Abstract. A 3-day period of strong, synoptic-scale stagnation, in which daytime boundary-layer winds were light and variable over the region, occurred in mid July of the 1995 Southern Oxidants Study centered on Nashville, Tennessee. Profiler winds showed light and variable flow throughout the mixed layer during the daytime, but at night in the layer between 100 and 2000 m AGL (which had been occupied by the daytime mixed layer) the winds accelerated to 5-10 m s-1 as a result of nocturnal decoupling from surface friction, which producect inertial oscillations. In the present study, we investigate the effects of these wind changes on the buildup and transport of ozone (03). The primary measurement system used in this study was an airborne differential absorption lidar (DIAL) system that profiled 03 in the boundary layer as the airplane flew along. Vertical cross sections showed that 03 concentrations exceeding 120 ppb extended up to nearly 2 km AGL, but that the 03 hardly moved at all horizontally, instead forming a dome of pollution over or near the city. The analysis concentrates on four meteorological processes that determine the 3-D spatial distribution of 03 and the interaction between urban and rural pollution: (1) daytime buildup of 03 over the urban area, (2) the extent of the drift of pollution cloud during the day as it formed, which controls peak 03 concentrations, (3) nighttime transport by the accelerated winds above the surface, and (4) vertical mixing of pollution layers the next day. Other consequences of very light-wind conditions were intra-regional differences in daytime mixed-layer depth over distances of 50 km or less, and indications of an urban heat-island circulation.
IntroductionOver relatively simple topography, such as that found near Nashville, Tennessee, the overriding control on the transport and dispersion of atmospheric pollutants is the large-scale (usually synoptic) wind speed and direction. Occasionally, the larger-scale flow becomes weak, indicating that synoptic controls on flow and transport are weak (except for the large-scale subsidence accompanying these conditions). Two implications of such stagnation are (1) that pollution emissions are confined to a much smaller volume than when stronger flow is present, often leading to the highest pollutant concentrations in a season, and (2) that smaller-mesoscale influences, such as surface heating differences, can express themselves as locally generated flows or variations in mixing properties.Because stagnation periods routinely produce the highest pollution concentrations, it is important to understand the key meteorological processes affecting the distribution and magnitude of the ozone buildup. We identify four processes that control the spread of the urban plume. The first three represent a sequence occurring through different stages of the diurnal heating cycle, and the fourth, advection, represents a daytime control that proved to be significant even under very light wind conditions. The processes are as follows: (1)
