Plume Rise and Dispersion of Emissions from Low Level Buoyant Sources in Urban Areas

The projected increase in distributed power generation (DG) has given rise to the concerns on the air quality impact of small power plants located in urban areas. In order to estimate this impact, there is a need for a model that can treat plume rise and dispersion of a buoyant release in an inhomogeneous urban boundary layer whose structure is governed by complex surface characteristics. Such a model requires three essential ingredients: 1) a realistic treatment of the interaction between the highly turbulent urban canopy layer and the turbulent plume spread, 2) a plume rise model that can accounts for the flow modifications caused by buildings, and 3) an appropriate estimate of the height of the nocturnal urban boundary layer. Comprehensive laboratory and field studies were conducted to investigate each of these elements separately. Ground level concentrations (GLC) associated with a modeled DG were measured inside the water channel under different surrounding building geometries. Results from these measurements indicated that surrounding buildings induce vigorous vertical mixing which increase the near source GLC. Further investigations focused on the plume rise from these sources. The results from plume rise measurements suggested that plume exiting the DG stack can be significantly impacted by the flows induced by surrounding buildings. In addition to dispersion and plume rise measurements, a field study was conducted in Riverside, CA and the structure of the nocturnal urban boundary layer was investigated over three different nights. Results from these measurements helped us to develop a semi-empirical model that can predict the height of the stable boundary layer. Although we were able to reasonably understand and develop models to predict the micro-meteorology as well as plume rise and spread; we concluded that simple Gaussian dispersion models have limited performance in predicting the concentrations associated with urban sources due to the substantial complexity involved with the urban dispersion process