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In this study,the particle size-resolved distribution from a China-3 certificated fight-duty diesel vehicle was measured by using a portable emission measurement system (PEMS).In order to examine the influences of vehicle specific power (VSP) and high-altitude operation,measurements were conducted at 8 constant speeds,which ranged from 10 to 80 km/hr at 10 km/hr intervals,and two different high altitudes,namely 2200 and 3200 m.The results demonstrated that the numbers of particles in all size ranges decreased significantly as VSP increased when the test vehicle was running at lower speeds (＜ 20 km/hr),while at a moderate speed (between 30 and 60 /hr),the particle number was statistically insensitive to increase VSP.Under high-speed cruising conditions,the numbers of ultrafine particles and PM2.5 were insensitive to changes in VSP,but the numbers of nanoparticles and PM1o surged considerably.An increase in the operational altitude of the test vehicle resulted in increased particle number emissions at low and high driving speeds;however,particle numbers obtained at moderate speeds decreased as altitude rose.When the test vehicle was running at moderate speeds,particle numbers measured at the two altitudes were very close,except for comparatively higher number concentrations of nanoparticles measured at 2200 m.