RELATIONS BETWEEN METEOROLOGY AND OZONE IN THE LAKE-MICHIGAN REGION

The held program phase of the Lake Michigan Ozone Study (LMOS) took place during the summer of 1991. Observed ozone concentrations and weather variables have been analyzed for the Lake Michigan region and the eastern United States for four 1991 LMOS ozone episodes covering 21 days. It is found that all LMOS episodes are associated with large polluted regions with dimensions of 1000-2000 km, located on the western side of high pressure systems centered over the eastern United States. Consequently, the air coming into the LMOS region contains significant amounts of ozone, haze, and other pollutants advected along trajectories with anticyclonic curvature originating in upwind source regions from St. Louis, through the Ohio River valley, and into the northeast megalopolis. The local sources in the Gary-Chicago-Milwaukee region then add to this already polluted air mass, where concentrations are influenced by the strong stability of the boundary layer over Lake Michigan and the associated lake-land-breeze circulations. However, the magnitudes of ozone concentrations in the LMOS region are often quickly and significantly reduced by rain and by fronts, which are present in some portion of the domain during nearly all of the days that were studied. The portion of the urban ozone plume that is not influenced by rain and fronts is observed to be advected into rural areas at downwind distances of 100-200 km or more from Chicago, and often has concentrations higher than those in Chicago. Whether maximum concentrations are located on the western or the eastern shoreline of Lake Michigan depends on whether winds have an easterly or westerly component.