Geoscience Reference
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studies (European Automobile Manufacturers Asso-
ciation (ACEA), 1999; Maricq et al., 1999) indicate
that most fossil fuel BC is emitted in particles smaller
than 100 nm in diameter, but ambient measurements
in Los Angeles, the Grand Canyon, Glen Canyon,
Chicago, Lake Michigan, Vienna, and the North Sea
show that accumulation mode BC often exceeds
emissions mode BC (McMurry and Zhang, 1989;
Hitzenberger and Puxbaum, 1993; Venkataraman and
Friedlander, 1994; Berner et al., 1996; Offenberg and
Baker, 2000). The most likely way that ambient BC
redistributes so dramatically is by coagulation and
growth. Similarly, the measured mean number diam-
eter of biomass-burning smoke less than four minutes
old is 100 to 130 nm (Reid and Hobbs, 1998); yet the
mass of such aerosol particles increases by 20 to 40 per-
cent during aging, with one-third to one-half the growth
occurring within hours after emissions (Reid et al.,
1998).
Transmission electron microscopy (TEM) images
support the theory that soot particles can become coated
once emitted. Katrlnak et al. (1992, 1993) show TEM
images of soot from fossil fuel sources coated with
sulfate or nitrate. Martins et al. (1998) show a TEM
image of a coated biomass-burning soot particle, and
Posfai et al. (1999) show TEM images (Figure 5.16a)
of North Atlantic soot particles coated by ammonium
sulfate. They found that internally mixed soot and sul-
fate appeared to comprise a large fraction of aerosol
particles in the troposphere. Almost all soot particles
found in the North Atlantic contained sulfate. Strawa
et al. (1999) took scanning electron microscopy (SEM)
images of black carbon particles in the Arctic strato-
sphere. One such image is reproduced in Figure 5.17.
The rounded edges of the particle seem to indicate that
the particle is coated. Katrlnak et al. (1993) report that
rounded grains on black carbon aggregates indicate a
coating. As soot aggregates become coated, they com-
press into a more spherical shape (Schnaiter et al., 2003;
Wentzel et al., 2003). As such, particle shapes change
over time intheair.
In sum, whereas emitted soot particles are relatively
distinct, or
externally mixed
from other aerosol par-
ticles, soot particles typically coagulate or grow to
become
internally mixed
with other particle compo-
nents. Although soot becomes internally mixed, it does
not become “well mixed” (diluted) in an internal mix-
ture because soot consists of a solid aggregate of many
graphite spherules. Thus, soot is a distinct component,
generally a core, in a mixed particle.
Figure 5.17.
Scanning electron microscopy image of a
coated soot particle from the Arctic stratosphere.
From Strawa et al. (1999).
5.6. Health Effects of Aerosol Particles
Aerosol particles contain a variety of hazardous inor-
ganic and organic substances. Some hazardous organic
substances include benzene, polychlorinated biphenyls,
and polycyclic aromatic hydrocarbons (PAHs). Haz-
ardous inorganic substances include metals and sul-
fur compounds. Metals cause lung injury, bronchiocon-
striction, and increased incidence of infection (Ghio and
Samet, 1999). Particles smaller than 10
mindiameter
(
PM
10
)have been correlated with asthma and chronic
obstructive pulmonary disease (MacNee and Donald-
son, 1999).
With respect to outdoor air, some studies found that
there may be
no low threshold for PM
10
-related
health problems
(Pope et al., 1995). Because most
mass of PM
10
is not hazardous, damage from PM
10
may be due primarily to small particles, particularly
ultrafine particles
,which are particles smaller than
100 nm in diameter. Such particles may be toxic to the
lungs, even when the particles contain components that
are not toxic when present in larger particles (MacNee
and Donaldson, 1999).
Studies in the 1970s found a link between cardiopul-
monary disease and high concentrations of aerosol par-
ticles and sulfur oxides. Subsequent studies found a
link between low concentrations of aerosol particles and
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