Performance comparison of SMPSs with soft X-ray and Kr-85 neutralizers in a humid atmosphere

Abstract Scanning mobility particle sizer (SMPS) is widely used for particle number size distribution measurements of submicron particles. In this work, two TSI® SMPSs equipped with a soft X-ray neutralizer and a Kr-85 neutralizer, respectively, were operated in parallel to measure particle number size distributions of ambient air. An independent TSI® ultrafine condensation particle counter (CPC) was used as a reference to measure the total concentration of particles in the ambient air. Our results show that the total particle concentrations measured by the two SMPSs were comparable with each other, while their size-resolved concentration ratios were size-dependent and relative humidity (RH)-dependent. Neutralizer was found to be essential to quickly bring small particles to charge equilibrium in field measurements, as indicated by the comparison of particle number size distributions measured by the Kr-85 SMPS with those inverted from naturally charged particles. The total particle concentrations measured by two SMPSs tend to exceed those by the reference CPC at high RH, hinting that the particle concentrations measured by SMPSs could be overestimated at high RH. This study urges a better understanding of the bipolar charging processes under high RH conditions.