It is well-documented that high surface ozone concentrations can have an adverse effect on many different types of vegetation in the upper Great Lakes region. A comprehensive analysis of daily maximum ozone concentrations in the region has been completed to determine the frequency of occurrence of high ozone episodes and the spatial and temporal patterns of ozone for a 11-year period between 1985 and 1995. A comprehensive climatology has been performed using meteorological fields from the National Center for Environmental Prediction / National Center for Atmospheric Research (NCEP/NCAR) reanalysis data to identify the meteorological conditions associated with the high ozone episodes. The main advantage of employing this particular data set is that it provides a long, complete, and consistent record of meteorological fields using a state-of-the-art data assimilation system.
Ozone concentrations exceeded 80 ppb at rural forest and agricultural locations for less than 5% of the days for most stations in Minnesota, Wisconsin, and the upper peninsula of Michigan. The frequency of occurrence of ozone concentrations above 80 ppb gradually increased towards the south and southeast, with 20-25% of the days exceeding this value along the Ohio River valley; however, the highest frequency of 25-30% occurred along the Lake Michigan shoreline. It was found that the number of high ozone episodes (defined as days in which widespread ozone concentrations exceeded 80 ppb in the region) as varied significantly from year to year, with the summers of 1988 and 1991 having the most high ozone episodes, because ozone production is influenced by meteorological conditions such as temperature, humidity, and solar radiation that also vary from year to year. During the high ozone episodes, positive anomalies of near surface temperature, humidity, and 850-mb geopotential heights were found to be centered over the Great Lakes. An empirical orthogonal function (EOF) analysis has been performed on the 850-mb geopotential height anomalies for the high-ozone episode days. Individual EOF's describe the most prominent geopotential height departure fields that characterize the regional-scale weather patterns during the episodes. The EOF analysis suggests that there are 5 dominant synoptic patterns associated with high ozone episodes in the Great Lakes region and that the position of the high pressure ridge for each of the synoptic patterns indicates that several transport pathways are responsible for the high ozone concentrations in the region.