acids reduce the pH of lakes and streams. Because
fish and microorganisms can survive only in particular
pH ranges, the change in a lake's pH kills off many