Critical Comparison of Brake Wear Particle Emissions Measured on a Brake Dynamometer and Under Real-Driving Conditions

Brake wear is an increasingly important source of non-exhaust traffic-related particulate matter, yet the relationship between emissions measured under standardized brake-dynamometer conditions and those observed during complete-vehicle operation remains insufficiently resolved. This study presents a systematic critical analysis and methodological synthesis of laboratory, chassis-dynamometer, and on-road evidence, emphasizing physical system boundaries, brake duty, thermal conditions, sampling efficiency, component conditioning, and measurement reproducibility. Only three experimental programs provided sufficiently comparable paired or quasi-paired road-to-dynamometer data for the principal quantitative comparison; their study-specific emission-factor ratios (EFratio) ranged from 0.33 to 1.44 and occurred on both sides of unity. The small, heterogeneous evidence base therefore does not support a pooled meta-analysis or a universal road-to-dynamometer correction factor. For particulate mass, braking energy, friction material, conditioning, and particle recovery are major determinants of cross-method differences. Particle number is particularly temperature-sensitive because thermally activated evaporation and decomposition can trigger nucleation of ultrafine particles. Mobile samplers may therefore both lose particles after generation and perturb the source by altering brake cooling. To structure these effects, the study applies a four-level system-boundary framework and proposes a three-stage validation sequence combining metrological benchmarking, thermal non-interference assessment, and matched road-laboratory testing. The proposed acceptance targets are explicitly presented as provisional research criteria requiring further interlaboratory validation.