During the summers of 2004 and 2006, extinction and scattering coefficients of particle emissions inside a San Francisco Bay Area roadway tunnel were measured using a combined cavity ring-down and nephelometer instrument. Particle size distributions and humidification were also measured, as well as several gas phase species. Vehicles in the tunnel traveled up a 4% grade at a speed of approximately 60 km hr-1. The traffic situation in the tunnel allows the apportionment of emission factors between light duty gasoline vehicles and diesel trucks. Cross-section emission factors for optical properties were determined for the apportioned vehicles to be consistent with gas phase and particulate matter emission factors. The absorption emission factor (the absorption crosssection per mass of fuel burned) for diesel trucks (4.4±0.79 m2 kg-1) was 22 times larger than for light-duty gasoline vehicles (0.20±0.05 m2 kg-1). The single scattering albedo of particles – which represents the fraction of incident light that is scattered as opposed to absorbed – was 0.2 for diesel trucks and 0.3 for light duty gasoline vehicles. These facts indicate that particulate matter from motor vehicles exerts a positive (i.e., warming)
radiative climate forcing. Average particulate mass absorption efficiencies for diesel trucks and light duty gasoline vehicles were 3.14±0.88 m2 gPM
-1 and 2.9±1.07 m2 gPM -1, respectively. Particle size distributions and optical properties were insensitive to increases in relative humidity to values in excess of 90%, reinforcing previous findings that freshly emitted motor vehicle particulate matter is hydrophobic.