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Abstract: Average particle number concentrations and size distributions from ∼61,000 light-duty (LD) vehicles and ∼2500 medium-duty (MD) and heavy-duty (HD) trucks were measured during the summer of 2006 in a San Francisco Bay area traffic tunnel. One of the traffic bores contained only LD vehicles, and the other contained mixed traffic, allowing pollutants to be apportioned between LD vehicles and diesel trucks. Particle number emission factors (particle diameter ) were found to be (3.9±1.4)×1014 and (3.3±1.3)×1015 #kg−1 fuel burned for LD vehicles and diesel trucks, respectively. Size distribution measurements showed that diesel trucks emitted at least an order of magnitude more particles for all measured sizes per unit mass of fuel burned. The relative importance of LD vehicles as a source of particles increased as decreased. Comparing the results from this study to previous measurements at the same site showed that particle number emission factors have decreased for both LD vehicles and diesel trucks since 1997. Integrating size distributions with a volume weighting showed that diesel trucks emitted 28±11 times more particles by volume (per unit fuel) than LD vehicles, consistent with the diesel/gasoline emission factor ratio for PM2.5 mass measured using gravimetric analysis of Teflon filters, reported in a companion paper. [Copyright &y& Elsevier]