Document submitted by the Informal Working Group (IWG) for the Particle Measurement Programme to inform and update the GRPE on work to amend UN GTR 15 Annexes 5, 6, 7 to:

• Modify the existing solid PN measurement methodology having a 50% cut-off size at 23 nm (SPN23) in order to allow the use of catalyzed volatile particle remover (VPR) and introduce minor improvements,
• Include as a second option a solid PN measurement methodology with a 65% cut-off size at 10 nm (SPN10).

Explanatory note from the PMP informal group addressing the proposed changes to the current methodology and the proposed changes for the second alternative option to extend the particle size detection range to 10 nm particles.