Environmental Engineering Reference
In-Depth Information
10
5
2
10
5
0
1000
500
0
60
40
20
0
0.01
0.10
1.00
10.00
Diameter (µm)
FIGURE 6.8
Three descriptions of the size distribution of “typical” urban airborne particles. The top panel
shows
number
size distribution; the middle shows
surface area
size distribution, and the lower panel shows
volume
size distribution. The
mass
size distribution has the same shape as the volume distribution. (From
Seinfeld, J.H. and Pandis, S.N.,
Atmospheric Chemistry and Physics
, 2nd edn., Wiley-Interscience, New York,
1232 p., 2006. With permission.)
Sampling applications
: Particle number size distributions are measured with electrical mobility
analyzers, condensation nuclei counters, differential mobility analyzers, electron microscopes, and
optical counters.
6.3.1.2 Surface Area
The middle section of Figure 6.8 shows the
surface area size distribution
for the typical urban aerosol
example. The peak in the surface area distribution occurs at larger particle size, within the accumu-
lation mode. Interactions between particles and luids take place at particle surfaces. Adsorption,
absorption, condensation, and reactions all occur at surfaces or in liquid ilms on particle surfaces.
The size dependence of concentrations of aerosol constituents can be used to infer mechanisms of
aerosol processing. For example, if the size dependence of concentrations of semi-volatile compounds
such as polycyclic aromatic hydrocarbons or photochemical reaction products matches the size depen-
dence of surface area, rather than particle number or mass, sorption or condensation is responsible.
Sampling surface area
: The epiphaniometer is an example of an aerosol instrument that mea-
sures particle surface area size distributions.
6.3.1.3 Volume and Mass
The lower section of Figure 6.8 shows the
mass size distribution
of the same urban aerosol example.
There are two peaks, one in the accumulation mode and one in the coarse mode. This is a bimodal
distribution. The minimum in mass concentration around 2 μm is usually quite pronounced in
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