Characterizing Reduced Counting Efficiencies of 23 nm Exhaust Emission Condensation Particle Counters Using Atomized Salt Particles

Helmut Krasa, Martin Kupper, Mario Anton Schriefl, ALEXANDER BERGMANN, Graz University of Technology

     Abstract Number: 305
     Working Group: Instrumentation and Methods

Abstract
The Euro 5b legislation for light-duty vehicles introduced particle number (PN) measurements as part of the vehicle homologation process, requiring condensation particle counters (CPCs) with a counting efficiency (CE) of 0.5 ± 0.12 at 23 nm mobility diameter. While regulation R83 defines the requirements for the sampling system, it does not specify a calibration aerosol [1]. With an increasing number of countries introducing PN measurements during periodic technical inspections, a fast, cheap and compact aerosol calibration method is desired. In this work we analyzed the counting behavior of a 23 nm exhaust emission CPCs with atomized salt particles.

The CE of an AVL 23 nm CPC was evaluated using a TSI 3775 CPC (d₅₀ = 4 nm) as reference instrument. In addition to commonly used NaCl, NaI, NaF and NaBr were dissolved and atomized. Furthermore, NaCl and NaBr particles were also generated in a tube furnace. The results showed that the CE is not only influenced by the particle composition, but also varies with the generation method. Compared to atomized particles, particles from the tube furnace showed increased CEs with d₅₀ values of 31.7 and 38.2 nm for NaCl, respectively. The composition of the particles also had a large influence, with the NaI particles being fully counted down to 18 nm. [2]

At low dilution and high salt concentration in the atomized solution, the CE in the transition region decreased significantly for all atomized salts. Particles with electric mobility diameters of up to 150 nm were not fully counted by the 23 nm CPC, resulting in d₅₀ values as large as 69.5 nm in the case of NaCl. When the dilution temperature was increased from 60 to 190 °C, the decrease in CE was mitigated and the CE was comparable to the aerosol generated in the tube furnace.

[1] Regulation No 83 of the Economic Commission for Europe of the United Nations (2015) Official Journal of the European Union. L172/1, 123-126.
[2] Krasa H, Kupper M, Schriefl M. A & Bergmann A (2023), Aerosol Sci. and Techn. 57:4, 329-341.