Characterizing Ventilation and Exposure in Street Canyons Using Lagrangian Particles

The residence time measures the rate at which a pollutant escapes from a region of interest. Previous studies of urban ventilation have estimated the mean residence time from Eulerian data by assuming a spatially homogeneous pollutant field. Using a large-eddy simulation and a Lagrangian particle mo...

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Bibliographic Details
Published in:Journal of applied meteorology and climatology 2017-05, Vol.56 (5), p.1177-1194
Main Authors: Lo, K. W., Ngan, K.
Format: Article
Language:English
Subjects:
Age
Air exposure
Air pollution
Atoms & subatomic particles
Brownian motion
Circulation
Computational fluid dynamics
Dispersion
Distribution functions
Dye dispersion
Entrainment
Exposure
Indoor air quality
Lagrangian models
Large eddy simulation
Mathematical analysis
Oceanic eddies
Outdoor air quality
Pollutants
Probability distribution
Probability distribution functions
Probability theory
Residence time
Simulation
Stochastic models
Street canyons
Theory
Turbulence
Turbulent flow
Urban areas
Urban studies
Ventilation
Vortices
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Summary:The residence time measures the rate at which a pollutant escapes from a region of interest. Previous studies of urban ventilation have estimated the mean residence time from Eulerian data by assuming a spatially homogeneous pollutant field. Using a large-eddy simulation and a Lagrangian particle model, the residence and exposure times are calculated for an idealized street canyon in the skimming-flow region and a deep street canyon within a realistic urban area. For both domains, the mean residence time is on the order of a canyon circulation time scale, while the mean exposure time, which includes re-entrainment and characterizes the total time spent by a pollutant in a region of interest, is about 20% longer. Intensive quantities such as the Lagrangian visitation factor and return coefficient indicate that re-entrainment is modest. Probability distribution functions of the exposure and residence times are nearly exponential for both domains, in accord with pure diffusion and single-time-scale, vertical-exchange parameterizations. It is argued that, by analogy with Brownian motion, the mean residence and exposure times are set primarily by the mean circulation rather than the turbulence when the flow approximates that within a two-dimensional street canyon.
ISSN:1558-8424
1558-8432
DOI:10.1175/jamc-d-16-0168.1