182 
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Discussion 
Dr. C. E. Junge—As I understand you, you 
evaluate the particle concentration using the 
microphotometer by which you measured the to- 
tal scattering? 
Dr. A. Goetz—We determine the surface scat- 
tering microphotometrically each time over an 
area of 6 X 10% cm® progressively along the 
spiral of the deposit pattern. 
Dr. Junge—In other words, you evaluate the 
particle concentration on the screen optically? 
Dr. Goetz—Yes, that is for the artificial aerosols 
T described. 
Dr. Junge—I would like to caution a little bit. 
If the same aerosol is deposited for instance in 
the absence and then in the presence of organic 
vapors, the aerosol droplets may spread on the 
foil surface with different contact angles. This 
will influence the light scattering characteristics 
of the individual particles which in turn would 
influence the analysis. The question of particle 
spreading is a complex one and I wonder how you 
took that into account. 
Dr. Goetz—In the micro-analyzer which I have 
briefly described individual counts can be taken 
over an area of 10-4 cm? along the deposit with 
reference to a reticule in the eye piece. Deposits 
which are dense enough to allow reliable scatter- 
ing determination are too crowded for counting, 
hence it is in general not possible to evaluate the 
same deposit in both ways. For comparison a 
brief and long exposure of the same aerosol are 
taken. This has so far been applied to a few cases 
only for the artificial NaCl aerosols with and 
without organic trace. Hence their general quan- 
titative interpretation in terms of numbers is not 
yet possible. These comparisons indicated that 
the presence of the organic trace definitely 
changed the numerical size distribution as in- 
dicated qualitatively by the scattering, however, 
it may also have affected the specific scattering 
