First comprehensive inter-comparison of aerosol electrometers for particles sizes up to 200 nm and concentration range 1000 cm-³ to 17 000 cm-³

The concentration of nanometre-sized particles is frequently measured in terms of particle number concentration using well-established measuring instruments, e.g. condensation particle counters. Traceability for these measurements can be achieved by means of calibrations using an aerosol electrometer (AE) as a reference. A number of national metrology institutes (NMIs) and expert laboratories provide such calibrations, but the metrological basis is at present not well established because the equivalence between the unit realizations has not been investigated by means of an inter-laboratory comparison. This paper presents the results of the first comprehensive comparison of AEs involving NMIs and expert laboratories worldwide. The comparison covered the particle size and charge concentrations ranges 20 nm to 200 nm and 0.16 × 10^-15 C cm^-3 to 2.72 × 10^-15 C cm^-3 (equivalent to 1000 cm^-3 to 17000 cm^-3 singly charged particles), respectively. The obtained results agreed to within about ±3%, which was within stated uncertainties, with only a few exceptions, such as at low concentrations. Additional measurements with sub-20 nm particles show that comparisons in this size range are more challenging and require special considerations, though agreement to within about ±5% was still found with 6 nm particles. This comparison is the first and vital step towards internationally recognized SI traceability in particle number concentration measurements.