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.

Vehicular emissions from arterials may present a risk to public health considering the type of surrounding built environments that can trap pollutants. In order to study the influence of urban morphometry on flow and dispersion of vehicular emissions, field measurements were performed in major arterials in 5 Southern Californian cities with different building geometries. Local mean wind, turbulence, virtual temperature, roadside fine particulate matter (PM2.5) concentration, and traffic flow data were collected in summer 2008. In each city, data were collected for three days, covering two hours during the morning and evening commute and lighter mid-day traffic. First, the observation shows the influence of building geometry on street level concentration of particulates. Tall buildings cause a strong downdraft which upon impinging the street level flushes street canyon from pollutants. Second, field experiments help us understand the influence of local meteorological variables and their interaction with urban canopy to particle concentration. Concentrations at the windward side of buildings within urban canopy are extremely sensitive to wind direction. In addition to wind direction, turbulent flux, sensible heat flux and turbulent velocity are also affecting concentrations by enhancing vertical transport.

Vehicular emissions from arterials may present a risk to public health considering the type of surrounding built environments that can trap pollutants. In order to study the influence of urban morphometry on flow and dispersion of vehicular emissions, field measurements were performed in major arterials in 5 Southern Californian cities with different building geometries. Local mean wind, turbulence, virtual temperature, roadside fine particulate matter (PM2.5) concentration, and traffic flow data were collected in summer 2008. In each city, data were collected for three days, covering two hours during the morning and evening commute and lighter mid-day traffic. First, the observation shows the influence of building geometry on street level concentration of particulates. Tall buildings cause a strong downdraft which upon impinging the street level flushes street canyon from pollutants. Second, field experiments help us understand the influence of local meteorological variables and their interaction with urban canopy to particle concentration. Concentrations at the windward side of buildings within urban canopy are extremely sensitive to wind direction. In addition to wind direction, turbulent flux, sensible heat flux and turbulent velocity are also affecting concentrations by enhancing vertical transport.

This paper provides preliminary evidence that the placement of buildings influences the concentration of fine particulates by altering wind flow. The authors collected measurements of fine particulate concentration, wind speed, wind direction, and traffic levels around five Southern California arterials selected to represent a range of building densities. In some cases the difference in average concentrations between opposite sides of the street was on the order of 10 μg/m3. In most cases the concentration was higher on the upwind side of the street, where the wind wakes of buildings limit the dispersion of particulates. Although this work is exploratory in nature, it reveals that fine particulate concentrations can vary even within a single city block, a scale finer than those used in current policy models. Given the trend towards infill development and densification in many places, this is an important topic that warrants further research to more fully understand the influence of the built environment on air quality.

Project included three major parts: 1) field measurements of particulate matter in five urban areas, 2) laboratory modeling of flow and dispersion within model urban areas, and 3) numerical modeling. Project website and database are located at http://emissions.engr.ucr.edu/.