Tetroon trajectories within the Los Angeles Basin in the autumn of 1973 show that, on non-stagnation days, air located in the Los Angeles area in the morning can pass over the Puente Hills and reach the San Bernardino-Riverside area by mid-afternoon of the same day. However, the enhanced vertical mixing associated with these Hills would be expected to dilute any pollution present. Of perhaps more importance is the evidence that, on stagnation days when the atmosphere is stable, air from the Los Angeles area may drift southward in the early morning katabatic flow, stagnate for 2–3 h in the industrialized and high vehicle-density region north of Long Beach, and then move rapidly eastward with the sea breeze flow through Santa Ana Canyon, reaching the Riverside-San Bernardino area in late afternoon. In this case there would seem to be more potential for severe pollution in the latter area. However, the frequency of occurrence of this particular trajectory pattern is uncertain.