Environmental Engineering Reference
In-Depth Information
TABLE 8.1
Number Concentration of Unit Density Monodisperse
Particles at a Mass Concentration of 15
μ
g m
-3
Particle Diameter (
μ
m)
Number Concentration (Number cm
-3
)
0.01
28,600,000
0.05
229,000
0.1
28,600
0.15
8,490
0.2
3,580
0.5
229
1
28.6
1.5
8.5
2.5
1.8
600
500
400
300
200
100
25
0
20
15
100
10
5
10
0
FIGURE 8.2
Particle size spectra collected over the course of a 24 h period at a rural location in Tuxedo, NY.
for the duration of the measurements. The particle size range was from 7 to 300 nm in 32 size bins.
Each clearly demonstrates both the temporal variability and the dominance of UFPs when ambient
particles are counted.
8.2 FORMATION
UFPs are formed by condensation reactions of precursor atmospheric gases and by nucleation of
gas-phase species. The boundaries of the ultraine region, as previously noted, are not exact, but
the upper boundary of the nuclei mode is between 50 and 100 nm, whereas the lower boundary of
the accumulation mode is roughly 100-200 nm. Nuclei mode particles are formed by nucleation,
condensation, and coagulation. The nuclei mode is sometimes further split into the nucleation
(diameter 10 nm) and Aitkin modes to indicate that only the smallest particles are formed directly
by nucleation. The particles are created when gas-phase species form condensed-phase species with
very low equilibrium vapor pressure. Those UFPs that are in the lower size range of the accumulation
mode are primarily formed by condensation and coagulation, and also from evaporation of fog and
cloud droplets in which dissolved gases have reacted [2,4].
Search WWH ::
Custom Search