Journal
SCIENCE OF THE TOTAL ENVIRONMENT
Volume 553, Issue -, Pages 474-485Publisher
ELSEVIER
DOI: 10.1016/j.scitotenv.2016.02.083
Keywords
Ultrafine particles; Built-environment; Aspect ratio; Turbulence; Pedestrian exposure; Transit-oriented development
Categories
Funding
- California Air Resources Board [12-308]
- U.S. National Science Foundation [CNS-1111971001]
- Korean Ministry of Environment through Climate Change Correspondence Program
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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 (Ar-area), developed in this study for real urban configurations, showed a strong relationship with block-scale [UFP]. Ar-area 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 sigma(w) 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 Ar-area 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. (C) 2016 Elsevier B.V. All rights reserved.
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