Metrology As a Driver for Emission Regulation: Aerosol Sensing Technologies from Homologation to Fleet-Wide Monitoring

Abstract Particulate matter (PM) emitted by combustion engines represents one of the most consequential anthropogenic pollutants, with demonstrable links to premature mortality, cardiorespiratory disease, and climate forcing. The transition from mass-based to particle-number (PN) metrics – and the progressive extension of regulated size thresholds from 23 nm toward 10 nm and beyond – has been enabled entirely by advances in aerosol sensing and metrology. This review, based on the IEEE I2MTC 2025 Keithley Award keynote by Alexander Bergmann, traces the metrological arc from laboratory homologation to portable real-driving measurement and fleet-scale periodic technical inspection (PTI). Three interconnected instrument families are examined in depth: condensation particle counters (CPC) with volatile particle removers (VPR) for type-approval testing, unipolar diffusion-charging (DC) sensors with modulated precipitation for portable and periodic technical inspection (PTI) applications, and photoacoustic soot sensors for remote emission sensing (RES). Key co-contributions by Giechaskiel and Bergmann on legislative harmonisation, and by Schriefl, Knoll, Fierz, and Bergmann on diffusion-charging physics, are integrated to illustrate how metrological rigour translates measurement science into regulatory impact. The conclusion identifies non-exhaust particles and the direct biological sensing of aerosol toxicity as the frontier challenges facing the field.