Geoscience Reference
In-Depth Information
black. Black carbon is the main component of diesel
soot particles, which also contain organic carbon, some
sulfate, and trace metals. Diesel soot particles appear
black because they are dominated by black carbon.
Black carbon also appears in soot particles from bio-
fuel and biomass burning. Such particles contain less
black carbon and more organic carbon, and so generally
appear brown, yellow, or gray.
All organic aerosol particle constituents absorb short
UV wavelengths, but only some absorb long UV wave-
lengths and short visible (blue and some green) wave-
lengths. The strongest long UV and short visible
wavelength-absorbing organics include certain
nitra-
ted aromatics
,
polycyclic aromatic hydrocarbons
(PAHs), benzaldehydes, benzoic acids, aromatic poly-
carboxylic acids, and phenols (Jacobson, 1999b). Sev-
eral of these compounds may be present in
tar balls
(Figure 7.9), which are amorphous, spherical aerosol
particles 30 to 500 nm in diameter that form downwind
of biomass or biofuel burning emissions and absorb UV
and short visible light wavelengths (P osfai et al., 2004).
Tar balls comprise about 15 percent of biomass burn-
ing particles by number 1.6 km from a fire. Although
individually they are weaker absorbers than black car-
bon aggregates (Figure 7.10), their size, relative abun-
dance, and strong absorption cause them to heat biomass
burning plumes only slightly less than do black carbon
emissions in the plumes (Adachi and Buseck, 2011).
Together, all visible-absorbing organics appear brown
Black carbon
1
Tar balls
4-Nitrophenol anion
0.1
Nitrobenzene
2-Nitrophenol
4-Nitrophenol
0.01
0.3
0.4
0.5
0.6
0.7
0.8
Wavelength (
ยต
m)
Figure 7.10.
Imaginary indices of refraction of black
carbon, nitrated aromatics, and tar balls versus
wavelength. Black carbon data are from Krekov
(1993) and references therein; nitrated aromatic data
are from Jacobson (1999b) and references therein; tar
balls data are from Alexander et al. (2008).
or yellow and are referred to as
brown carbon
.Biofuel
and biomass burning particles often contain both black
carbon and brown carbon, although others, such as tar
balls, contain only brown carbon.
Another important aerosol particle type that absorbs
solar radiation is soil dust. Soil dust particles contain
different proportions of Fe
2
O
3
(s), Al
2
O
3
(s), SiO
2
(s),
CaCO
3
(s), MgCO
3
(s), clays, and other substances.
Such particles often appear brown or red because they
contain hematite. Hematite is also found in industrial
particles.
Hematite
[Fe
2
O
3
(s)] strongly absorbs blue,
moderately absorbs green, and weakly absorbs red
wavelengths. Because it reflects red and some green,
it appears red or reddish-brown in high concentrations.
Because of hematite primarily, soil dust particle absorp-
tion increases from the visible to the UV spectra (e.g.,
Gillette et al., 1993; Sokolik et al., 1993).
Soil dust particles also contain
aluminum oxide
[Al
2
O
3
(s), alumina], which absorbs moderately to
weakly across the whole visible spectrum and appears
silvery white in pure form. Aluminum oxide is also
found in combustion particles.
Silicon dioxide
[SiO
2
(s), silica], which is a crys-
talline compound found in quartz, sand, and other min-
erals present in soil dust, is a weak absorber of visible
and UV radiation. Because it appears white or color-
less, it does not contribute much to the color of soil dust
particles.
Most other components in soil dust and other parti-
cles are weak absorbers of visible and UV radiation. For
Figure 7.9.
Transmission electron microscope image
of tar balls on fibers of lacey carbon substrate,
collected on March 22, 2006, in Mexico. From Adachi
and Buseck (2011).
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