Fine particle emissions from engine exhaust have attracted attention, because of concern of higher deposition fraction in alveoli. Solid particle number (PN) emission regulations have been implemented mainly in Europe in order to reduce fine particle emissions and improve sensitivity of conventional gravimetric particulate matter mass measurement methodology. Since it was observed that sizes of solid particles in exhaust of conventional internal combustion engine technologies are mainly distributed above 30 nm and sensitivity to volatile particles can be reduced, the current PN measurement methodology can identify particle number emissions just to bigger particle than 23 nm. Necessity in measurement of smaller particles than 23 nm is still under discussion. Despite of implementation of stricter emission regulations, pollutant levels in environment have not been improved sufficiently. It is surmised that there is difference between emissions under regulatory defined test cycles and real driving conditions. In these days, implementation of further real driving emission regulations utilizing portable emissions measurement systems (PEMS) are actively discussed in some regions. A commercially available PEMS for PN was modified to expand detection limit to smaller 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. Application of condensation particle counter methodology was feasible to adjust the lower detection efficiency and ideal to determine PN concentration with existence of plateau region of detection efficiency under sufficiently large particle sizes. A heated catalytic stripper improved volatile particle removal performance and prevented re-condensation of volatile fractions at the particle counter. One of technical challenges was higher dependency of detection efficiency on particle size with wider detectable particle size range of the counter due to higher diffusion losses of small particles.