Coastal boundary layer transport of urban pollution in New England

The coastal boundary layer plays a key role in transporting large concentrations of ozone, aerosols, and other pollutants from the major urban areas of the northeastern U.S. to New Hampshire and Maine. During pollution episodes, the air over land in daytime is warmer than the sea surface, so air transported from land over water becomes statically stable and separates into possibly several layers, each transported in a different direction. We present two such episodes in the summer of 2002 (22-23 July and 11-14 August). In both cases, the pollutants that affected coastal New Hampshire and coastal southwest Maine were transported over coastal waters in stable layers at the surface. These layers were at least intermittently turbulent, but retained their chemical constituents. The lack of deposition or deep vertical mixing on the overwater trajectories allowed pollutant concentrations to remain strong. The polluted plumes came directly from the Boston area. In the 22-23 July case, the trajectories were relatively straight and dominated by synoptic-scale effects, transporting pollution to the Maine coast. On 11-14 August, sea breezes brought polluted air from the coastal waters inland into New Hampshire. Observations from radar wind profilers, lidars, and surface chemical and meteorological instruments answer some questions and raise others. A 3D mesoscale model (COAMPS) including a tracer emitted from Boston provides a more complete picture of the transport.