Geoscience Reference
In-Depth Information
North America in the mid-1970s, compared
with the mid-1950s, showed a twofold to
threefold increase in hydrogen ion deposition
and rainfall acidity. Sulfate concentrations in
rainwater in Europe increased over this 20-
year period by 50 percent in southern Europe
and 100 percent in Scandinavia, although there
has been a subsequent decrease, apparently
associated with reduced sulfur emissions
in both Europe and North America. The
emissions from coal and fuel oil in these
regions have high sulfur contents (2-3 percent)
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 mainly deposited locally. SO
2
and
NO
x
have atmospheric resident times of one to
three days. SO
2
is not readily dissolved in cloud
or raindrops unless oxidized by OH or H
2
O
2
,
but dry deposition 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 readily dissolves.
In the western United States where there are
fewer major sources of emission, H+ ion
concentrations in rainwater are only 15-20
percent of levels in the east, while sulfate and
nitrate anion concentrations are one-third to
one-half of those in the east. In China, high-
sulfur coal is the main energy source and
rainwater sulfate 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 contain more
nitrate and less sulfate than rain, apparently
because falling snow scavenges nitrate faster
and more effectively. Consequently, nitrate
accounts for about half of the snowpack
acidity. In spring, snowmelt 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 indicate pH values averaging
3.4 in fog. This is a result of several factors.
Small fog or cloud droplets have a large surface
area, higher levels of pollutants provide more
time for aqueous-phase chemical reactions,
and the pollutants may act as nuclei for fog
droplet condensation. In California, pH values
as low as 2.0-2.5 are not uncommon in coastal
fogs. Fog water in Los Angeles usually has high
nitrate concentrations due to automobile
traffic during the morning rush-hour.
The impact of acid precipitation depends
on the vegetation cover, soil and bedrock
type. Neutralization may occur by addition
of cations in the vegetation canopy or on the
surface. Such buffering is greatest if there are
carbonate rocks (Ca, Mg cations); otherwise
the increased acidity augments normal
leaching of bases from the soil.
4 Aerosols
There are significant quantities of
aerosols
in
the atmosphere. These are suspended particles
of sulfate, sea salt, mineral dust (particularly
silicates), organic matter and black carbon.
Aerosols enter the atmosphere from a variety of
natural and anthropogenic sources (
Table 2.2
).
Some originate as particles that are emitted
directly into the atmosphere - mineral dust
particles from dry surfaces, carbon soot from coal
fires and biomass burning, and volcanic dust.
Figure 2.1B
shows their size distributions. Others
are formed in the atmosphere by gas-to-particle
conversion processes (sulfur from anthropogenic
SO
2
and natural H
2
S; ammonium salts from NH
3
;
nitrogen from NO
x
). Sulfate aerosols, two-thirds
of which come from coal-fired power station
emissions, played an important role in countering
global warming effects by reflecting incoming
solar radiation during the 1960s-1980s, but
that so-called 'global dimming' has subsequently
been reversed ('global brightening') (see Chapter
13). Other aerosol sources are sea salts and
organic matter (plant hydrocarbons and anthro-
