Hygroscopic properties of inorganic-salt aerosol with surface-active organic compounds

This paper investigates the hygroscopic characteristics of inorganic-salt aerosols with surface-active organic compounds (SAOCs). The experimental system includes a Tandem Differential Mobility Analyzer and a Scanning Mobility Particle Sizing System. The aerosols were prepared by mixing calcium chloride with SACs at five mass fraction of SAOCs, which are 0, 10, 16.7, 20, 25%. Four SAOCs, sodium dodecyl sulfate (SDS), sodium oleate (SO), Tween 80, and Span 80 were chosen according to the hydrophile-lipophile balance (HLB) values. First, the hygroscopic growth of SAOCs aerosol was observed. Then the hygroscopic growth of calcium chloride with SAOCs were observed from 7.5% to 85% relative humidity in this study. The results show that the maximum growth ratio of four SAOCs in only 7.5% at relative humidity of 85%. Thus, four SAOCs used in this study have low hygroscopic growth. However, the relative hygroscopic growth ratio of inorganic-salt aerosols compared with inorganic-salt aerosols with SAOCs is between 56 to 83% even at SAOCs mass fraction of 25%. The relative hygroscopic growth ratio of inorganic-salt aerosols with SAOCs appears to decrease with increasing SAOCs concentration but to increase with the HLB values of SAOCs. In other words, SAOCs may inhibit the hygroscopic growth ratio but 56 to 83% relative hygroscopic growth ratio can still be observed. Although the relative hygroscopic growth ratio appears to decrease with increasing SAOC concentration, the growth ratio tends to be at stable value as the mass fraction of SAOCs greater than 20%. Moreover, the spreading rate of SAOCs was compared with the surface growth rate of aerosols calculated in this study. The presence of surface organic film can not inhibit the absorption of water vapor by calcium chloride completely should be caused by the high surface growth rate of aerosols in phase transition period. On the contrary, the spreading rates of SAOCs are much larger than surface growth rate of calcium chloride aerosol in the stable growth period, therefore, the absorption of water vapor can be inhibited significantly by the surface organic film. (C) 1999 Elsevier Science Ltd. All rights reserved.