Geoscience Reference
In-Depth Information
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
10
9
kg N/year).
Sulphur species
. Reactive species are sulphur
dioxide (SO
2
) and reduced sulphur (H
2
S, DMS).
Atmospheric sulphur is almost entirely anthropogenic
in origin: 90 per cent from coal and oil combustion, and
much of the remainder from copper smelting. The major
sources are sulphur dioxide (80-100
10
9
kg S/year),
hydrogen sulphide (H
2
S) (20-40
and
H
2
O + HSO
3
→ H
+
+ SO
4
2-
+ H
2
O (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 concentra-
tions, atmospheric transport and chemical activity,
cloud type, cloud microphysical processes, and type
of precipitation. Observations in northern Europe and
eastern North America in the mid-1970s, compared with
the mid-1950s, showed a twofold to threefold increase
in hydrogen ion deposition and rainfall acidity. Sulphate
concentrations in rainwater in Europe increased over
this twenty-year period by 50 per cent in southern
Europe and 100 per cent in Scandinavia, although there
has been a subsequent decrease, apparently associated
with reduced sulphur emissions in both Europe and
North America. The emissions from coal and fuel oil in
these regions have high sulphur content (2-3 per cent)
and, since major SO
2
emissions occur from elevated
stacks, SO
2
is readily transported by the low-level
winds. NO
x
emissions, by contrast, are primarily from
automobiles and thus NO
3
- is deposited mainly locally.
SO
2
and NO
x
have atmospheric resident times of one
to three days. SO
2
is not dissolved readily in cloud or
raindrops unless oxidized by OH or H
2
O
2
, but dry depo-
sition is quite rapid. NO is insoluble in water, but it is
oxidized to NO
2
by reaction with ozone, and ultimately
to HNO
3
(nitric acid), which dissolves readily.
In the western United States, where there are fewer
major sources of emission, H
+
ion concentrations in
rainwater are only 15 to 20 per cent of levels in the east,
while sulphate and nitrate anion concentrations are
one-third to one-half of those in the east. In China,
high-sulphur coal is the main energy source and rain-
water sulphate concentrations are high; observations
in southwest China show levels six times those in
New York City. In winter, in Canada, snow has been
found to have more nitrate and less sulphate than rain,
apparently because falling snow scavenges nitrate faster
and more effectively. Consequently, nitrate accounts for
about half of the snowpack acidity. In spring, snow-melt
runoff causes an acid flush that may be harmful to fish
populations in rivers and lakes, especially at the egg or
larval stages.
In areas with frequent fog, or hill cloud, acidity
may be greater than with rainfall; North American data
10
9
g S/year) and
dimethyl sulphide (DMS) (35-55
10
9
kg S/year).
DMS is produced primarily by biological productivity
near the ocean surface. SO
2
emissions increased by
about 50 per cent between 1940 and 1980, but declined
in the 1990s. Volcanic activity releases approximately
10
9
kg S/year as sulphur dioxide. Because the lifetime
of SO
2
and H
2
S in the atmosphere is only about one
day, atmospheric sulphur occurs largely as carbonyl
sulphur (COS), which has a lifetime of about one year.
The conversion of H
2
S gas to sulphur particles is an
important source of atmospheric aerosols.
Despite its short lifetime, sulphur dioxide is readily
transported over long distances. It is removed from the
atmosphere when condensation nuclei of SO
2
are pre-
cipitated as acid rain containing sulphuric acid (H
2
SO
4
).
The acidity of fog deposition can be more serious
because up to 90 per cent of the fog droplets may be
deposited.
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 sulphur
dioxide and nitrogen dioxide. For sulphur dioxide, rapid
pathways are provided by:
≤
HOSO
2
O
2
→ HO
2
+ SO
3
H
2
O + SO
3
→ H
2
SO
4
(gas phase)
