The deliquescence and efflorescence relative humidity values of 6-to 60-nm NaCl particles were measured using a tandem nano-Differential Mobility Analyzer. The deliquescence relative humidity (DRH) increased when the dry particle mobility diameter decreased below approximately 40 nm. The efflorescence relative humidity (ERH) similarly increased. For example, the DRH and ERH of 6-nm particles were 87% and 53%, respectively, compared to 75% and 45% for particles larger than 40 nm. + 44, which are calibrated for 6 < d m < 60 nm with less than 1% RH uncertainty and where d m is the dry particle mobility diameter (nm). Two independent methods were used to generate the aerosol particles, namely by vaporizing and condensing granular sodium chloride and by electrospraying a high-purity sodium chloride aqueous solution, to investigate possible effects of impurities on the results. The DRH and ERH values were the same within experimental uncertainty for the particles generated by the two methods. The physical explanation for the nanosize effect of increasing DRH and ERH for decreasing dry particle mobility diameter is that the free energy balance of NaCl increasingly favors smaller particles (i.e., those without water) because the surface areas and hence surface free energies per particle are less for small, anhydrous particles than for bloated, aqueous particles. [Supplementary materials are available for this article. Go to the publisher's online edition of Aerosol Science and Technology for the following free supplemental resources: Graphs and data of the size distribution measurements of the deliquescenceand the efflorescence-mode experiments of the 6-, 8-, 15-, 20-, 30-, and 60-nm dry mobility diameter particles.]

Ice freezing of aqueous ammonium sulfate particles containing hematite or corundum mineral dust cores is studied by aerosol flow tube infrared spectroscopy (AFT-IR). The cores induce freezing heterogeneously at temperatures warmer than homogeneous nucleation. Heterogeneous nucleation rates (j) vary from 102 to 106 cm-2 s-1, depending on the mode diameter of the hematite or corundum core (50−250 nm), temperature, and aqueous mole fraction composition. The rates are rationalized by the equations of classical heterogeneous nucleation theory, which yields contact angles of 90° for ice/hematite and ice/corundum interfaces and temperature-dependent (215−235 K) surface tensions of ice against aqueous ammonium sulfate solutions. The slope of the temperature dependence is positive for pure water but is progressively negative as the ammonium sulfate content increases. These results quantify a potentially important role for mineral dusts as initiators of cirrus cloud formation by heterogeneous nucleation.

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