Seabreeze circulations have the ability to significantly modify the atmospheric boundary layer by replacing the existing air mass with low level incursions of cool, moist air, elevated wind speed, and changes in wind direction that can either increase or diminish the concentrations of chemical constituents depending on the relative difference between the in-situ and upstream properties. The NE-OPS Philadelphia site is near the western fringe of mid-Atlantic seabreeze intrusions (~ 50-100 km), and several propagated through the site during the 1999 and 2002 summer field intensives and were documented using the suite of instruments, including Raman lidar, tethersondes, profiler/RASS, and surface particulate and trace gas analyzers. Common to all seabreeze events is the sudden (~ 1-10 minutes) rise in relative humidity to near saturation, and the concurrent increase in optical extinction (total and back scatter coefficients) with the formation of haze droplets on existing aerosols. Measurements show that temperatures declined by several degrees and wind speed increased by factors of 2-3 as the shallow (100 – 300 m) air mass propagated through the site. The effect on ozone and other trace gas concentrations depends on the relative differences between the two air masses. Case studies are presented that show some episodes where ozone concentrations were reduced by nearly a factor of two (e.g., 165 to 95 ppbv), and other cases where ozone increased as the seabreeze front replaced drier, cleaner continental air. Doppler radar displays show that as the seabreeze continues westward, its length, intensity, and signature reflectivity diminishes, eventually becoming indistinguishable in the surrounding air mass, while conditions within the cool, moist air mass are sustained for hours after passage. The depth, propagation speed, and timing of the seabreeze are crucial to prediction of pollution episodes. Operational numerical models are capable of simulating the seabreeze circulation, but generally fail to accurately capture the depth and timing of these events at locations well removed from coastal regions, where they still can exhibit a significant influence on the local meteorology and chemistry.