An experimental study of ion-induced nucleation using a drift tube ion mobility spectrometer/mass spectrometer and a cluster-differential mobility analyzer/Faraday cup electrometer

An experimental system employing a drift tube ion mobility spectrometer/mass spectrometer (DT-IMS/MS) and a cluster-differential mobility analyzer with Faraday cup electrometer (C-DMA/FCE) was developed for the study of ion-induced nucleation. Using this, both the time scales and size scales of ions to be measured can be extended significantly. The system offers the potential for tracing the entire process of ion-induced nucleation initiated by the radiative ionization of air. The main processes are the formation of stable cluster ions and their growth to nanometer-sized ultrafine particles. Negative ions and negatively charged nano-particles generated by α-ray radiolysis in SO2/H2O/air mixtures were examined. High-resolution mobility spectra in which ions are distributed in the relatively high mobility range of 2.1– were obtained by DT-IMS/MS. Alternatively, more aged ions with mobilities as low as were observed in the C-DMA spectra. The differences between both spectra reflect the nature by which cluster ions grow into nano-particles. In addition to mobility measurements, mass spectra of ions can be obtained by DT-IMS/MS, which allows the identification of nucleating ions and relevant ion chemistry to be investigated. In the mass spectra at low SO2 and H2O concentrations, , and were observed as major ions, indicating that SO2 is preferentially ionized by ion–molecule reactions with primary negative ions. When the concentrations of SO2 and H2O were increased, and ions became prominent, suggesting that the oxidization of SO2 into H2SO4 is enhanced, probably due to the enhanced production of OH radicals by ion–molecule reactions involving H2O. This suggests that the role of SO2 in particle formation differs depending on the concentrations of SO2 and H2O. At low SO2 and H2O concentrations, SO2 tends to form core ions that lead to ion-induced particle formation, whereas more SO2 molecules are transformed into gaseous H2SO4 and homogenous nucleation involving H2SO4 and H2O is enhanced at higher concentrations of SO2 and H2O.