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.

扫描迁移率粒度仪（SMPS）广泛用于亚微米颗粒的粒度数分布测量。在这项工作中，两个分别装有软X射线中和器和Kr-85中和器的TSI®SMPS并联运行，以测量环境空气的粒径分布。独立的TSI®超细冷凝颗粒物计数器（CPC）被用作测量环境空气中颗粒物总浓度的参考。我们的结果表明，两种SMPS测得的总颗粒浓度彼此可比，而它们的尺寸分辨浓度比取决于尺寸和相对湿度（RH）。发现中和剂对于在现场测量中快速使小颗粒达到电荷平衡至关重要，如通过Kr-85 SMPS测量的粒径分布与从自然带电粒子倒置的粒径分布的比较所示。在高RH下，由两种SMPS测量的总颗粒浓度往往会超过参考CPC的浓度，这表明在高RH下，由SMPS测量的颗粒浓度可能被高估了。这项研究敦促对高RH条件下的双极性充电过程有更好的了解。