Single Charging of Nanoparticles by UV Photoionization at High Flow Rates

Abstract

The feasibility of UV photoionization for single unipolar charging of nanoparticles at flow rates up to 100 l· min ⁻¹ is demonstrated. The charging level of the aerosol particles can be varied by adjusting the intensity of the UV radiation. The suitability of a UV photocharger followed by a DMA to deliver monodisperse nanoparticles at high aerosol flow rates has been assessed experimentally in comparison to a radioactive bipolar charger ( ⁸⁵ Kr, 10 mCi). Monodisperse aerosols with particle sizes in the range of 5 to 25 nm and number concentrations between 10 ⁴ and 10 ⁵ cm ⁻³ have been obtained at flow rates up to 100 l· min ⁻¹ with the two aerosol chargers. In terms of output particle concentration, the UV photoionizer performs better than the radioactive ionizer with increasing aerosol flow rate. Aerosol charging in the UV photoionizer is described by means of a photoelectric charging model that relies on an empirical parameter and of a diffusion charging model based on the Fuchs theory. The UV photocharger behaved as a quasi-unipolar charger for polydisperse aerosols with particles sizes less than 30 nm and number concentrations ∼10 ⁷ cm ⁻³ . Much reduced diffusion charging was observed in the experiments, with respect to the calculations, likely due to ion losses onto the walls caused by unsteady electric fields in the irradiation region.

Acknowledgments

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the Collaborative Research Centre on “Nanoparticles from the gas phase: formation, structure and properties” (SFB 445). Dr. Esther Hontañón was supported by Ministerio de Educación y Ciencia (MEC) of Spain.