ABSTRACT Road grade is an important factor affecting vehicle emissions, but relatively few studies studied the impact of road grade on exhaust emissions of heavy-duty vehicles. This paper investigated the effect of road grade on the emissions of three vehicles, including a light-duty gasoline vehicle and two heavy-duty diesel vehicles. On-road emissions from three vehicles were recorded by Portable Emission Measurement System (PEMS) on an urban-traffic route. Vehicle Specific Power (VSP) analysis showed that ignoring road grade would overestimate the emission rate when VSP is negative and underestimate the emission rate when VSP is positive. Besides, in the grade range of -5 -- 5%, the exhaust emissions of the three vehicles are highly correlated with grade changes. The light-duty vehicle’s emission is more sensitive to road grade, while the absolute increment of NOx emission for the heavy-duty vehicle is much greater than that of the light-duty vehicle. Keywords: emissions; road grades; PEMS; heavy-duty vehicle; light-duty vehicle

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