Aerosol along the Harbor Freeway in Los Angeles, California, was sampled and measured with the Air Resources Board Mobile Air Pollution Laboratory. Simultaneously with the taking of filter and impactor samples for chemical analysis, the aerosol particle size distribution was measured with four continuous instruments over the particle size range from approx. 0.003-40 μm. From comparisons of measurements when the wind was directly from the freeway with measurements when the wind was blowing toward the freeway, it was possible to calculate by difference the direct contribution of the freeway traffic to the aerosol mixture. It was found that morning rush hour traffic contributes about 17.1 μm3 cm-3 to the aerosol volume, predominantly in the particle size range below 0.15 μm. The freeway aerosol size distribution exhibits a typical strong combustion mode at about 0.02 μm particle size.