Atmospheric aerosol particles have been sampled near a Los Angeles freeway and analyzed by proton-induced X-ray emission, PIXE, as a sulfur baseline study in September 1974 before 1975-model auto mobiles, equipped with catalytic emission control devices, appeared in large numbers. The sampling plan was optimized a c cording to the requirements of X-ray analysis techniques, fluctuations in air flow across the freeway, and the time and particlesise resolution of the aerosol sampling equipment. During five selected two-hour intervals with cross wind conditions, particles were collected as a function of size by six cascade impactorsope rating simultaneously on three towers at 2 and 7 meters above road level 35 meters upwind and 35 meters downwind of the traffic lanes. Particles as a function of time were sampled continuously, for the 100-hour duration of the experiment, from the three 7-meter heights using time series streaker filter samplers with two-hour time resolution. Precise analyses of S, Cl, K, Ca, Fe, Br, and. Pb established relationships which permit estimates to be made of future increases in particulate sulfur as a consequence of increased sulfate emissions from catalytic automobiles.