Boundary layer processes associated with three pollution events during the North Atlantic Regional Experiment (NARE) 1993 field campaign are examined using airborne measurements and a Lagrangian particle dispersion model in conjunction with a mesoscale model employing four-dimensional data assimilation. This nonphotochemical modeling system was able to qualitatively reproduce many of the observed features during a 15-day period of the NARE field campaign. Simulated particle releases from urban regions within the daytime convective boundary layer were transported to heights up to 2 km above ground level. Particles located in the upper part of the residual layer in the early evening hours were quickly advected by higher wind speeds aloft to the sampling domain; however, particles released within the nocturnal stable boundary layer or the marine boundary layer remained within 200 m of the surface and often exhibited complex circulation patterns. As a consequence of the diurnal boundary layer characteristics, emissions released within a relatively close time interval were often found to have significantly different trajectories. Mixing of particles from various source regions results in a plume that does not have a unique age but is better characterized by a distribution of ages which vary with altitude. It is shown that much of the layering over Yarmouth is well established by convective boundary layer processes and vertical wind shears prior to the air masses leaving land. As expected, sea surface temperatures were found to play an important role in defining the vertical gradient of potential temperature and hence the amount of vertical mixing over the Gulf of Maine. Peak particle concentrations within 1 km of the ocean were often associated with a low-level jet over the Gulf of Maine. A common feature during the analysis period is synoptic-scale lifting in advance of low-pressure systems which appears to be partially responsible for lifting particles to the heights observed over Yarmouth. ¿ American Geophysical Union 1996