Eastern North American transport climatology during high- and low-ozone days

A synoptic scale transport climatology during days with high and low ozone concentrations was established for five summers from 1991 to 1995 over eastern North America. The airmass transport patterns were estimated from source impact regions derived from forward airmass histories. Daily maximum ozone concentrations were used to define locally and regionally high- (90th percentile) and low- (10th percentile) ozone days. Examination of transport during locally high-ozone days revealed that there is poor transport in the central part of the domain, i.e. from Tennessee to northern Indiana, and in the South from Texas to South Carolina due to stagnating or recirculating air masses. However, high ozone concentrations over the western and northern sections of the domain occurred during strong and persistent southerly and westerly winds, respectively. In addition, on average, high ozone concentration surrounding the central part of the domain were associated with transport from this region. These results support the notion that ozone exceedances in the central and Southeastern US are predominately “homegrown” while the western and northern section of the domain are also influenced by regional transport. In contrast, on low-ozone days, the transport was predominantly from outside (e.g., Canada and the Gulf of Mexico) into the Eastern US. The transport conditions during regionally high-ozone days were characterized by slow meandering transport over Kentucky, Tennessee, and West Virginia, with a strong clockwise transport around this region of stagnation.