University of California Transportation Center

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.