Condensation Diffusion Charging – Particle Number Measurement of High Concentrations Down to 2.5 nm

Abstract. Particle number (PN) measurement of particles smaller than 10 nm is challenging and has so far primarily relied on condensation particle counters (CPCs). In this work, we present a concept that combines a condensational growth stage with a diffusion charger to allow for PN measurement with a lower particle cut-off diameter of 2.5 nm with the ability to measure PN concentrations exceeding 10⁶ cm^-3. We use diethylene glycol as working fluid to magnify ultrafine particles into monodisperse µm-sized droplets, which are then charged by a corona charger and finally detected with a Faraday cup electrometer. The sensor developed in this work, the Condensation Diffusion Charger (CDC), shows a size-independent counting efficiency above 10 nm, similar to CPCs. Finite element simulations were performed to model the particle activation and subsequent droplet growth. The particle activation was verified experimentally and showed a counting efficiency of 50 % for particles with 3 nm mobility diameter. The CDC was tested on exhaust emissions at a chassis dynamometer for category L-vehicles to demonstrate its viability for vehicle emission measurements. The results closely correlate with a 2.5 nm reference CPC. Our findings indicate that this method offers an approach for a compact and portable PN measurement system for ultrafine particles at very high concentrations without the need for dilution.