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in diameter and contains liquid organic material (often
lubricating oil and unburned fuel oil) and sulfuric acid.
The second mode often peaks between 20 and 100 nm
in diameter and contains primarily aggregates of black
carbon spherules coated with lubricating oil, unburned
fuel oil, and sulfuric acid. Small nucleated or emitted
particles increase in size by coagulation (collision and
coalescence of particles) and growth (condensation of
gases onto particles). Only a few gases, such as sulfu-
ric acid, water, and some heavy organic gases, among
others, condense onto particles as they age in urban and
background air. Molecular oxygen and nitrogen, which
comprise the bulk of gas in the air, do not condense.
Growth and coagulation move nucleation mode par-
ticles into the accumulation mode ,where diameters
are 0.1 to 2.5
10 6
300
10 5
250
m 2 cm -3 )
x=a (
µ
10 4
n
200
10 3
m 3 cm -3 )
x=v (
µ
150
10 2
100
10 1
10 0
50
10 -1
0
0.01
0.1
1
10
Particle diameter (D, µm)
Figure 5.2. Number ( n ,particles cm −3 ), area ( a ,
m 2
cm −3 ), and volume ( v ,
m 3 cm −3 )concentration size
distribution of particles at Claremont, California, on
the morning of August 27, 1987. Sixteen model size
bins and four lognormal modes were used to simulate
the distribution (Jacobson, 1997). The distribution
does not include the smallest nucleation mode
particles (those less than 20 nm in diameter).
m(100-2,500 nm). Some of these par-
ticles are removed by rain, but they are too light to fall
out of the air by sedimentation (dropping by their own
weight against the force of drag). The accumulation
mode sometimes consists of two submodes with mean
diameters near 200 nm and 500 to 700 nm (Hering and
Friedlander, 1982; John et al., 1989), corresponding to
newer and aged particles, respectively. The accumula-
tion mode is important for two reasons. First, unlike
larger particles, accumulation mode particles are likely
to affect health by penetrating deep into the lungs. Sec-
ond, accumulation mode particles are close in size to
the peak wavelengths of visible light and, as a result,
affect visibility (Chapter 7). Particles in the nucleation
and accumulation modes together are fine particles.
The coarse mode consists of particles larger than
2.5
noticeable in the area concentration distribution and
invisible in the volume concentration distribution.
5.2. Sources and Compositions
of New Particles
New aerosol particles originate from two sources: emis-
sions and homogeneous nucleation. Emitted particles
are called primary particles .Particles produced by
homogenous nucleation, a gas-to-particle conversion
process, are called secondary particles .Primary parti-
cles may originate from point, area, or mobile sources.
mindiameter. These particles originate from
windblown dust, sea spray, volcanos, plants, fossil fuel
combustion, tire erosion, and other sources. Coarse
mode particles are generally heavy enough to sediment
out rapidly within hours to days. The emission sources
and deposition sinks of fine particles differ from those
of coarse mode particles. Fine particles usually do not
grow by condensation to much larger than 1
5.2.1. Emissions
Aerosol particle emission sources may be natural or
anthropogenic. Natural emission sources include sea
spray, soil dust, volcanos, natural biomass fires, and bio-
logical materials. Major anthropogenic sources include
fugitive dust emissions (dust from road paving, passen-
ger and agricultural vehicles, and building construc-
tion/demolition), fossil fuel and biofuel combustion,
anthropogenic biomass burning, industrial emissions,
and tire erosion.
In the United States, about 4.4 million tonnes of
anthropogenic particles smaller than 2.5
m, indi-
cating that coarse mode particles originate primarily
from emissions.
In general, the nucleation mode has the highest num-
ber concentration of aerosol particles, the accumulation
mode has the highest surface area concentration, and the
coarse mode has the highest volume (or mass) concen-
tration. Figure 5.2 shows a quadramodal distribution,
fitted from data at Claremont, California, for the morn-
ing of August 27, 1987. All four modes (one nucleation
mode, two subaccumulation modes, and one coarse par-
ticle mode) are most noticeable in the number concen-
tration distribution. The nucleation mode is marginally
mindiam-
eter were emitted in 2008 (Figure 3.14). On a global
scale, more than half of all particle emissions are anthro-
pogenic in origin.
Table 5.2 summarizes the natural and anthropogenic
sources of the major components present in aerosol
 
 
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