Sizing of neutral sub 3Â nm tungsten oxide clusters using Airmodus Particle Size Magnifier

Measurement of the size distributions of neutral sub 3 nm aerosol particles is technically a challenging task. Here we present laboratory experiments on sizing of neutral tungsten oxide clusters with two independent methods: Airmodus A11 Particle Size Magnifier (PSM) and a combination of a high resolution differential mobility analyzer (DMA) and an electrometer. The test particles were generated with a hot wire generator, and their mass and composition were measured with a high resolution mass spectrometer. The particles were shown to be (WO3)x up to 12,000 amu or approximately 2 nm. As particle counters are usually calibrated with electrically charged particles but often measure mostly neutral particles, we quantified the enhancement due to charge to the PSM cut-off diameter to be approximately 0.1 nm. To compare directly the two methods, charged fraction of the sample particles and effect of organic impurity ions to the measured concentrations were estimated, and the PSM was shown to measure similar size classified concentrations as the combination of DMA and electrometer. The results confirm that with a sound characterization of the particle source and the particle counter, sub 3 nm neutral particle distributions can be measured with good precision with a method based on supersaturation scanning.