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-l evel ozone concentrations have exceeded the National Ambient Air Quali ty Standard level of 0.12 ppm for hourly ozone throughout the ozone tr ansport region in the northeastern United States. Analyses of the mete orological conditions conducive to the ozone formation and accumulatio n reveal that the above ozone exceedances in the Northeast are associa ted with such meteorological features as the stagnant high pressure sy stem, the Appalachian leeside trough, the frontal trough, the sea bree ze, the channeling effects induced by the topography, and the stratifi ed boundary layers. Also, aircraft measurements provide evidence for t he buildup of ozone levels in the nighttime residual layer under south westerly flows from 12 to 15 July. In this paper, it is shown that hig h ozone trapped aloft mixes downward, elevating the ground-level ozone concentrations as the daytime mixed layer starts to grow in the morni ng. Simulations using a photochemical box model (the Ozone Isopleth Pl otting 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 o zone downwind of urban areas is presented using the data from surface as well as aircraft measurements.