Environmental Engineering Reference
In-Depth Information
15
N
n
=7.7× 10
4
DG
N
n
=0.013
σ
gn
=1.7
10
5
N
a
=1.3× 10
4
DG
N
a
=0.069
σ
ga
=2.03
(a)
0
S
a
= 535
DG
S
a
=0.19
600
400
S
n
= 74
DG
S
n
=0.023
S
c
= 41
DG
S
c
= 3.1
σ
gc
= 2.15
200
0
(b)
0.001
0.01
0.1
Particle diameter (µm)
1.0
10
100
FIGURE 8.1
Frequency distribution of particles averaged over 1000 measured ambient particle size distribu-
tion in the United States: (a) by number and (b) by surface area. DGN and DGS, geometric mean diameter by
number and surface, respectively;
D
p
, geometric diameter. (From U.S. EPA, Air quality for particulate matter,
Vols. I, II, III, EPA/600/P-95/001aF, EPA600/P-95/001bF, EPA/600/P-95/001cF, 1996.)
The grand average concentration of over 1000 particle size distributions of ambient particles
measured in the United States [6] is shown in Figure 8.1. The size distributions are shown as a
number or surface concentration. It is clear that UFPs dominate the atmosphere when particles
are counted and provide a dominant fraction of particle surface. The number in the nuclei mode is
clearly an order of magnitude greater than the number in the larger size ranges. Taking an upper
boundary diameter at 150 nm, almost all observed particles are in this ultraine size range.
In the 1997 revision of the U.S. National Ambient Air Quality Standards, the U.S. EPA set
15 μg m
3
as an average annual mass concentration that should not be exceeded for particles with
diameters less than 2.5 μm in aerodynamic diameter.
The annual average standard was retained in their 2006 revision, although the 24 h average limit
was reduced from 65 to 35 μg m
3
.The number of airborne unit density particles per cm
3
of a speciic
diameter that would result in this mass concentration is shown in Table 8.1. The number of 0.1 μm
particles would be 3 orders of magnitude greater than the number of 1 μm diameter particles.
Very detailed data have become available on the ambient number concentration segregated by
particle size. This resulted from the development of the scanning mobility particle size analyzer
(SMPS). The data are acquired into narrow size classes by automatically selecting particles in a
speciic size range that penetrate a differential mobility analyzer (DMA) and counting them with a
condensation nucleus counter (CNC). Automation of this combination of instruments has permitted
semi-continuous acquisition of particle size spectra.
A few examples of such data are shown in Figures 8.1 through 8.3. These size distribution mea-
surements were made with an SMPS system (TSI, St. Paul, MN) averaging three scans every 30 min
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