How early UK particle research helped shape the PMP Programme and particle number legislation
22 September 2026
Twenty-five years ago, Ricardo issued the final report of the DETR/SMMT/CONCAWE Particulate Research Programme (PRP). At the time, road-vehicle particle legislation was based only on particulate mass (PM). That project’s measurements made it clear that the PM metric did not tell the whole story of exhaust particle emissions.
In this anniversary year, I thought I would share how the PRP contributed to the creation of the Particle Measurement Programme (PMP), link its contribution to the development of particle number legislation in Europe and comment on how the findings still have relevance to Euro 7.
The PRP was not established to develop particle number regulations! Rather, it identified which particles could be measured reproducibly, which were strongly influenced by the sampling system, where existing instruments could provide misleading data and why a future regulatory method would have to define the particle metric it intended to count and control.
In retrospect, the route leading from the PRP—along with evidence from some other projects running in a similar timeframe, such as the FP5 PARTICULATES project—to the UN Economic Commission for Europe Particle Measurement Programme, and then on to European particle number legislation, is clear.
PRP drivers
The PRP was a collaboration of the UK Government (Department of the Environment, Transport and the Regions), the UK/European motor industry represented by the Society of Motor Manufacturers and Traders, and the oil industry represented by Concawe. It examined six light-duty vehicles, four heavy-duty engines and 13 diesel and gasoline fuels. The technologies included conventional diesel, diesels with particulate filters (DPF), multi-point-injection gasoline, gasoline direct injection, LPG and a heavy-duty CNG engine.
The PRP focused on the measurement of regulated filter-based particulate matter plus mass-weighted and number-weighted particle size distributions from a constant volume sampler (CVS) dilution tunnel. Two scanning mobility particle sizers (SMPS) covered approximately 7 to 710 nm, while a micro-orifice uniform deposit impactor (MOUDI) covered mass up to 18µm. Testing included steady-state and dynamic operation because the transient conditions by which an engine reaches a stabilized operating condition can be as important to emissions as the condition itself.
The PRP’s objective was not to explore the potential of particle number legislation, it was to understand what mass, number, size and chemical composition factors revealed—and to consider the reliability of each measurement metric.
What did the PRP find?
- Mass and number effects were not necessarily aligned: reductions in PM generally led to lower accumulation mode particle numbers but, under some conditions, reducing the carbonaceous mode allowed a highly numerous, low-mass, nucleation mode to form in the dilution system.
- Nucleation and accumulation mode particles, measured by SMPS, behaved differently: accumulation mode particles, nominally above 50 nm, were primarily carbonaceous. Nucleation mode particles, ≤ 50 nm, were mainly condensed volatile material. When abundant carbon nuclei were present, volatile material condensed on them, limiting the particle count to the number of accumulation mode particles present. Where carbon nuclei were limited, a dominant nucleation mode could form.
- DPFs produced the largest of any technology effects: they reduced regulated particulate mass substantially and accumulation mode particle number by orders of magnitude. However, at some high-temperature conditions, substantial nucleation mode emissions were measured post-DPF. Those particles had minimal mass and formation was strongly dependent on the presence of sulphate nuclei and hydrocarbons, plus specific sampling conditions.
- Instrument measurement conditions could change the results: nominally identical SMPS instruments agreed within about 20% when configured identically, but selecting different size ranges changed modal locations and integrated totals. Two studies could therefore test the same emissions and reach different total PN conclusions if their instruments covered different ranges.
- Preconditioning and transients can influence later measurements: three nominally equivalent idle points produced different size distributions because of the preceding engine and dilution tunnel prehistory. During heavy-duty steady-state cycles, most of the nucleation mode PN production occurred in the transitions between modes, outside the regulated PM sampling-time windows. The ~2-minute scanning SMPS was not suitable for resolving rapidly changing transient particle size distributions.
These findings led to some critical conclusions, PN was a potentially important alternative metric, particularly as emissions levels post-DPF fell below the practical sensitivity of gravimetric mass measurement, but several factors rendered it currently unsuitable for legislation: volatile nucleation particles were legitimate constituents of the exhaust aerosol, but their measured level was highly dependent on dilution, temperature, preconditioning and instrument configuration to constitute a robust type-approval metric.
How the findings contributed to PMP
A UNECE GRPE informal group had met since 1998 to exchange information on vehicle particulate emissions, and in May 2000 it highlighted that regulated filter-based particulate limits were approaching the limit of detection, and that emerging health concerns might require more robust emissions control. France, Germany, the Netherlands, Sweden and the United Kingdom then proposed coordinated research under UNECE. At its 41st session, GRPE gave the “PMP” group a formal mandate, with the UK as Chair, to develop a harmonised measurement method for (initially) light-duty diesel vehicles. (Legislation for heavy-duty engines, GDI gasoline and non-road mobile machinery later built upon outputs, with specific modifications also made more recently for Euro 7.)
PMP Phase I, conducted in 2001 and 2002, records that the UK and Switzerland were the principal research contributors to PMP Phase I, that considered multiple alternative measurement approaches and metrics, building on earlier government-commissioned work, including PRP.
PRP observations that volatile nucleation particles were highly sampling-sensitive contributed to the decision to concentrate the regulatory method on solid, or more precisely method-defined non-volatile, particles. The eventual PMP system used sample conditioning to remove volatile material before particle counting. In practical terms, the measurement procedure now defines the “regulatory particle” quantified by the legislative process.
The absence of fast particle-sizing instruments suitable for monitoring changing transient aerosols supported the move toward rapid particle counting rather than regulatory size-distribution scanning. PMP Phase I identified a condensation particle number counter (PNC), used with dilution and volatile-particle removal, as the leading candidate. The lower limit of this particle counter was set near the primary carbon sphere diameter of ~20nm (for pragmatic purposes, 23 nm was used) so that the size range “PN23” focused on the soot-dominated accumulation mode of vehicle exhaust. Also recommended was the use of instruments with suitable time resolution and the application of transient cycles compatible with existing type-approval procedures.
The PRP report's suggestions on instrument range, calibration, dilution and preconditioning also helped shape the requirements of PMP, advancing them from their prior status as recommended experimental considerations. PMP specified counting efficiency, particle losses, volatile removal, dilution and system validation. Inter-laboratory exercises then tested whether nominally equivalent systems gave comparable results in different facilities.
The post-DPF emissions results of PMP supplied the technology case that could be required to further reduce exhaust particle emissions. Gravimetric mass was becoming increasingly difficult to resolve at post-filter emission levels. A properly conditioned particle-number method remained sensitive and could discriminate between filtered and unfiltered diesel vehicles. That gave regulators a practical way to require the very high filtration performance already demonstrated by wall-flow DPFs, and ultimately enabled the limit value (of 6 × 1011 #/km) to be set at a level only achievable by them.
PMP was a multi-stage sustained international effort involving governments, the European Commission, research organisations, industry and instrument manufacturers. However, the 2001 PRP report, funded by government, the oil industry and the motor industry in partnership, supplied experimental evidence and practical insights that PMP used to create a robust framework for a repeatable, practical regulatory method.
Is the PRP still relevant?
One of the main lessons of the PRP is that a regulatory particle metric cannot be separated from the method used to create it. At low emissions, dilution conditions, thermal prehistory, volatile removal, particle losses, counting efficiency and calibration influences are part of the measured quantity.
The PRP report also provides a reminder that reducing one metric does not guarantee reduction of every particle population under every condition. Mass and number metrics provide different information, while composition and size distributions impact the interpretation of results. Results of light-duty and heavy-duty technologies, and their trends, might not be simply translated from one experimental regime to the other.
Those observations were important in 2001, and remain so, even as PN methods have been extended to a lower particle size limit (PN10), new powertrains and new sources. In particular, even though the exhaust emissions methodology and calibration has been impressively stabilized under the stewardship of JRC, the forthcoming Euro 7 brake particle controls—that may well include volatile particle emissions—and following that potential solid and volatile tyre particle number emissions legislation, will benefit from lessons learned in the PRP as well as in the PMP.
Research may still be needed...
Much of particle sampling and measurement research in the past 25 years has focused on accurately and repeatably measuring non-volatile particles, and perhaps the potential future need for a regulatory measurement of volatile and semi-volatile particles has been neglected. Maybe it is time to start looking at those topics now...
Download the DETR/SMMT/Concawe Final Report | This story was first published on LinkedIn