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ABSTRACT
The uptake of vapor molecules by nanometer scale aerosol particles (clusters) is of fundamental importance in aerosol science; uptake is the first step of condensational growth in both the ambient as well as in condensation based particle detectors. However, uptake is not well understood at the nanometer scale. We examined the uptake of organic vapor molecules by nanometer scale sodium chloride cluster ions ((NaCl)x(Na+)z and (NaCl)x(Cl−)z) using a differential mobility analyzer coupled with a time-of-flight mass spectrometer. Through monitoring cluster ion inverse mobilities as functions of solvent vapor pressure in the mobility analyzer, the extent of uptake was monitored for 1-butanol, ethanol, methyl ethyl ketone (MEK), and toluene. With butanol vapor pressures in the <300 Pa range, shifts in inverse mobility in excess of a factor of 2 were observed for nearly all examined clusters. Ethanol and MEK uptake led to shifts for positively charged cluster ions upwards of a factor of 1.5. Ethanol exposure led to similar sized shifts for negatively charged clusters ions, while MEK exposure led to negative ion inverse mobility shifts less than a factor of 1.3. Toluene was sorbed much less efficiently than the other solvents; toluene exposure led to shifts in inverse mobility below a factor of 1.2. In total, relative inverse mobility shifts, which are direct functions of the extent of vapor uptake, were found to be only weakly dependent on cluster ion size when compared to the influence of vapor molecular structure and cluster ion charge polarity. Classical (Kelvin-based) models are found inadequate to explain the observed mobility shifts, and we instead used a site-specific, Langmuir type model to describe the vapor uptake behavior by the cluster ions.
© 2017 American Association for Aerosol Research
EDITOR:
Funding
This work was supported by the University of Minnesota Grant-in-Aid, as well as the Center for Filtration Research Consortium.
