Comment on ar-2025-39

<strong class="journal-contentHeaderColor">Abstract.</strong> An inter-laboratory comparison (ILC) involving optical particle counters (OPCs) was organized at the French national level. The aim of this study was to make an inventory of the metrological capabilities of particle number size distribution (PNSD) measurements using OPCs. This laboratory study took place over a period of 18 months and involved 16 partners and 35 OPCs. Rather than focusing on the actual capability of the tested OPCs, this paper aims to reveal good laboratory practices when using standard OPCs. For that, each partner applied the same pre-defined experimental protocol on the OPC(s) to be tested, operated together with a common control OPC. Three different powder-borne test aerosols were involved, and their PNSDs were measured: (1) &ndash; a monodisperse amorphous silica sample, (2) &ndash; glass beads and (3) &ndash; a green cornstarch powder. This article presents the measured PNSD using the 35 OPCs associated with the description of the experimental set-up, sample preparation protocol and comparison with Scanning Electron Microscopy measurements.