In 2011, the Euro 5b light-duty vehicle legislation introduced particle number (PN) limits for vehicle homologation. To measure PN concentrations, specific condensation particle counters (CPC) with a counting efficiency (CE) of 50% at 23 nm are required. The usage of such automotive CPCs will significantly increase with the upcoming legislation for PN measurements during periodic technical inspections, which comes along with the need for simple calibration methods for particle number counters using salt particles. In this work, we examined the counting characteristics of an automotive AVL 23 nm CPC with inorganic salt particles. We further investigated the usability of different atomized inorganic salt particles for calibration purposes focusing on CE and linearity. The CE varies greatly depending on the chemical salt composition: While NaI was fully activated down to 18 nm and linear over the full concentration range, the CE of other atomized salt solutions showed a strong concentration dependency with a d(50) diameter ranging from 38.2 nm to 69.5 nm for atomized NaCl with increasing concentration. Such effects were not found for tube-furnace generated salt particles within similar concentration ranges. When a hot dilution was applied, the non-linear behavior was minimized and the CE overall increased, leading to a d(50) of 27.3 nm for atomized NaCl. Based on those findings, we propose a simplified calibration method for automotive PN counters without the need for a differential mobility analyzer due to the high CE of atomized NaI particles.
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