Observations of Transport Processes for Ozone and Ozone Precursors during the 1991 Lake Michigan Ozone Study

Abstract

The Lake Michigan Air Quality Region (LMAQR) continues to experience ozone concentrations in urban and rural areas above the federal standard of 125 ppb. During the summer of 1991, the LMAQR states sponsored the Lake Michigan Ozone Study, which included a comprehensive field measurement program to gather data to understand the complex meteorology and air quality of that region and to verify predictions from air quality models. Air quality and meteorological data were collected by aircraft, surface monitors, rawinsondes, and radar wind profilers to characterize pollutant distributions and understand transport regimes. The unique meteorology of the LMAQR and the stable conditions over the lake produce several transport regimes. This paper presents observations of these transport regimes on two days with maximum surface ozone concentrations above 160 ppb in Wisconsin and Michigan. Transport of air with the highest ozone concentrations occurred in a conduction layer, which is a shallow layer of cool air over Lake Michigan. The conduction layer provided a stable environment where morning emissions remained concentrated and allowed reaction at high precursor concentrations. Southerly and southwesterly winds transported pollutants in this layer northward. Eventually this air flowed onshore and produced many nearshore ozone exceedances along the Wisconsin and Michigan shorelines. To successfully model ozone in the LMAQR, meteorological and air quality models must simulate these transport processes and stable conditions over the lake.