Daytime buildup and nighttime transport of urban ozone in the boundary layer during a stagnation episode

A 3-day period of strong, synoptic-scare 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 produced inertial oscillations. In the present study, we investigate the effects of these wind changes on the buildup and transport of ozone (O-3) The primary measurement system used in this study was an airborne differential absorption lidar (DIAL) system that profiled O-3 in the boundary layer as the airplane flew along. Vertical cross sections showed that O-3 concentrations exceeding 120 ppb extended up to nearly 2 km AGL, but that the O-3 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 O-3 and the interaction between urban and rural pollution: (1) daytime buildup of O-3 over the urban area, (2) the extent of the drift of pollution cloud during the day as it formed, which controls peak O-3 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.