Abnormally high concentrations of O3 have been observed in rural locations on the shore of Lake Michigan and on the Atlantic coast in Maine, at a distance of 300 km or more from major anthropogenic sources. We hypothesize that this O3 is associated with transport from major urban centers and with the suppression of vertical mixing as urban plumes are transported over water. A dynamical/photochemical model is developed that represents formation of O3 in shoreline environments and is used to simulate case studies for Lake Michigan and the northeastern United States. Results suggest that a broad region with elevated O3, NOx, and volatile organic carbon (VOC) forms as the Chicago plume travels over Lake Michigan, a pattern consistent with observed O3 at surface monitoring sites. Near-total suppression of dry deposition of O3 and NOx over the lake is needed to produce high O3. Results for the east coast suggest that the observed peak O3 can only be reproduced by a model that includes suppressed vertical mixing and deposition over water, 2-day transport of a plume from New York, and superposition of the New York and Boston plumes. An investigation of the sensitivity of O3 to emissions of NOx and VOC suggests that results vary greatly between cities, even when the composition of urban emissions is similar. An index for VOC versus NOx sensitivity is shown to correlate with total reactive nitrogen (NOy) at the time of peak O3. ¿ American Geophysical Union 1992