Improving the Comparability of Brake Wear Solid and Total Particle Number Emission Measurement

Brake wear emissions are a significant contributor to particle mass (PM) emissions originating from road transport. In Europe, this is taken into consideration by including emission limits for brake wear particles in the legislation. UN GTR (United Nations Global Technical Regulation) No.24 is a technical description of how to measure the particle number (PN) and PM emissions of brakes. PN measurement includes solid particle number (SPN) and total particle number (TPN), meaning excluding and including the volatile particle matter, respectively. In this study, we examine over 500 TPN and SPN emission factors, in terms of SPN-TPN ratio. To interpret the emission factor data, we present results of a characterization of SPN and TPN measurement instruments in a laboratory setting. We discuss the benefits of using a flow splitter in the PN measurement and present an experimental demonstration of its suitability for measurement of brake wear PN. Combining the results of this investigation with previously published research, we draw the conclusion that seemingly implausible ratios of SPN and TPN (i.e., SPN emissions higher than TPN emissions) can be observed, and this observation can be attributed to three main reasons: 1) contribution of background concentration, 2) poor mixing of particles in the sample transport tunnel of the measurement system, and 3) inherent uncertainty of the PN measurement instrument. Additionally, we conclude that a properly designed flow splitter will not adversely affect the PN measurement and utilizing one will improve the comparability of SPN and TPN emission results.