In this work, we investigated the influence of different types of soot aerosol on the counting efficiency (CE) of instruments employed for the periodic technical inspection (PTI) of diesel vehicles. Such instruments report particle number (PN) concentration. Combustion aerosols were generated by a prototype bigCAST, a miniCAST 5201 BC, a miniCAST 6204 C and a miniature inverted soot generator (MISG). For comparison purposes, diesel soot was generated by a Euro 5b diesel test vehicle with by-passed diesel particulate filter (DPF). The size-dependent counting efficiency profile of six PN-PTI instruments was determined with each one of the aforementioned test aerosols. The results showed that the type of soot aerosol affected the response of the PN-PTI sensors in an individualised manner. Consequently, it was difficult to identify trends and draw conclusive results about which laboratory-generated soot is the best proxy for diesel soot. Deviations in the counting efficiency remained typically within 0.25 units when using laboratory-generated soot compared to Euro 5b diesel soot of similar mobility diameter (~50–60 nm). Soot with a mobility diameter of ~100 nm generated by the MISG, the lowest size we could achieve, resulted in similar counting efficiencies as that generated by the different CAST generators for most of the PN-PTI instruments, showing that MISG may be a satisfactory – and affordable- option for PN-PTI verification.

The concentration of nanometre-sized particles is frequently measured in terms of particle number concentration using well-established measuring instruments, such as condensation particle counters. Traceability for these measurements can be achieved by means of calibration against a reference aerosol electrometer starting at concentrations > 1 000 cm ⁻³ . Here, two independent methods for extending traceability down to 1 cm ⁻³ are described. The first method relies on a custom-made, reference optical particle counter while the second method combines electrometer measurements with a series of dilution steps. An inter-comparison of the two methods was carried out using polystyrene spheres with a nominal diameter of 100 nm in the concentration range 1 cm ⁻³ - 100 cm ⁻³ . A condensation particle counter Model 3752 (TSI Inc, USA) was used as transfer standard. The obtained results showed a deviation of 1 % - 4 % between the two methods, which was in agreement with the stated uncertainties.

This article provides a report of the recent workshop on “The metrology of quantities which can be counted” organised jointly by the International Committee for Weights and Measures’ Consultative Committees for Amount of Substance (CCQM) and for Units (CCU). The workshop aimed to trigger a discussion on counting and number quantities across the metrological community so that a common understanding of counting and a common nomenclature could be achieved and there was clarity on the differences between these increasingly important concepts. This article details the background to the workshop, provides a summary of the presentations given and the discussions on the topics raised. It also reports the conclusions, agreed actions and next steps resulting from the workshop.

Main text This report presents the results of CCQM-P189, a pilot comparison between 10 laboratories which tested the participants’ capability to measure particle number concentration (in the range of 100 to 20 000 cm ⁻³ ) using condensation particle counters (CPCs), and particle charge concentration (in the range of 0.15 to 3 fC cm ⁻³ ) using aerosol electrometers (AEs). Measurements of aerosol particle number concentration are needed to demonstrate compliance to vehicle emission legislation and are becoming increasingly important in other areas such as ambient air and workplace monitoring. The measurements are typically carried out using condensation particle counters, which are calibrated using either reference CPCs or reference AEs. An analogous report is available for the CCQM-K150 comparison. CCQM-K150 was identical to, and used the same experimental data as CCQM-P189 with one exception: data from TROPOS, which is not a National Measurement Institute (NMI) or Designated Institute (DI), were only included in CCQM-P189. CCQM-P189 was an amount-of-substance Track C comparison. To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/ . The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Main text This report presents the results of CCQM-K150, a key comparison between nine National Measurement Institutes (NMIs) which tested the capability of the NMIs to measure particle number concentration (in the range of 100 to 20 000 cm ⁻³ ) using condensation particle counters (CPCs), and particle charge concentration (in the range of 0.15 to 3 fC cm ⁻³ ) using aerosol electrometers (AEs). Measurements of aerosol particle number concentration are needed to demonstrate compliance to vehicle emission legislation and are becoming increasingly important in other areas such as ambient air and workplace monitoring. The measurements are typically carried out using condensation particle counters, which are calibrated using either reference CPCs or reference AEs. An analogous report is available for the CCQM-P189 comparison. CCQM-P189 was identical to and used the same experimental data as CCQM-K150 with one exception: data from TROPOS, which is not an NMI or Designated Institute (DI), were only included in CCQM-P189. CCQM-K150 was an amount-of-substance Track C comparison. To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/ . The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).