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

Abstract 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 SO 2 / H 2 O / air mixtures were examined. High-resolution mobility spectra in which ions are distributed in the relatively high mobility range of 2.1– 1.5 cm 2 V - 1 s - 1 were obtained by DT-IMS/MS. Alternatively, more aged ions with mobilities as low as 0.05 cm 2 V - 1 s - 1 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 SO 2 and H 2 O concentrations, SO 2 - , SO 3 - , SO 4 - , and SO 5 - were observed as major ions, indicating that SO 2 is preferentially ionized by ion–molecule reactions with primary negative ions. When the concentrations of SO 2 and H 2 O were increased, HSO 4 - and HSO 4 - · H 2 SO 4 ions became prominent, suggesting that the oxidization of SO 2 into H 2 SO 4 is enhanced, probably due to the enhanced production of OH radicals by ion–molecule reactions involving H 2 O . This suggests that the role of SO 2 in particle formation differs depending on the concentrations of SO 2 and H 2 O . At low SO 2 and H 2 O concentrations, SO 2 tends to form core ions that lead to ion-induced particle formation, whereas more SO 2 molecules are transformed into gaseous H 2 SO 4 and homogenous nucleation involving H 2 SO 4 and H 2 O is enhanced at higher concentrations of SO 2 and H 2 O .