Enhanced unipolar charging of concentration-enriched particles using water-based condensational growth

A novel methodology with high efficiency for charging fine and ultrafine particles was developed and evaluated. The technique has been realized by combining the versatile aerosol concentration enrichment system (VASES) developed by our group and a newly developed unipolar charger equipped with carbon fiber ionizers. Particles are grown to super-micron droplets via condensation of ultrapure deionized water and then concentrated by virtual impaction in the VACES. The grown droplets are charged with negligible ozone generation in the carbon fiber unipolar charger, and subsequently dried to original particle size distribution using a diffusion dryer, while preserving the acquired charges. This new methodology was investigated for different particle sizes, chemical compositions and concentrations. The number of charges on particles was highly dependent on particle concentration as well as particle size; larger particles and smaller particle concentrations led to a greater number of charges per particle. The average electrical mobility of the charged particles exceeded 1.4 × 10^{- 1} cm² / (V s), corresponding, on average, to more than 250 elementary charges per particle for concentrations in the range of 1.4 - 1.9 × 10⁵ particles / cm³ for ammonium nitrate, ammonium sulfate and glutaric acid. A lower number of charges per particle was observed for PSL particles, probably due to their hydrophobic nature, and thus, number of charges per particle averaged to about 98, 191 and 349 for polystyrene latex (PSL) particles of 99, 130 and 170 nm, respectively, at particle concentrations of 1.1 - 1.7 × 10⁵ particles / cm³. Even in the case of PSL, these charges far-exceed those obtained by traditional corona chargers. © 2008 Elsevier Ltd. All rights reserved.