COMPARISON OF MOBILITY EQUIVALENT DIAMETER WITH KELVIN-THOMSON DIAMETER USING ION MOBILITY DATA

Mobility distributions of both positive and negative cluster ions were studied in nitrogen gas enriched by ethanol, isopropanol, n-butanol a nd acetone vapours, respectively. The ions were generated into the sys tem by a radioactive source normally used as a bipolar aerosol charger . The ion mobility distributions were measured with a time-of-fight mo bility spectrometer (TOF). The measured ion mobility can be used to es timate the mobility equivalent diameter via conventional Millikan-Fuch s formula based on Stokes law corrected by molecule slip effects. When the vapour concentration is increased the cluster ions will get large r due to enhanced molecule attachment onto ions. Here the vapour conce ntration is assumed to be high enough so that the chemical tracer effe cts would become of minor importance. Therefore the Kelvin-Thomson equ ation for ion induced nucleation is first assumed to be valid. The Kel vin-Thomson diameter of the ions in all cases was found to be systemat ically smaller than the corresponding mobility equivalent diameter acc ording to Millikan-Fuchs, indicating of inconsistencies between the fo rmulas based on macroscopic quantities at 1 nm size range. Thereafter, other available mobility vs diameter relationships were surveyed. The closest agreement between mobility equivalent and Kelvin-Thomson diam eters at this size was obtained using the relationship given by Tammet [J. Aerosol Sci. 26, 459 (1995)]. (C) 1996 American Institute of Phys ics.