Fine particle emissions from engine exhaust have attracted attention because of concern of their higher deposition fraction in alveoli. Since it was observed that sizes of solid particles in exhaust of conventional internal combustion engine technologies are mainly distributed above 30 nm and the mainly irreproducible sensitivity to volatile particles can be reduced, the current solid particle number (PN) measurement methodology was targeted to PN emissions particles larger than 23 nm. The necessity of the measurement of particles smaller than 23 nm is now under discussion. It is also surmised that there is difference between emissions under regulatory defined test cycles and real driving conditions. Currently, implementation of further real driving emission regulations utilizing portable emissions measurement systems (PEMS) is in place for the EU and being actively discussed in other regions. In this study, a commercially available PEMS for PN was modified to extend the detection limit to particles below 23 nm and its feasibility to on-board testing was investigated by performing measurement system performance checks, correlation exercises with a laboratory instrument and on-road tests. The application of condensation particle counter methodology was able to adjust the lower detection efficiency and ideal to determine the PN concentration due to the existence of a plateau region of detection efficiency over a sufficiently large range of particle sizes. A heated catalytic stripper improved volatile particle removal performance and prevented re-nucleation of volatile fractions at the particle counter. One of the technical challenges was higher particle losses below 23 nm.