UC Riverside

The built environment surrounding arterials impacts the dispersion of vehicular emissions in urban areas, modifying the potential risks to public health, which are not incorporated in current models. In order to study the influence of urban morphometry on flow, turbulence and dispersion of vehicular fine particulate matter emissions, water channel experiments and field measurements were performed. The research started from the investigation of flows through urban-like rectangular arrays of cubical obstacles. Then a mock downtown Los Angeles building cluster was built to simulate the dispersion of vehicular emissions within a complex built environment. Meanwhile, field experiments were carried out in five study areas in the Greater Los Angeles area.

For rectangular arrays, Particle Image Velocimetry (PIV) was used for comprehensive flow measurements. A novel flow feature, lateral channeling, observed and quantitatively measured within regular 3 × 3 and 5 × 5 arrays. Low pressure in the wake region drew the fluid through the array, which led to formation of the strongest lateral inflow in front of the last row of buildings.

For the complex urban setup, simultaneous Particle Image Velocimetry/Planar Laser Induced Fluorescence (PIV/PLIF) technique was applied to study the impact of an individual tall building on flow characteristics and plume dispersion in built environment. The results suggested that the presence of the tall building increase 16% of vertical plume spread by adding 0.02H* to characteristic length H*. The larger integral time scale of concentration fluctuations Tc below the height of H* indicated that it took longer to get plume well mixed when the tall building was present. The removal of plume from the canyon was driven mainly by the advective process.

Field measurements suggested that the observed meteorological data at surface level within the urban canopy has a reasonable agreement with the Monin-Obukhov similarity theory. The generalized additive models showed that urban mixing dominated the variation of roadside particle concentrations regardless of urban mophometry. In Los Angeles, the increasing vertical velocity fluctuation and vertical mean wind speed reduced fine particle concentrations at street level. The estimated fine particle emission factor was 0.021 g/(vehicle⋅km) in the Los Angeles area.