Geoscience Reference
In-Depth Information
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 con-
densation. In California, pH values as low as 2.0 to 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 sea-salt,
mineral dust (particularly silicates), organic matter and
smoke. Aerosols enter the atmosphere from a variety of
natural and anthropogenic sources (Table 2.2). Some
originate as particles - soil grains and mineral dust from
dry surfaces, carbon soot from coal fires and biomass
burning, and volcanic dust. Figure 2.1B shows their size
distributions. Others are converted into particles from
inorganic gases (sulphur from anthropogenic SO
2
and
natural H
2
S; ammonium salts from NH
3
; nitrogen from
NO
x
). Sulphate aerosols, two-thirds of which come
from coal-fired power station emissions, now play an
important role in countering global warming effects by
Table 2.2
Aerosol production estimates, less than 5 µm radius (10
9
kg/year) and typical
concentrations near the surface (µg m
-3
).
Concentration
Production
Remote
Urban
Natural
Primary production
Sea
salt2300
5-10
Mineral particles
900-1500
0.5-5*
Volcanic
20
Forest fires and biological debris
50
Secondary production (gas
→
particle):
Sulphates from H
2
S
70
1-2
Nitrates from NO
x
22
Converted plant hydrocarbons
25
Total natural
3600
Anthropogenic
Primary production:
Mineral particles
0-600
Industrial dust
50
Combustion (black carbon)
10
}100-500
†
(organic carbon)
50
Secondary production (gas
→
particle):
Sulphate from SO
2
140
0.5-1.5
10-20
Nitrates from NO
x
30
0.2
0.5
Biomass combustion
(organics)
20
Total anthropogenic
290-890
Notes
: *10-60 µg m
-3
during dust episodes from the Sahara over the Atlantic.
† Total suspended particles.
10
9
kg = 1 Tg
Sources
: Ramanathan
et al.
(2001), Schimel
et al
. (1996), Bridgman (1990).
