Measurement of Automotive Nonvolatile Particle Number Emissions within the European Legislative Framework: A Review

Abstract

In 2011, the European Commission introduced a limit for nonvolatile particle number (PN) emissions >23 nm from light-duty (LD) vehicles and the stated intent is to implement similar legislation for on-road heavy-duty (HD) engines at the next legislative stage. This paper reviews the recent literature regarding the operation-dependent emission of PN from LD vehicles and HD engines, and the measurement procedure used for regulatory purposes. The repeatability of the PN method is of the order of 5% and higher scatter of the results can easily be explained by the effect of the vehicles or the aftertreatment devices on the PN emissions (e.g., the fill state of the diesel particulate filters). Reproducibility remains an issue since it may exceed 30%. These high-variability levels are mainly associated with calibration uncertainties of the PN instruments. Correlation measurements between the full-flow dilution tunnels (constant-volume samplers, CVS) and the proportional partial-flow dilution systems (PFDS) showed agreement within 15% for the PN method down to 1 × 10¹¹ p/kWh. At lower concentrations, the PN background of the CVS and/or the PFDS can result in larger inconsistencies. The filter-based particulate matter (PM) mass and the PN emissions correlate well down to 1–2 mg/km for LD vehicles and to 2–3 mg/kWh for HD applications. The correlation improves when only elemental carbon mass is considered: it is relatively good down to 0.1–0.3 mg/km or mg/kWh.

Copyright 2012 American Association for Aerosol Research

Notes

The term particle is conventionally used for the matter being characterized (measured) in the airborne phase (suspended matter), and the term particulate for the deposited matter, according to the definitions in the European regulation. The letter ‘p’ will be used to refer to ‘particles’ in the graphs.

The sample preconditioning includes hot dilution at 150°C, followed by an evaporation tube at 300°C. Any surviving particles large enough to be detected with a PNC having a 50% counting efficiency at 23 nm are defined as solids in the PMP protocol. Accordingly, these operationally defined solid particles may also include semivolatile material not evaporating at 300°C (e.g., heavy molecular hydrocarbons) or at least not shrinking to a sufficiently small size that would not activate inside the PNC. In this paper, we use the term nonvolatile material instead. Similarly, we use the term volatile for particles that evaporate below 300°C, even though semivolatiles are also included. The term PN method refers to the measurement procedure defined by the PMP for the measurement of nonvolatile particles larger than 23 nm. Similarly, the term PN (measurement) system refers to PMP compliant system, unless otherwise specified.

Nevertheless, the terms PMP or Regulation 83 for LD (or Regulation 49 for HD)-compliant PN measurement systems are used interchangeably.

The legislation requires concentration greater than 5000 p/cm³; at the moment, linearity is proven for PNCs that measure approximately up to 20,000 p/cm³.

The combined uncertainties of all the factors are calculated by adding them in quadrature (i.e., taking the square root of the sum of their squares).