Morphology hypothesized to influence aerosol particle deliquescence

The deliquescence phase transitions of populations of internally mixed particles of solid ammonium bisulfate and letovicite, having overall extents of neutralization between 0.60 and 0.75 and diameters between 15 and 60 nm, were studied using a hygroscopic tandem nano-differential mobility analyzer (HTnDMA). Populations having particles sufficiently small or large, as well as particles sufficiently acidic or neutralized, behaved as expected, with prompt transitions of all particles at a single relative humidity. Populations having particles of simultaneously intermediate diameter and acidity, however, behaved differently and unexpectedly. Subpopulations of individual particles did exhibit prompt deliquescence but did so over a broad range of relative humidity (RH) for the entire population. For example, some particles of 20-nm diameter and an extent of neutralization of 0.65 deliquesced promptly at as low as 38% RH, whereas some particles in the same experiment deliquesced promptly at as high as 59% RH. Across this 21% RH span, prompt deliquescence of particle subpopulations was observed, which resulted in the continuous deliquescence of the population as the aerosol progressed from dominantly solid to dominantly aqueous particles for increasing RH. Morphological differences among particles in different subpopulations that drive variable rates of water uptake, such as coatings of letovicite of varying thickness, porosity, and hence water permeability on cores of ammonium bisulfate, are hypothesized to explain these observations.