Size-resolved chemical composition analysis of ions produced by a commercial soft X-ray aerosol neutralizer

Abstract

Electrical mobility and mass of atmospheric ions are of great importance, since both properties jointly determine their charging states. In this work, different carrier gases of bottle N2, bottle air, and particle-free lab air were passed through a TSI® soft X-ray aerosol neutralizer to produce both positive and negative ions. The electrical mobilities and mass-to-charge ratios of these ions were measured with a high-resolution half-mini differential mobility analyzer (DMA) coupled to an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer. The effects of carrier gas, sample flowrate, and relative humidity (RH) on the ion mobility and mass-to-charge ratio were evaluated. Most detected ions could be traced back to the impurities in the carrier gases, or the compounds used to manufacture the system components. However, severe fragmentation of DMA-classified clusters was observed, especially under high RH conditions, which likely occurred inside the APi-TOF and complicated the identification of the chemical composition of the DMA-classified clusters. The mobility spectra and ion identities were comparable when using bottle N2 and bottle air as the carrier gas, whereas different features on ion mobility and mass-to-charge ratio were found in the case of particle-free lab air. Residence time of the carrier gas inside the neutralizer as a function of the sample flowrate, when at the order of a few seconds, exerted a weak impact on the ion mobility spectra. The effect of RH on the electrical mobility was contingent on the polarity and chemical composition of ions. The presence of water vapor would grow the acid radical ions and hydronium ions to larger sizes by forming clusters, whereas the positively charged polydimethylsiloxanes in the case of particle-free lab air were marginally impacted. In addition, the concentration of neutral clusters formed through the ion-ion recombination or radiolysis was measured with a particle size magnifier (PSM). Compared with those in the case of bottle gases, the neutral cluster concentrations were significantly larger when particle-free lab air was used as the carrier gas. Higher concentrations of neutral clusters were observed in the high RH experiments, likely because high RH could either enhance the ion-ion recombination or the detection efficiency of PSM. Our work indicates that RH and the impurities in the carrier gas have a significant effect on the measured electrical mobilities and mass spectra for ions generated by a soft X-ray aerosol neutralizer.