Coastal regions are among the most difficult to model from an air-quality perspective. Many of the largest air pollution sources are located in coastal regions and the transition from land to sea makes the meteorological modeling extra difficult. Thus where the challenges are the largest, the task is also the most difficult. The most commonly discussed cases with severe air pollution problems relate to large coastal cities surrounded by high terrain. Stagnant synoptic weather, sea breeze circulations and topographic trapping of local emissions are then the main issues, and prominent examples are Los Angles in USA, or Athens in Greece.

Along the New England coastline the situation is very different. The local sources are minor and there is no significant coastal terrain. Instead the major pollution sources are located farther south, for example in New York and Boston, and pollution episodes occur when the sea surface temperature is substantially lower than inland temperatures for much of the diurnal cycle. Pollution from remote sources is transported from land over water, where the air becomes statically stable and pollution plumes separate into possibly several layers, each transported in a different direction. Over the ocean, concentrations remain high due to a weaker turbulent mixing until the plume is advected inland again, either through a curved coastline turning into the plume or by enhanced cross-coast transport by sea breeze circulations. When this happens, concentrations along the coast, where the majority of the population reside, can rapidly become quite high; this happens regularly in summer.

We present fully 3D mesoscale simulations of tracer transport using the COAMPS model, for two cases from the NEAQS 2002 experiment. In one, the dispersion is dominated by synoptic scale flow while in the other a sea breeze is the main transporting feature. In an attempt to clarify several aspects of the cross-coast flow, simpler exploratory 2D simulations with the MIUU model were also performed. In particular, we examine the questions: - How quickly does the boundary layer stabilize after crossing from land to water (?) and what fluxes are required to accomplish the stabilization? - What physical processes create the surface-based inversion? Is shear-driven turbulence sufficient or does advected turbulence also play a role? How intense is the turbulence? - Are multiple distinct layers formed from the continental boundary layer after it passes over the water, and if so, how many and how deep is each of the layers? - Under what conditions is part of the polluted continental air lofted above a clean marine layer? - Does a mixed layer form again downwind, and if so, how far downwind? - What is the structure of the pollution plume from the continent in three dimensions? How does it vary with time?