Dispersion from ground-level sources in a shoreline urban area

We present results from a field study conducted in Wilmington, a suburb of Los Angeles, during 8 days of the period 26 August–10 September 2004. The tracer, sulfur hexafluoride, was released at a height of 3 m from a power plant site on the shoreline, and the concentrations of the tracer were sampled on five arcs at 100, 400, 1000, 3000, and 5000 m from the source during 6 h of the day starting at 7 a.m. This resulted in 40-h-long experiments, out of which, 21 had concentration measurements that could be interpreted with models. The meteorological conditions that governed dispersion were measured using sonic anemometers and sodars. The data analysis indicates that even during summer, the stability of the onshore flow is strong enough to keep the height of the convective internal boundary layer below 150 m at distances of 5000 m from the shoreline. However, the turbulence levels in the stable boundary layer are not smaller than those in the surface convective layer suggesting the presence of a shear generated boundary layer, which is advected with the onshore flow. A simple Gaussian dispersion model was used to interpret the ground-level concentrations measured in the experiment. The model uses expressions for plume spreads that depend on meteorological information at a height of 50 m from the surface. The vertical spread of the plume is limited to the height of the shear generated boundary layer. The height of this boundary layer is proportional to σw/N, where σw is the standard deviation of the vertical velocity fluctuations, and N is the Brunt–Vaisala frequency of the stable layer capping the surface-based convective layer. This result is based on indirect evidence: model performance improves significantly when vertical plume spread is limited to this height.