Charged particle formation by the ionization of air containing sulfur dioxide

Abstract

Experimental investigation of charged particle formation by the ionization of air containing sulfur dioxide (SO 2) was performed using a nano-DMA (differential mobility analyzer) and an atmospheric pressure ionization mass spectrometer. A radioactive ion source of 241Am and a negative dc corona discharge were used to ionize SO 2/H 2O/air mixtures. The results showed that the number of charged particles that formed had increased as H 2O concentration increased (ca. 20–3 × 10 3 ppm) for both ion sources, but also that the number of charged particles produced when using the negative corona discharge was more than two orders of magnitude greater than what was produced using 241Am. During ionization by α-ray irradiation, SO 4 −(H 2O) n ions predominated coincident with the formation of charged particles. The negative corona discharge produced a more complicated ion mass spectrum, which included ion groups of NO 3 −, SO x − ( x = 2–5) and HSO x − ( x = 3–5). The relative abundance of the ion groups varied depending on H 2O concentration and ion reaction time. The ions with an HSO 4 − core surpassed the ions of other groups as H 2O concentration increased. The formations of NO 3 − ions and cluster ions containing HNO 3 also were enhanced at higher H 2O concentrations. Possible ion-molecule reactions responsible for the observed mass spectra are discussed in detail.