Meteorological Processes and Ozone Exceedances in the Northeastern United States during the 12–16 July 1995 Episode*

The North American Research Strategy for Tropospheric Ozone-Northeast field measurements taken during 12–16 July 1995 indicate that ground-level ozone concentrations have exceeded the National Ambient Air Quality Standard level of 0.12 ppm for hourly ozone throughout the ozone transport region in the northeastern United States. Analyses of the meteorological conditions conducive to the ozone formation and accumulation reveal that the above ozone exceedances in the Northeast are associated with such meteorological features as the stagnant high pressure system, the Appalachian leeside trough, the frontal trough, the sea breeze, the channeling effects induced by the topography, and the stratified boundary layers. Also, aircraft measurements provide evidence for the buildup of ozone levels in the nighttime residual layer under southwesterly flows from 12 to 15 July. In this paper, it is shown that high ozone trapped aloft mixes downward, elevating the ground-level ozone concentrations as the daytime mixed layer starts to grow in the morning. Simulations using a photochemical box model (the Ozone Isopleth Plotting Package, Research Oriented Version) confirm that ozone trapped aloft in the nocturnal residual layer has a significant impact on the temporal evolution as well as on the peak ozone concentration near the surface. Evidence for the photochemical production and transport of ozone downwind of urban areas is presented using the data from surface as well as aircraft measurements.