The Particle Measurement Programme (PMP) was established to improve the sensitivity of the Particulate Matter (PM) mass measurement system for heavy and light duty vehicle emissions. PMP developed a particle number (PN) counting method for ultrafine solid particles and enhancements to the PM measurement procedure that were incorporated into UN R83 and R49. Since 2013, PMP has focused on PN as a function of particle size (mainly on the difference between the number of particles measured with the existing PMP methodology (d50=23 nm) and with systems with lower d50s, investigating the possibility to modify the existing d50 methodology from 23 to about 10 nm.