The Effect of Lake Temperatures and Emissions on Ozone Exposure in the Western Great Lakes Region

Abstract

A meteorological-chemical model with a 12-km horizontal grid spacing was used to simulate the evolution of ozone over the western Great Lakes region during a 30-day period in the summer of 1999. Lake temperatures in the model were based on analyses derived from daily satellite measurements. The model performance was evaluated using operational surface and upper-air meteorological measurements and surface chemical measurements. Reasonable agreement between the simulations and observations was obtained. The bias (predicted - observed) over the simulation period was only - 1.3 ppb for the peak ozone mixing ratio during the day and 5.5 ppb for the minimum ozone mixing ratio at night. High ozone production rates were produced over the surface of the lakes as a result of stable atmospheric conditions that trapped ozone precursors within a shallow layer during the day. In one location, an increase of 200 ppb of ozone over a 9-h period was produced by chemical production that was offset by losses of 110 ppb through vertical mixing, horizontal transport, and deposition. The predicted ozone was also sensitive to lake temperatures. A simulation with climatological lake temperatures produced ozone mixing ratios over the lakes and around the lake shores that differed from the simulation with observed lake temperatures by as much as 50 ppb, while the differences over land were usually 10 ppb or less. Through a series of sensitivity studies that varied ozone precursor emissions, it was shown that a reduction of 50% in NOx or volatile organic compounds would lower the 60-ppb ozone exposure by up to 50 h month-1 in the remote forest regions over the northern Great Lakes. The implications of these results on future climate change and air quality in the region are discussed.