Four per day backward trajectories were computed from Chebogue Point, Nova Scotia, for the month of August 1993 using two different three-dimensional approaches: diagnostic vertical velocity fields (kinematic) and an isentropic assumption. Essentially, 90% of all the kinematic-isentropic trajectory pairs were displaced within ¿75 hPa of each other and differed in the horizontal by about 10% of the travel distance. The greatest vertical displacement differences occurred with flow from the northwest and only three time periods were associated with the greatest displacements; a cyclonic system was to the north of the trajectory starting location in each case. Precipitation was associated with two of these events. A moist isentropic departure was computed for both kinematic and isentropic cases and departures were typically about 5 ¿K. Comparison of CO measurements and upwind trajectories suggested by temporal groupings of trajectories were consistent with transport from major urban areas. However, the peak measured CO values seemed to occur more frequently with flow transition periods, when trajectory uncertainty is the greatest. ¿ American Geophysical Union 1996