A coupled meteorological and chemical model with a horizontal grid spacing of 12 km was employed to simulate the production/destruction, turbulent mixing, transport, and deposition of ozone over the western Great Lakes region between 15 July and 14 August 1999. Lake temperatures in the meteorological model were derived from daily satellite observations made at 3 km intervals. The meteorological model was evaluated with operational National Weather Service surface and upper observations, while the chemical model results were evaluated with U.S. EPA AIRS monitor data. During July of 1999, there several periods in which the meteorological conditions were conducive to local ozone production and regional-scale ozone transport into the region. After 1 August, however, the region was under the influence of a series of continental high-pressure systems so that relatively lower ozone mixing ratios occurred during this period. Ozone exposure was computed to determine cumulative effect of relatively high ozone concentrations in rural areas and the impact of pollutants on vegetation.

The simulated surface ozone compared well to observations in most locations. High ozone mixing ratios were frequently produced over the lake surfaces, even when meteorological conditions over land are not conducive to ozone production. Ozone mixing ratios over the lakes often exceeded the NAAQS of 120 ppb within in a shallow layer near the surface. High ozone mixing ratios were only produced only a few km inland as a result of dilution within the convective boundary layer, although ozone was advected far inland after the collapse of the convective boundary layer on several days. A sensitivity simulation was made that employed climatological lake temperatures instead of the daily observations. Ozone production over the lakes was very sensitive to lake temperature. Changing the lake temperatures in the model by 5 C changed ozone mixing ratios by as much as 50 ppb; however, the effect was smaller when stronger synoptic forcing was present after 1 August. The lake temperatures also had an indirect effect on ozone mixing ratios over land.