AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
DMA-MS Measurement of Water Vapor Uptake by Charged Clusters Under Sub-Saturated Conditions
DEREK OBERREIT, Carlos Larriba, Peter McMurry, Christopher Hogan Jr., University of Minnesota
Abstract Number: 26 Working Group: Aerosol Nucleation: From Clusters to Nanoparticles
Abstract Vapor molecule uptake by charged particles in the nanometer size range is an important yet not well understood phenomenon observed in atmospheric chemistry, materials synthesis, and aerosol instrumentation. Existing methods used to study these effects for aerosols lack the sensitivity required for small particles. We use DMA-MS (Differential Mobility Analysis- Mass Spectrometry) to study water vapor uptake by charged particles/clusters by introducing controlled vapor concentrations to the DMA sheath gas. Electrical mobilities of charged salt cluster particles composed of Cs+(CsI)n, Rb+(RbI)n, K+(KI)n, and Na+(NaI)n are measured at varying saturation ratios of water vapor at different temperatures where the value for n ranges from 0 to 13 neutral cation-anion pairs. The charged salt clusters are generated using electrospray ionization with methanol as the solvent. The DMA is high resolution (R~60), parallel-plate DMA operated such that particles/clusters of a specified electrical mobility are constantly provided to the mass spectrometer inlet. The mobility range is decreased stepwise to obtain mass-mobility spectra. With a mass spectrometer as the detector, we can clearly distinguish between cluster ions of similar electrical mobility but different mass to charge ratio (singly versus doubly charged). From these measurements, we find that the electrical mobility of all clusters examined decreases with increasing water saturation ratios. The relative shift in electrical mobility varies for different numbers of neutral cation-anion pairs present in the original cluster and does not depend monotonically on dry cluster size. The measured electrical mobility of the clusters corresponds to their average size when equilibrium is established with the surrounding vapor; thus, electrical mobility shifts allow us to infer the equilibrium constants for water vapor uptake by clusters of controlled size and chemical composition.