Ozone production in urban plumes transported over water: Photochemical model and case studies in the northeastern and midwestern United States

Abstract

Abnormally high concentrations of O 3 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 O 3 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 O 3 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 O 3 , NO x and volatile organic carbon (VOC) forms as the Chicago plume travels over Lake Michigan, a pattern consistent with observed O 3 at surface monitoring sites. Near‐total suppression of dry deposition of O 3 and NO x over the lake is needed to produce high O 3 . Results for the east coast suggest that the observed peak O 3 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 O 3 to emissions of NO x and VOC suggests that results vary greatly between cities, even when the composition of urban emissions is similar. An index for VOC versus NO x sensitivity is shown to correlate with total reactive nitrogen (NO y ) at the time of peak O 3 .