Geoscience Reference
In-Depth Information
sulfur, nitrogen and chlorine. These play key roles
in acid precipitation and in ozone destruction.
Sources of these species are as follows:
which has a lifetime of about one year. The
conversion of H
2
S gas to sulfur particles is an
important source of atmospheric aerosols.
Despite its short lifetime, sulfur dioxide is
readily transported over long distances. It is
removed from the atmosphere when conden-
sation nuclei of SO
2
are precipitated as acid
rain containing sulfuric acid (H
2
SO
4
). The
acidity of fog deposition can be more serious
because up to 90 percent of the fog droplets
may be deposited.
•
Nitrogen species
. The reactive species of
nitrogen are nitric oxide (NO) and nitrogen
dioxide (NO
2
). NO
x
refers to these and other
odd nitrogen species with oxygen. Their
primary significance is as a catalyst for
tropospheric ozone formation. Fossil fuel
combustion (approximately 40 percent for
transportation and 60 percent for other energy
uses) is the primary source of NO
x
(mainly
NO) accounting for ~25 × 10
9
kg N/year.
Biomass burning and lightning activity are
other important sources. NO
x
emissions
increased by some 200 percent between 1940
and 1980. The total source of NO
x
is about
40 × 10
9
kg N/year. About 25 percent of this
enters the stratosphere, where it undergoes
photochemical dissociation. It is also removed
as nitric acid (HNO
3
) in snowfall. Odd
nitrogen is also released as NH
x
by ammonia
oxidation in fertilizers and by domestic
animals (6-10
•
Acid deposition
includes both acid rain and
snow (wet deposition) and dry deposition of
particulates. Acidity of precipitation represents
an excess of positive hydrogen ions [H
+
] in a
water solution. Acidity is measured on the pH
scale (1 - log[H
+
]) ranging from 1 (most acid)
to 14 (most alkaline), 7 is neutral (i.e., the
hydrogen cations are balanced by anions of
sulphate, nitrate and chloride). Peak pH
readings in the eastern United States and
Europe are
4.3.
Over the oceans, the main anions are Cl-
and SO
4
2
- from sea salt. The background level
of acidity in rainfall is about pH 4.8 to 5.6,
because atmospheric CO
2
reacts with water to
form carbonic acid. Acid solutions in rainwater
are enhanced by reactions involving both gas-
phase and aqueous-phase chemistry with
sulfur dioxide and nitrogen dioxide. For sulfur
dioxide, rapid pathways are provided by:
HOSO
2
+ O
2
→
×
10
9
kg N/year).
•
Sulfur species
. Reactive species are sulfur
dioxide (SO
2
) and reduced sulfur (H
2
S, DMS).
Atmospheric sulfur is almost entirely anthro-
pogenic in origin: 90 percent from coal and oil
combustion, and much of the remainder from
copper smelting. The major sources are sulfur
dioxide (80-100 × 10
9
kg S/year), hydrogen
sulfide (H
2
S) (20-40
HO
2
+ SO
3
H
2
O + SO
3
→ H
2
SO
4
(gas phase)
10
9
g S/year) and
dimethyl sulfide (DMS) (35-55 × 10
9
kg
S/year). DMS is primarily produced by
biological productivity near the ocean surface.
SO
2
emissions increased by about 50 percent
between 1940 and 1980, but declined in the
1990s. China is the largest source of emissions
although the USA makes the largest per
capita contribution. Volcanic activity releases
approximately 10
9
kg S/year as sulfur dioxide.
Because the lifetime of SO
2
and H
2
S in the
atmosphere is only about one day, atmospheric
sulfur occurs largely as carbonyl sulfur (COS),
×
H+ + SO
4
2
- + H
2
O
and
H
2
O + HSO
3
→
(aqueous phase)
The OH radical is an important catalyst in gas-
phase reaction and hydrogen peroxide (H
2
O
2
)
in the aqueous phase.
Acid deposition depends on emission
concentrations, atmospheric transport and
chemical activity, cloud type, cloud micro-
physical processes, and type of precipitation.
Observations in northern Europe and eastern
