title: The effects of the built environment, traffic patterns, and micrometeorology on street level ultrafine particle concentrations at a block scale: Results from multiple urban sites.
abstract: This study attempts to explain explicitly the direct and quantitative effects of complicated urban built-environment on near-road dispersion and levels of vehicular emissions at the scale of several city blocks, based on ultrafine particle concentrations ([UFP]). On short timescales, ultrafine particles are an excellent proxy for other roadway emissions. Five measurement sites in the greater Los Angeles with different built environments but similar mesoscale meteorology were explored. After controlling for traffic, for most sampling days and sites, morning [UFP] were higher than those in the afternoon due to limited dispersion capacity combined with a relatively stable surface layer. [UFP] at the intersection corners were also higher than those over the sampling sites, implying that accelerating vehicles around the intersections contributed to [UFP] elevation. In the calm morning, the areal aspect ratio (Ararea), developed in this study for real urban configurations, showed a strong relationship with block-scale [UFP]. Ararea includes the building area-weighted building height, the amount of open space, and the building footprint. In the afternoon, however, when wind speeds were generally higher and turbulence was stronger, vertical turbulence intensity σw was the most effective factor controlling [UFP]. The surrounding built environment appears to play an indirect role in observed [UFP], by affecting surface level micrometeorology. The effects are substantial; controlling for traffic, differences in Ararea and building heterogeneity were related to differences in [UFP] of factors of two to three among our five study sites. These results have significant implications for pedestrian exposure as well as transit-oriented urban planning.
authors.affiliation: University of California, Los Angeles, Department of Atmospheric and Oceanic Sciences, 405 Hilgard Ave., Los Angeles, CA 90095, USA; University of California, Los Angeles, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, Los Angeles, CA 90095, USA. Electronic address: wschoi@atmos.ucla.edu.
authors.forename: Wonsik
authors.initials: W
authors.lastname: Choi
authors.affiliation: University of California, Los Angeles, Department of Atmospheric and Oceanic Sciences, 405 Hilgard Ave., Los Angeles, CA 90095, USA.
authors.forename: Dilhara
authors.initials: D
authors.lastname: Ranasinghe
authors.affiliation: University of California, Los Angeles, Department of Atmospheric and Oceanic Sciences, 405 Hilgard Ave., Los Angeles, CA 90095, USA.
authors.forename: Karen
authors.initials: K
authors.lastname: Bunavage
authors.affiliation: University of California, Los Angeles, Luskin Center for Innovation, Luskin School of Public Affairs, 3250 Public Affairs Bldg., Los Angeles, CA 90095, USA.
authors.forename: J R
authors.initials: JR
authors.lastname: DeShazo
authors.affiliation: University of California, Los Angeles, Luskin Center for Innovation, Luskin School of Public Affairs, 3250 Public Affairs Bldg., Los Angeles, CA 90095, USA.
authors.forename: Lisa
authors.initials: L
authors.lastname: Wu
authors.affiliation: University of California, Los Angeles, Department of Atmospheric and Oceanic Sciences, 405 Hilgard Ave., Los Angeles, CA 90095, USA; Center for Environmental Sciences, Faculty of Sciences, University of Chile, Las Palmeras 3425 Ñuñoa, Santiago, Chile.
authors.forename: Rodrigo
authors.initials: R
authors.lastname: Seguel
authors.affiliation: University of California, Los Angeles, Fielding School of Public Health, Environmental Health Sciences Department, 650 Charles Young Dr., Los Angeles, CA 90095, USA.
authors.forename: Arthur M
authors.initials: AM
authors.lastname: Winer
authors.affiliation: University of California, Los Angeles, Department of Atmospheric and Oceanic Sciences, 405 Hilgard Ave., Los Angeles, CA 90095, USA; University of California, Los Angeles, Institute of the Environment and Sustainability, La Kretz Hall, Suite 300, Los Angeles, CA 90095, USA.
authors.forename: Suzanne E
authors.initials: SE
authors.lastname: Paulson
keywords: Aspect ratio; Built-environment; Pedestrian exposure; Transit-oriented development; Turbulence; Ultrafine particles