_ ROTATING-DISK-SAMPLER MEASUREMENT OF AEROSOLS
227
where k,, the deposition velocity, is defined by the preceding expres-
sion and n(r) dr is the ambient concentration of particles in the range
r to r-+ dr. The deposition flux may be expressed in terms of the cumulative distribution of particles measured on the electron micro-
graphs, ZX(r):
dJ =—
d Z(r)
Ar
(5)
where A is the area examined on the grid and is the total sampling
wo --[S22(ch)
time of the rotating disk. Hence, from Eqs. 4b and 5,
_
td Zr)
1
where n(r) is the time-averaged distribution defined by Eq. 3.
°
The particle diffusion coefficient, D, can be estimated from the
Stokes — Einstein expression
_kT
(7)
Dey
where k is Boltzmann’s constant and T is the absolute temperature,
The coefficient of frictional resistance, f, can be calculated (assuming
a Spherical particle) by the expression
_ 6rur
where pis the viscosity of the medium (air), The semiempirical cor-
rection factor recommended by Davies® is
C=1+2 (1.257 + 0.400078 9")
(9)
when the mean free path of the air, A, is given by
nM\%
A= 0.708v 24)
(10)
where M is the molecular weight, v is the kinematic viscosity, and R is
the gas constant,
.
Since each sample grid was examined at two magnifications, there
are two 2(r) curves for each run, one covering the size range below
about 0.02 u and the other the range 0,015 to 0.08 u. In principle, the
two sets of X(r) data can be combined to form one Set by the application
of a correction factor involving the areas examined at each magnification. This method was rejected since the uncertainties of low particle
count around 0.015 u for the smaller particle set and inaccurate sizing
around 0.015 » for the larger particle set would magnify the error in
the
overlap
region.
For these reasons, it was decided to seek an