State-of-the-Art Chamber Facility for Studying Atmospheric Aerosol ...

time for diffusion to the wall of the present chamber is 1
min. However, FEP Teflon film is nonabsorptive to most
hydrocarbons; measured losses of C 6F 6 and m-xylene are
0.001 h -1 (loss rate includes potential leaks).
Acknowledgments
This work was supported by the U.S. Environmental Protection
Agency Center on Airborne Organics and U.S. Environmental
Protection Agency Agreement CR827331-01-0.
FIGURE 10. Illustration of different processes occurring within the
chamber. Pathways include the following: (1) condensation, (2)
gas-phase wall adsorption, (3) gas-deposited particle transport, (4)
particle-wall transport, and (5) coagulation.
the number concentration. Because coagulation is a secondorder
process, the rate of reduction in the number concentration
by coagulation decreases much more rapidly than
does particle loss to the walls.
Table 2 gives characteristic times for chamber processes
important in gas-to-particle conversion. Note that the only
significant change in particle number concentration occurs
through wall deposition processes (except when nucleation
occurs). Therefore, the number of particles in the chamber
can be described by the first-order wall loss rate as N ) N oe -βt .
As the mass transfer rate to suspended particles greatly
exceeds that of those deposited on the wall, the amount of
organic material associated with each particle at the time of
deposition is assumed to be constant. Thus, the total organic
material produced is estimated to be the sum of that still
suspended in the aerosol phase plus that deposited to the
wall. The particle size increases due to condensation of
organic vapors (and any nucleation products). The final size
of the particles less the initial size of the seed particles provides
an estimate of the volume of organic material suspended in
the chamber. Figure 10 illustrates the competing processes.
Adsorption of gas-phase compounds on the wall might
possibly decrease their concentrations since the characteristic
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Received for review November 29, 2000. Revised manuscript
received April 3, 2001. Accepted April 5, 2001.
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