2. Giorgio Martini introduced the presentation from JRC reminding the group that this was a literature survey study to determine the current state of play and knowledge on sub-23nm particles. The PMP group had delegated this activity to the JRC at the 27th meeting. Mr Martini also explained the basis of the existing legislation – which was developed to measure solid particles from diesel engines where a significant amount was known about the characterisation of the particles. The need for a robust solid particle number measurement and the state of the art of reliable measurement equipment led to the decision for a 50% cut off size of 23nm. As the PN (particle number) measurements are being extended to other technologies, so it is important to review the state of knowledge regarding the existence, characterisation and measurement possibilities for sub-23nm particles.

3. Barouch Giechaskiel gave the JRC presentation: Key points are summarised below:

• Nucleation mode is liquid mostly 10-20 nm. PN measurement to the PMP procedure evaporates the liquid particles and nucleation mode more or less disappears.
• Diesel literature in last 5 years shows separate modes (nucleation and accumulation) of solid particles in larger older diesel vehicles at low load, rarely seen in other diesel engines. Must be noted that these are from direct tailpipe measurements not from the dilution tunnel / CVS.
• Are the particles really solid or only defined as solid because they are measured after the VPR (evap tube 350 deg C)? TSI mentioned that if formed during combustion at same size as primary particles then this raises the question of why they don’t agglomerate. Ricardo suggested maybe more likely to see this in low soot engines. Others assume might be particles formed from combustion of lube oil.
• Primary particles approx. 25nm. GDI primary particle distribution is wider (7-60nm). Therefore % sub 23nm probably higher for GDI.
• GDIs show typically a shoulder at 20nm, a separate solid nucleation mode is not typical but size distribution can peak at 20nm in some operation modes (highly unlikely).
• Conventional gasolines show sub 23nm from metal additives of lube oil or fuel.
• Mopeds – quite a lot of solid sub-23nm particles seen even down to 6nm although this can also be a sampling artifact.

JRC has run some testing: not on NEDC, but using WLTP but using PMP protocol, PCRF 1000 and measuring in parallel sub and above 23 nm particles. No additional loss correction was included.

• Diesels less than 20% particles below 23nm
• GDI slightly higher age of solid sub 23nm particles compared to diesels but typically < 60
• GDI nucleation mode (as shoulder) peaked at 10nm
• Low temperature testing (-7 deg C) increased the percentage.
• Regeneration (one car, 2 regenerations) showed similar percentages and no distinct solid nucleation mode particles

• TSI mentioned to be very careful not to overemphasize the 3nm as many measuring issues with sizes this small.

• Even at tailpipe didn’t see high levels of solid nucleation particles. Thanasis Mamakos (AVL) wondered exactly what these particles were and what their origin was.

• Measurement Feasibility PNC:
  • Lowering cut off of PNC is potentially advantageous and reduces material dependency
  • Down to 10nm possible with existing PNCs but below would need different model and thus big modifications for existing PN systems. Might need for appropriate calibration material and would mandate use of an electrometer for both calibration and validation of the PMP system.
• Measurement Feasibility – VPR:
  • Penetration / losses: losses of VPR increase significantly as 50% cut off size reduced – feasible only as indication of existence. Generation of monodisperse particles < 30nm problematic but not impossible for characterising the VPR efficiency.
  • Formation of solid particles from volatiles (e.g. pyrolysis) – there are very few studies available – indications it may occur in the presence of sulphuric acid.
  • Hydrocarbons – Theoretical estimations show that they will renucleate at concentration of 3 mg/m³
  • Sulphuric acid renucleates at 0.7 to 3.5ug/m3 – fuel less than 10ppm and lube conversion translates to 300 ug/m³ – same order of magnitude.
  • Even when considering dilution in CVS and primary diluter – renucleation is likely – experimentally it has been shown in some cases sub 23nm were renucleated particles.
  • There was some discussion that presence of ammonia would increase the risk of renucleation. Note: Request for data on formation of solid particles from volatile material.
• Summary:
  • With existing equipment reducing cut off size is not recommended.
  • Decreasing size to 10nm is possible but addition of catalytic stripper to replace or complement the evaporation tube in the VPR is recommended.
  • Decreasing size below 10nm – Uncertainty very high – possible for research but not for legislation.
  • Open issues – regeneration, pyrolysis, catalytic stripper performance and update of calibration procedures (VPR and PNC).