OZONE PRODUCTION DURING AN URBAN AIR STAGNATION EPISODE OVER NASHVILLE, TENNESSEE

The highest O-3 levels observed during the 1995 Southern Oxidants Stud y in middle Tennessee occurred during a period of air stagnation from July 11 through July 15. Extensive airborne (two fixed wing and one he licopter) and ground-based measurements of the chemistry and meteorolo gy of this episode near Nashville, Tennessee, are presented. In situ a irborne measurements include O-3, NOy, NO, NO2, SO2, CO, nitrate, hydr ocarbons, and aldehydes. Airborne LIDAR O-3 measurements are also util ized to map the vertical and horizontal extent of the urban plume. The use of multiple instrumented research aircraft permitted highly detai led mapping of the plume chemistry in the vertical and horizontal dime nsions. Interactions between the urban Nashville plume (primarily a NO x and hydrocarbon source) and the Gallatin coal-fired power plant plum e (primarily a NOx and SO2 source) are also documented, and comparison s of ozone formation in the isolated and mixed urban and power plant p lume are presented. The data suggest that during this episode the back ground air and the edges of the urban plume are NOx sensitive and the core of the urban plume is hydrocarbon sensitive. Under these worst ca se meteorological conditions, ambient O-3 levels well over the level o f the new National Ambient Air Quality Standard (NAAQS) for ozone (80 ppb) were observed over and just downwind of Nashville. For example, o n July 12, the boundary layer air upwind of Nashville showed 60 to 70 ppb O-3, while just downwind of the city the urban plume maximum was o ver 140 ppb O-3. With a revised ozone standard set at 80 ppb (8 hour a verage) and upwind levels already within 10 or 20 ppb of the standard, only a slight increase in ozone from the urban area will cause diffic ulty in attaining the standard at monitors near the care of the urban plume during this type of episode. The helicopter mapping and LIDAR ai rcraft data clearly illustrate that high O-3 levels can occur during s tagnation episodes within a few kilometers of and even within the urba n area. The extremely light boundary layer winds (1-3 m s(-1)) contrib uted to the creation of an ozone dome or blob which stayed very near t o the city rather than an elongated plume. The small spatial scale of the zone of high O-3 concentrations is mapped in detail demonstrating that the regulatory monitoring network failed to document the maximum O-3 concentrations. Modelers using such regulatory data to test photoc hemical algorithms need to bear in mind that magnitude and frequency o f urban ozone may be underestimated by monitoring networks, especially in medium-sized urban areas under slow transport conditions. Finally, this effort shows the value of collaborative field measurements from multiple platforms in developing a more complete picture of the chemis try and transport of photochemical O-3.