Environmental Engineering Reference
In-Depth Information
needs disulfide bridges in order to make long three-
dimensional polypeptide chains whose complex folds
allow proteins to be engaged in countless biochemical
reactions. Sea spray is the largest (140-180 Mt S) natural
input of the element into the atmosphere, but 90% of
this mass is promptly redeposited in the ocean (fig.
11.10). Some volcanic eruptions are very large sources
of SO
2
, others have S-poor veils. The long-term average
is about 20 Mt S/a, and dust, mainly desert gypsum,
may contribute as much as that. Biogenic sulfur flows
may be as low as 15 Mt S/a and as high as 40 Mt S/a;
they are produced on land by both sulfur-oxidizing and
sulfate-reducing bacteria present in waters, muds, and
hot springs.
Fossil fuel combustion generates more than 90% of all
anthropogenic sulfur (fig. 11.10). Its global emissions
rose from 5 Mt in 1900 to about 80 Mt in 2000, match-
ing or perhaps even surpassing the natural volcanic, dust,
and biogenic flux (Lefohn, Husar, and Husar 1999). The
remainder is emitted largely by the smelters of color met-
als (mainly Cu, Zn, Pb). Emitted SO
2
is rapidly oxidized
to sulfates whose deposition is the leading source of acid
deposition (dry or as rain, snow, and fog). Before 1950
emissions from households and from low industrial chim-
neys caused only local acidification. Tall stacks of post-
1950 coal-fired plants emitted hot flue gases that could
rise into the mid-troposphere and be carried considerable
distances downwind. During their time aloft (up to three
or four days, average
<
40 h) the acidifying gases can
travel 10
2
-10
3
km before being deposited on distant
ecosystems. Because of the short atmospheric residence
time of S compounds, the element does not have a true
global cycle as do carbon and nitrogen.
Deposited sulfates and nitrates acidify aquatic ecosys-
tems and reduce or eliminate sensitive species of fish,
amphibians, gastropods, crustaceans, and invertebrates.
Chronic acidification dissolves aluminum hydroxide, and
Al
3
þ
irritates fish gills and destroys their protective mu-
cus. Acidification also mobilizes abnormally high levels
of all heavy metals. However, the role of acid deposition
in reduced productivity and die-back of some forests is
not as clear (Tomlinson 1990; Godbold and H¨tter-
mann 1994). Acid deposition also accelerates corrosion
of metals and deterioration of mortar, limestone, and
marble (the Parthenon and the Taj Mahal are two nota-
ble examples). But prevailing levels of acid deposition in
North America and Europe have not caused any measur-
able reductions of crop yields. Europe and eastern North
America have successfully reduced SO
2
emissions, but
the Asian flux has been rising. The cooling effect of air-
borne sulfates has been noted; it is most pronounced
over eastern North America, Europe, and East Asia, the
regions with the highest sulfate levels.
The 16-fold increase in global commercial energy
consumption during the twentieth century has been the
most important cause of human interference in the bio-
spheric cycles of doubly mobile elements. By 2000 the
combustion of fossil fuels generated at least 75% of all
CO
2
emissions, 90% of the anthropogenic mobilization
of sulfur, and about 20% of the anthropogenic releases
of reactive nitrogen. Further substantial growth of global
TPES means that we must consider an unprecedented
possibility: that future limits on human energy use may
arise, not from resource shortage but from the necessity
to keep these cycles compatible with the long-term hab-
itability of the biosphere.
