Document QkEnKjvdq1g9Qkwww8EQ1M75v

4 DANIEL-BRACKETT--AIRBORNE, CHARGED PARTICLES 513 gravity settling, the fractional charge loss (fractional decrease in area of the chargedweighted distribution curves) will always be less than unity in an aerosol with other than equal magnitudes of positive and negative charges. Under such conditions, knowing the original ratio of positive to negative charge, we could calculate the (fractional) asymptote of curve 1 of figure 4 for high concentration. Under actual conditions this asymptote should be unity, because, with such large aggregation, gravity will effectively remove the unneutralized charge (this is still regarded as an effect of aggregation rather than gravity, since the final removal by gravity is con ditional on previous aggregation). However, for the middle portion of the curve, Compilation of Data Derived, from, the Charge-Weighted Distribution Curves of Silica, Sodium Chloride and Mixed Aerosols* Charge Ratio ( + Area ). Fractional Charge Position of ( . Amo ) JjOSS ,------*------, ( ~ Area 1 Area 2 ) Concentra Maximum r--------------- AA-. _____ .... Area A Positive ( Area 1 ) % fClhhnarrtgroe 1------ --------- -- --A-- . -- -... Aerosol tion, Mg./M. Row Positive Negative Particles Particles Positive Particles Negative Particles Negative Charge. Positive Negative Particles Particles SIOa......................... 33 1 2 5.0 6.8 7.0 8.2 77.5 46.5 59.0 35.9 1.31 1.30 .40 .39 SiOs........................... 42 1 5.7 7.4 109 63.2 1.72 .38 .40 2 8.0 9.5 68.0 38.1 1.78 810........................... 47 1 7.3 8.4 83.6 67.1 1.25 .40 .40 ( 2 8.6 9.2 49.9 40.6 1.23 810a........................... 52 1 6.2 8.2 106 73.6 1.44 .42 .43 2 8.6 9.8 61.7 41.9 1.47 SiOa............................ 60 1 7.0 9.3 no 75.2 1.46 .43 .48 2 9.3 11.5 62.9 39.2 1.60 NaCl (8% solution). 42 1 11.0 10.0 443 2 30.0 31.5 . 220 366 173 1.21 .50 .53 1.27 NaCl (8% solution). 56 1 12.5 12.5 435 2 33.0 35.0 202 321 155 1.35 .54 .52 1.30 NaCl (2% solution). 14.8 1 4.5 6.0 389 2 30.0 23.0 183 287 124 1.36 .53 .57 1.47 8i0a + NaCl 17.0 1 29.1 2 ... 290 139 242 113 1.20 .52 .53 1.23 SiOa + NaCl 33.2 1 31.4 2 258 211 142 101 1.22 .45 .52 1.41 SiOa + NaCl. 36.8 13.5 Residue (see text)... below detection 1 2 1 2 191 155 1.23 .47 ... ... 101 82.6 . 1.23 .32 .26 ' 76.4 85.4 .90 .65 .80 .70 26.6 26.2 1.01 .47 .69 1 * The figures In row 1 for a given aerosol are values for that aerosol taken direct from the exposure chamber, effec- s tively nine minutes after dispersal in that chamber. Row 2 for the aerosol gives values found after It has passed from ! the exposure chamber through a 20 1. settling chamber at the rate of 1.45 l./min. (effectively 14 minutes in the settling 4 chamber or 23 minutes after dispersal). The charge density p (in electrostatic units per cubic centimeter) of the aerosol = 6.28 X 10' X area. different ratios of positive and negative charge, as well as different sizes, will lead to somewhat different values of ordinate for the same aggregation rate. A second restriction arises in case collisions occur in which particle charge's are neutralized but the particles themselves do riot stick.16 In such cases a fractional charge loss is produced which does not represent aggregation. If a sufficient num ber of such collisions should occur, it might be possible (see below) to detect the change in shape which they would produce in-the aggregation distribution curves. Neither of these restrictions affects the determination of relative settling out due to gravity, or that due to diffusion, since these are extrapolations to zero concentra- 16. Aside from the probability of charge neutralization, collisions in which particles do not stick are usually considered unlikely with particles of this size, whatever their composition. See article by R. Whytlaw-Gray (Disperse Systems in Gases, Tr. Farad. Soc. 32:1042, 1936). r... r.. v! ;* P U ig II